For a trial run, you can build one make-shift unsealed tank. Try it and see how hot the water becomes. Or better yet, see how quickly it heats, because that's what's most important if you want to direct warmed waters to your regular hot water tank. But for the money-saving purposes of the last section of the last chapter, we need sealed tanks. Can we build out own? Is it safe to build sealed tanks in a rectangular shape filled with the sort of water pressures used typically in homes? Can home-built rectangular tanks is two pieces or more be shut tight by bolts, and yet prevent leakage?
A hot liquid under pressure is not as dangerous as a gas under pressure. This is due to the incompressibility of a liquid. In other words, virtually as soon as the heated liquid under pressure finds an escape route, the explosive potential or elasticity of the water body is gone. The only thing remaining will be the water pressure force out the escape route. Theoretically, it can be dangerous if for example a small escape route the size of a pin head shoots hot water into your eye. Otherwise, I can't see a danger aside from what damage the hot water can do if it leaks in large bulk with a total breakdown of the tank. Therefore, if you're going to build a tank, you need to assure that there is no total break down or large leak. Any danger to humans or household items from such a possibility is greatly minimized with the tank located on the roof.
To this it can be added that the authorities allow the use a steel hot water tanks that rust out over years until they may leak hot water onto a floor. The reason that your hot water tank is cylindrical in shape may be partially due to reduced metal cost, for a cylindrical shape is stronger than a square/rectangular shape. Moreover, rectangular tanks require ribbing (very expensive for the manufacturer) to keep pressures from bowing the sides. But if we have the time, we can make rectangular tanks take the pressures needed.
But why? Why not purchase cylindrical tanks and be done with it? They won't roll side-ways off a roof, though they might look funny. The problem comes when we've got to get inside them to add nipples in preparing heater tanks for the cockpit situation of the last chapter. You don't have to do this if all you want is to save money with your gas / electricity bills; just put a metal tank on the roof and you're off to the races because it already has a pipe inlet and outlet inserted for you. However, this is a book on tribulation preparation, not on saving you money. The point of saving you money is to help get your money back for preparing solar heaters now that you can use in the trib. Everybody likes a win-win.
Homes on wells may use water pressure in the order of 40 pounds per square inch, while city water can be more than twice that. But one can just get a small typical well pump ($150 or not much more) and small cold-water tank, placing them both between the solar-heated storage tank and the regular household hot water tank. Done! You do not need reinforced sealed tanks. You do not need sealed tanks at all...providing you have electricity for the pump or other means of pressurization. The pressure vessel pump (webpage below) looks like a mini version of a well-pump and water-tank combination.
The pumps above eliminate the need for pressurized water after the solar tanks, but does not account for the pressurized water feeding the solar tanks. In other words, we've got to reduce the pressure from the household water before it enters the first solar tank. This can be done by an automatic shut-off valve; see last chapter for that topic. There may also be valves triggered shut when climbing to a certain low pressure.
Excellent. The extra cost on building the sealed tank may as well go toward the pump-and-tank combination above...that eliminates the need for sealed tanks. Is that not sweet?
I suggest roof tanks of a rectangular shape six feet long, 21 inches wide, and three to four inches deep (about 24 gallons at 4" deep). I read that about four inches of water depth over a black solar-collector sheet of metal is an ideal depth, but you should check this out and make it to your liking.
Now that sealed tanks no longer appear necessary, why use any metal sides for it...aside from the solar collector that lies on the bottom of a transparent tank? Yes, a transparent top side to let sun in. The bottom side can be plywood or plastic, but I strongly recommend that the frame between the top and bottom be of plastic so that water deterioration is not an issue. Before committing to building a whole system, however, you may want a cheap trial-run tank, in which case 2 x 4's for the frame would be the way to go.
For plastic parts to build the tank, google "plastic lumber" or "plastic plywood." At the Taylors Plastic webpage below, a 12-foot length of 1.5" x 3.5" rectangular plastic rod is under $50 (about $4 per foot), but for $32 (January 2013 price) we can use 1" x 3.5" just fine. I would not use a plastic sheet for the bottom as it's very expensive, as you can see in the chart. A 4' x 8' sheet of plastic 1" thick is $343 in grey. It's a Canadian company, showing some pricing that I haven't seen elsewhere, but Canadian prices are typically more than American.
Here's the availability from American Plastic Lumber; they just wrote back telling of a 500-pound minimum order (drats). They sent along a data sheet: "An ultraviolet stabilizer is also incorporated at the time of manufacture, which protects the plastic from ultraviolet light degradation. This insures that the outside of the product will not degrade in exterior applications." It weighs .93 that of water (or about 2 pounds per linear foot for a 1.5" x 3.5" piece), and is structurally superior to wood (at least we're getting something for the higher costs). They sent a price of $3 per foot for 2" x 4" (actual size) and $5 per foot of 3" x 4" in premium-grade black. Extrapolating, the 1.5" x 3.5" from an American source should be well under $3 per linear foot. The email said: "UTILITY [grade] IS GENERALLY LOWEST IN COST, BUT IS NOT APPROPRIATE IN THIS [solar-heater] APPLICATION"
I can't find information to tell how much less efficient an all-metal solar collector is, if at all, compared to a transparent tank. My bet, the tank with plexiglas gets more heat. It's called a Flat-plate collector. This is what I'm going to build for myself, and so you may as well know how I'm planning to do it. Go ahead, copy me, make my day.
I'm not going to use circulated waters passed along the metal-plate collector due to the extra costs, work and pump involved. Water in the tank will sit idle, making the tanks easy to build. If the tanks lie on a roof, I don't see need in insulating the bottom; if the tanks are tilted toward a better sun angle so that the bottom sides are elevated above a surface, then insulating them is a good choice.
Thickness of plexiglas depends on width of the tank, but I wouldn't go less than 1/4" thick due to the heat and weight of water. If your tank will be horizontal, thickness is not an issue, but if it's going to be tilted or on a roof to maximize sun angle, the plexiglas will be urged to bulge a little at the bottom half due to weight of water.
Don't worry much about the non-transparency of the tank sides (= the frame) because they will block sunlight only when the sun is low and cool in the morning and evening sky. To minimize this situation and to decrease tank depth simultaneously, one can lay the tanks horizontally across the roof. However, tank positioning for the cockpit invention is by far best in the vertical. When building your tank now for use as a money saver, keep in mind that you may need the cockpit invention later. An example of a mistake may be to build your tanks 1' wide and 8' long with a very thin plexiglas of 1/16" or 1/8" due to the low pressure of water only one foot deep. But, later, when you want these tanks vertical for to separate cold water from hot water in the same tank, the thin plexiglas may compromise the seal of the tank.
Yes, you can have long tanks positioned horizontal now, and positioned vertical later, not a bad idea at all for capturing maximum sun now. Yes, a tank just 1' wide will capture much less sun in a vertical position, which is exactly why you don't want it 1' wide if you want it for trib purposes too. A tank just 1' wide positioned vertically makes for too much blocked sunlight from the sides of the tank. I think a good compromise in the conflict between plexiglas cost and maximizing low-sky sunlight is a tank width of 18-24 inches.
Next conflict: very large tank to reduce building costs versus the difficulty of getting it to the roof. That is, instead of building multiple tanks side by side and linked by small pipes, why not just make one tank the size of all tanks combined, and insert plastic dividers (vertical direction) that make for a multi-tank situation? It's the same thing, except that it uses less material, has fewer points at which to leak. Another benefit is that each divider is wholly in the water, meaning that each one will absorb sunlight when struck by it, instead of blocking sun as it would when used as outer side of a separate tank. It's win-win-win-win. The only downside is handling the larger tank.
A large tank the size of three or four manageable ones would not be monstrous. Its predictable size would be 4' x 8' (roughly 80 gallons or less than 700 pounds of water) so that a single sheet of plywood could be used for the back side. I would probably go with four vertical dividers to divide it into five 18" sections. However, rather than having five tanks -- that will increase the number of valves, sensors and gauges at the cockpit -- I'd cut the lower end of two dividers short by a few inches so that, in reality, there would only be three tanks separated from one another, one 18" wide, and twice that width.
And now, instead of fiddling with connecting separate tanks with short pipes, we just drill holes lickity-split in the dividers. Bonus. We now have a win-win-win-win-win situation. The only downsides are handling the larger tank. You and a friend or two could raise the tank partially assembled to the roof. The entire frame could be affixed to the plywood backing at the time. The metal collector plates could come up separately, and the 4' x 8' plexiglas is light enough. The plexiglas would be already been drilled to receive the bolts coming through the plastic frame pieces from the back side of the plywood. Yes, these bolts are going to sandwich the front and back together nice and tight to make the whole thing leak proof. Otherwise, it won't be a win-win-win-win-win situation anymore.
I would not use a plastic sheet as the back side sitting directly on a roof due the breakdown in plastic integrity as per high heat. The plastic frame will get sun, for which reason it should get covered with a light-colored material (vinyl siding sounds good), with a space or insulation between it and the plastic. The plastic frame must only be capable of handling the constant water temperatures to the point of maintaining the seal.
The high expense of plastic lumber, because it's made of recycled plastic, makes us wonder just how much municipalities are making off of us...as we do the work in giving our plastic bottles to it for free, even taking it out to the roadside for pick-up. Why don't we get a cut? How in tarnation can plastic lumber be several times the cost of wood lumber when no one needs to go cut plastic trees, or to drag plastic trunks of multiple thousand pounds to a saw mill for heavy-machine handling? Government-paid trucks just drive the plastics to a slave house where workers separate plastics, and its then transferred quite easily to plastic manufacturers who in turn transfer it in a different form to those who melt it for casting into plastic lumber. It doesn't require extreme temperatures to melt it, as it does iron ore, nor does any one need to mine plastic rocks with huge machinery and explosives, and yet cast plastic is roughly the same price per pound as iron rods and tubes. Does someone smell a rip-off industry here, with the municipalities getting quite a large cut for pop bottles that we purchased? Must we now buy them back at 30 times the price just because its in the form of lumber that won't rot?
What makes it worse is, we pay for the recycling truck, the gas, and the drivers. The town pays for nothing and yet gets all the income, never sending us a note telling what's being done with the profits, and never offering us a dime in return for our loyal slavery. Those who refuse to recycle are now being forced in some places to pay extra money for the extra garbage bag. Yes, because some places have one-bag limits now per household, with a small fee for extra bags, which can only have the purpose of forcing us to recycle, forcing us to work for free for them. It's just a small indication of the over-all wickedness that shameless governments have conformed to. They even make us pay for filling out their tax returns, if that has ever crossed your mind. Shouldn't they pay for it since they are getting the money?
There is talk in taxing our use of solar energy, if you can believe it, because from the global levels on down they are planning to force us all on that. It used to be that a government had to prove it permitted us a thing, like a job or a business, in order to justify taxing us for it, but now even the sun, even the air, can be taxed in the world of shameless government wickedness. The governments of the world class will join the 666 club, guaranteed, but when they come to tax your solar panels, just tell them, give to caesar what is caesar's, give to God what is God's, and since when did caesar own the sunlight? I'm not paying these taxes, take me to court if you want.
Sing the Praises of your Project on a Pedestal
Having a plywood back directly on a roof is asking for wood rot. But at least it's light and manageable, and a lot less money, as compared to a sheet of steel. For wood directly on a roof surface, you can either try to seal the wood from precipitation getting in, with a high probability of failure eventually, or raise the tank on a floor-less platform. In fact, there's a good chance that your roof pitch is not quite perfect for the best sun angles, meaning that you platform can be built at just the right angle. It's definitely worth a few hours work, isn't it?
There is a significant difference of 23 solar degrees between the sky of a March-21 sun verses a June-21 sun. On the northern border of the United States, the middle between those two dates is a noon sun as high as about 75-80 degrees (where the horizon is zero, and directly above our heads is 90). However, as there will be warmer air in June than in March, angle the panels with a bias toward the March sun, say at about 70 degrees. Even so, it means that the tanks should face almost straight up since you won't be using them in the cold seasons. You should NOT take your tank-angle info from people in the solar-electricity industry whose purpose is to find a happy medium for their solar-panel angle between December 21 and June 21.
The further south you are from the northern border of the U.S. (Europeans and others can extrapolate here), the more your solar heaters should be angled upward. If I'm not mistaken, the sun actually shines on a north side of a house in a Texas summer. What this means is that your roof platform will be so near flat that it'll be easy to work on. That is, you could build the entire tank up there and have your sunny vacation at the same time.
If you don't use plywood on the joists of your platform, then at least run them directly under the dividers in the tank. If the weight of the tank is 1,000 pounds filled, and it's eight feet long, it's only 100 pounds (a young teenager) per linear foot; roughly 50 pounds per foot will transfer to one side of the platform, and 50 to the other side...all of it transferred to the roof rafters that you're going to make sure will not bend over time under the weight.
Ordinary spruce lumber is often curved, or, if it's straight when you buy it, it can curve later as it dries (pressure-treated is prone to developing curves). The curve on one side of a piece up lumber goes in, toward the wood; the curve on the other side going out and away is called, the crown. All joists should be installed crowns up. But you should not install a large crown directly beside a small crown or straight piece, unless you want waves in your floor. Ask yourself how many waves, and how big, you would like under your solar tank.
The best thing to do is pick out your own straight lumber, and barring that, plane or belt-sand the crowns out. The trouble is, sometimes a piece of lumber is so straight you can't predict which way the crown will develop as it dries, and sometimes no crown develops at all...which may sound too good to be true but it actually happens. The good news is, a tank just four feet wide doesn't need very long joists, and short joists have very little crown problems. Just the same, mark your crowns before cutting joists to short pieces, because you may not be able to detect the crowns after that. Consider it unlikely that crowns will develop once the weight of water is in the tank.
If your roof is too steep for a platform, you can build it on the ground. I intend to add a chapter after this one speaking on ground level tanks connected to attic storage tanks. Don't become dismayed. You can have your water tank and eat it too, one way or another.
What other benefit do you see with tanks nearly horizontal most of the year? The weight of water seeking to break through the seals or to bow the plexiglas is greatly minimized, meaning that the tank will last longer in regards to that score.
But there's more win-win coming. Now that the tank is on a platform, it can be tilted a little more by hand, toward the horizon, to catch good sun for the August-to-November stretch, and for the February-to-April stretch, because many southern states get some hot sun starting at the first of February 1, and it can be scalding hot even into the first of November. So, after tilting them by hand in August, drain the tanks for the winter, and leave the tilt just as it is for the February-April stretch. You can thus maximize your money savings due to heating hot water. Don't forget to send me my 2% commission.
The power of a January sun in the north has just (as I write) melted 1 1/2" of over-night snow on my electric solar panels while its 7 degrees F out at 11:30 am (it was still colder overnight).
By the way, you cannot lift the tank itself. You must lift the upper ends of the platform joists. It can be done. All upper ends of the 2 x 6 joists are nailed to a 2 x 10 header (use joist hangers for a long last), which itself sits on the roof (the header should not sit on the roof unless it is wrapped in a waterproof material; aluminum siding material sounds good). If you bolt some handles, two per man and a total of four for two men, to the two ends of the header, the men should be able to lift the platform and tank together a matter of inches, after the tank is allowed to drain. Just let the header extend a foot or so past the platform on both sides. A third man puts whatever is needed under the header to keep the tilt just where you want it.
Smile: You're Framing Things Up
I have in mind to build a proto-type tank just as soon as this winter passes. Its only purpose is to assure a water-tight seal before expending myself on the real tank. The proto-type needs only to be a 2 x 4 wood frame about two feet wide and 4 feet deep, covered top and bottom with 1/2" particle board, with rubber gaskets between the boards and 2 x 4s. It's the rubber gaskets that I would like to test most of all. Until then, I have theoretical methods for applying the gaskets and making the seal, all discussed below. I'll report back here with the results, Lord willing, if life doesn't call me to something else.
The plywood needs to be backed with a single piece of rubber. Rubber comes in rolls, and should be widely available in four-foot wide rolls. At the page below, rubber at 1/16" thickness and 4' wide is under $9 per linear foot. I would not pay extra for their adhesive-back rubber; a little contact cement will do.
The rubber covering the plywood will act as the seal against the plastic 2 x 4s, but there is the space between any two pieces of 2 x 4 that likewise need sealing. Each corner consists of a 2 x 4 face butted up against by the end of another 2 x 4, and so four pieces of rubber the size of the 2 x 4 ends need to be cut, though they alone are not enough. Waterproof glue on the rubber gaskets makes sense.
Drill holes and insert two fat wood bolts (sharp-tipped kind) through one 2 x 4 piece extending into the end of the other; it's important to cut all ends machine square with an accurate electric saw. If the cut end is porous, consider using a highly-spongy rubber that fills the pores. The fat bolts pull the two pieces of plastic tightly together at the end of the bolt stroke, so just make sure they do the job without stripped threads (which releases the tightness). You want the plastic squeezing hard against the rubber. But this is not enough for the long term.
The ends of the 2 x 4's need pre-drilled holes to keep them from splitting due to the fat bolts, but if the hole is too large, the threads might be unable to bite well. For good measure, use 1/4" bolts long enough to make them tight enough for ratchet tightening without stripping threads. If you feel a strip taking place, remove the bolt, get one twice as long and perhaps the next size up, and try again. Or use a new piece of plastic. It's not wise to leave things be at a high risk for a leak that needs much effort later to fix. Probably, the thicker the rubber here, the better.
Later, when the frame is bolted to the plywood, there may yet be, due to poor workmanship, four pin-point locations, one at the bottom end of each inside corner (where 2 x 4s meet the plywood's rubber), where tank water may yet pass through to the outside. That is, between the rubber on the plywood and the piece of rubber between the 2 x 4s, there is a small seam the thickness of the rubber. Let's assume that after pulling the 2 x 4s tightly together, you shave off the excess rubber that squeezes out so that it ends up perfectly flush with the bottom of the 2 x 4. There may be a seal at first after the 2 x 4s are bolted hard to the plywood, but what happens over time if the some small distance is created between the two 2 x 4s? The rubber will find some relief from pressure, but in the process, the bottom of the rubber will move from what it once kissed hard against, thus opening the seam to water passage.
The first line of defense for this problem is to cut the small pieces of rubber perfectly flush at the bottom and top of the 2 x 4 before pulling it tight. If the rubber has expandable quality, it will "grow" (a good thing) all around the edge of 2 x 4 when pulled tight. You would do well to get a rubber with large expandable quality, meaning that it may be 1/8" or more thick. But after rubber has squeezed out due to the pull, do not shave it flush with the 2 x 4 top or bottom. You decide on how much, but my senses are that about 1/100" (a stroke a ball-point pen) of rubber should be left past flush, perhaps 2/100s if the rubber is gummy. If one leaves too much, however, the corner can become over-jammed with material, causing a passage for a leak in that way. You can always shave off more later if there are leaks on the first test.
If the above fails to produce a seal, good old silicone applied over the pin-point area will definitely seal it...until the silicone no longer sticks. To prolong the silicone seal, something needs to be applied over it that sticks forever. An inside corner bracket exactly the 3.5" length of the 2 x 4 face seems ideal here. Just stick it in by screwing it into the plastic (but don't crack the plastic; pre-drill the holes).
A compromise is to use rubber over half the end of the 2 x 4, and highly-flexible silicone on the other half (the one closest to the tank) that is applied just as the two pieces of 2 x 4 are being pulled together. That seems like a very good idea.
A consideration here is how well or how little the plastic can hold wood-bolt threads when it's very hot. I've written to American Plastic Lumber (California) asking about the temperature capability, but even before getting an answer, one already knows from their webpage that their plastic can handle the summer California noontime sun on a deck floor screwed into the deck joists. That's sounds promising. But just in case, don't use bolts too short or too loose for the inside corner brackets. Some threads bite better than others; deck screws are made with good-biting threads.
It would be better for the short term to glue rubber to the back side of each inside corner bracket, but will the bracket still hold the silicone tightly against the pin-hole threat if or when the rubber deteriorates? If the rubber is rated for the long term in the sun and heat, I'd use rubber behind the brackets, seeking to have it alone take care of the pin-hole threat.
Your corner brackets are probably rounded slightly at the corners rather than being a perfect square. It means that a small seam of silicone can fit between a bracket and a corner. After the dab of silicone at the pin-point area has dried, why not silicone the entire inside corner too just as the corner bracket is being installed? Its installation will squeeze out excess silicone and assure a better seal in case of an issue at the 2 x 4 joint. You may as well silicone (black if your area has it) over-top of the brackets at that pin-point location. But this is still not enough to assure leak-proof corners for the long term.
The inside brackets can be a short wrap past the corners because their job is to hold the silicone in place. It's then a must to install larger outside corner brackets to keep the integrity of the corners in the changing temperatures. The outer brackets should wrap 3.5" or more past each corner; I wouldn't skimp here because large brackets act as secure back-up to the long bolts through the ends of the 2 x 4s. Or put it this way, that while the bottoms of two adjacent 2 x 4s will be very secure and unable to expand at their joint due to their being bolted into the same sheet of plywood, the tops of the 2 x 4s will not have plywood to hold them together.
It won't hurt to use 3/8" bolts into the brackets (1" to 1.25" deep into the plastic), but assure, when drilling the long bolts, that they will not interfere with the insertion of bracket bolts.
Once you get the long bolts into the corners, you'll be an expert for doing the same into every 2 x 4 divider. You'll want the frame to be square.
Then assure that, of the four outer 2 x 4s, the upper and lower two end up perfectly parallel after the dividers are located between them. I would suggest that the upper and lower 2 x 4s be the ones with rubber on their ends (in which case the length of each divider must be shorter than the other two outer 2 x 4s). It's your choice on whether to apply rubber to the ends of the dividers. I probably wouldn't.
Before screwing in the dividers, you need to have decided which one(s) will be a few inches shorter than other dividers in order to turn two tank sections into one tank for draining purposes. Alternatively to cutting one or two dividers shorter, you can just cut a notch into their bottom sides. Let the notch be 1" deep and a few inches up. This way, you can still insert some long bolts into the dividers for keeping the frame stronger. The idea is for two tank sections on either side of the notched divider to share waters from their lower ends. Two sections sharing waters like this need only one drain hole between them (to the shower head and water mixer). You get it.
Upper ends of all dividers need small holes on the order of 1".
Well, were just about finished. Or maybe I'm just telling a fib to make you fell better. We'll be fixing the plexiglas top on just as soon as we talk piping. Well, almost. How many pipe holes do you have into the tank through the 2 x 4 framing? Are the holes positioned where you want them? Is it more advantageous in your case to bring pipes through the plywood bottom? If you later want a third and fourth hole, per tank, for the air hole and drain hole as per the "cockpit" situation, you might just have these holes drilled and looked after now. In fact, you need to drain "each tank" before winter, and so rather than installing a small drain hole in each one, why not make them all large drain holes acceptable for the cockpit situation? (By "per tank" and "each tank," I'm referring to the multiple tanks built between the dividers.)
You may as well include now any hole(s) to a storage tank(s) for the purposes of "thermosiphon" circulation. Although that term was not mentioned in the last chapter, this method of circulation was discussed, where heat from a roof tank rises naturally into a storage tank. If you're sure you only want one storage tank now or later, put a large hole in the top 2 x 4 as far from the cold-water inlet as possible. If you have your drawing on paper, this discussion will be easier on you.
A hole to fit a 3" pipe would be ideal for thermosiphon heat transfer, but if its size is a little daunting for the purpose of sealing the hole, two smaller holes taking two pipes to a larger one (before entering the storage tank) will have to do. Two 2" pipes are equivalent in volume to one 3" pipe.
You should probably drill the holes on the ground, before the tank is on the roof. American Plastic Lumber claims that their plastic can be tooled just like wood, so that a typical hole saw (the kind used for door knobs) can be used for large holes.
It would be wise to locate the top of the roof tank low enough on the roof to allow for a higher storage tank in the attic as well as a water-mixing storage tank lower in the attic than the bottom of the tank. You probably wouldn't want to make holes in the roof early for any pipes scheduled for later use, unless making the holes now is much simplified as compared to later.
For example, if you're locating the "drain" pipes (actually they're supply pipes to the shower head and water-mixer tanks) directly under the platform for the tank, it would be much simplified to make the holes in the roof, and to fit the pipes through the holes, during the installation of the tank. Just cap all pipes in the attic until you need them. All pipes going straight down from the plywood already have rubber (on the plywood) at their disposal for the waterproof seal. However, this requires a hole in the metal heat collector (that lies on the plywood).
Of course, don't make the pieces of metal collector fit tight between the dividers. Allow room for expansion. It may be that 1/32" thick metal sheet is just as good as four times that thickness, for the metal needs only to absorb sunlight, giving up the resulting heat immediately to the water.
The only trick to installing the nipples through the holes is to make a good seal. Your local plumbing department has fully-threaded water-pipe "nipples" (galvanized) more than long enough to fit through the 2 x 4 plastic. Get them just the right length so that two water-pipe couplings (made for the nipples) screw on tight against a 2 x 4 on both the inside and outside of the tank. Use pipe dope in the threads to assure a seal in the threads.
These couplings are quite thick to act as both washer and nut, but if the flat end of the coupling is not smooth, file or grind it smooth. Make and locate rubber washers between the plastic and couplings. Make the holes in the rubber washers slightly smaller than the outside diameter of the nipples minus their threads; this may be overkill, but a rubber washer should best squeeze into the threads when the coupling is tightened. For extra security, squeeze putty or silicone into the joint between the nipple and 2 x 4.
Bolt Holes: Just Humming Right Along
It's plexiglas time. Just fibbing, but we're getting close. First, let's talk bolt holes. Blech. They penetrate multiple things and should be perfectly aligned through all. There's only one way to do this: 1) Assemble the frame to finality with outer brackets and all; 2) screw the plywood bottom temporarily to the frame and to the plastic 2 x 4 dividers with no more than five or six screws; 3) flip the tank over (plywood down) and lay rubber strips, acting as a gasket, on the top sides of the frame and dividers; 4) assuming you decide to use the full plexiglas sheet, lay it into position on the gaskets; 5) lay thin strips of a "cool" material on the plexiglas directly over every frame and divider piece; 6) lay the metal strips discussed above (that act as batten-down for the plexiglas) on top of all the cool material.
If the metal comes pre-drilled with holes of about 1/4" to 3/8", use a drill bit of the same size. Decide which holes to use (if you don't need them all), and drill into one hole, clear through all materials including the plywood. Drop a tight bolt (it should slide through easily) temporarily through the first hole. Don't drill another hole until that bolt is in. Drill away, hole after hole, assuring that the materials at each drill location are positioned just right. Drop bolts in the holes after each drill to assure that nothing moves out of position. You have only one main task here, to make all holes align with all the various materials being drilled. It's important.
There is an argument to be made (later) that the plexiglas not be bolted over the frame, in which case you won't be drilling through the plexiglas over the frame.
If the metal does not come pre-drilled, get a new drill bit for the purpose (I suggest one small size more than 1/4" bolts), and dip it into a can of water as needed cool it somewhat. Drill the metal separately (i.e. not on/at the tank). The hole spacing along the dividers can be increased in distance if you're not too concerned about waters passing through from tank section to tank section. It's the bolt holes along the frame that must assure a good seal. One suggestion is bolt holes every eight inches penetrating right through the tank. If the tank leaks at that point between bolts, it may need to be disassembled for more bolts between them. It would be nice to stick screws instead between them that penetrate only into the plastic (rather than penetrating through the entire tank), as that eliminates the drilling of holes and the taking of the tank fully apart, but then there is an expansion issue where the plywood may not expand at nearly the same rate as the plastic.
When done drilling, remove bolts and set the metal aside. Re-drill all plastic-lumber holes larger, but leave the metal holes and plywood as is (bolts in plywood are best snug to keep insure corner seals). What I would like to see at my project, to eliminate any threats from uneven expansion between materials, is 1/2" to 9/16" holes re-drilled in the plexiglas, the cool strips, the rubber, and in the plastic lumber (see expansion details end of chapter). Take the greatest care in re-drilling plexiglas holes, especially when they are near the edges of the plexiglas. Assure no contact, or no near-contact, between plexiglas and bolts. Assure (as best you can, anyway) that the re-drilling goes straight down the center of the previous holes.
The holes need to be larger as distance increases toward the ends of the tank, but theoretically, all holes in all materials at the center of the tank can be the same size as in the metal, then enlarged to only 3/8" midway between the tank center and ends of the tank. It's your choice after you read up on the expansion details.
Once the holes are enlarged, lay the metal heat collectors in the tank, and re-position all tank materials as above, right up to the metal strips. It's assumed that all work in the tank has been done at this point). Insert two bolts temporarily per divider, and 2 bolts per piece of frame (don't tighten). These bolts keep everything in place as the tank is lifted on one side about three feet or more in height until there is a good tilt. Use a piece or two of lumber to keep the tank securely propped, because you're going to get under it to insert the bolts from the back side...because bolt heads need to go against the plywood, and the nuts need to go on top. This way, you can get into the tank, if ever need be, by simply removing the nuts. Use large washers under the bolt heads, unless you decide to use metal strips also on the plywood as you have it on the plexiglas.
Due to what I've seen the sun do to screws on hot metal roofs, I might not only use lock washers under the nuts, but also locknuts. If rusty nuts is a concern to you, get fully anti-rust nuts (most nuts are merely coated with a non-rusting metal that later rust anyway).
We now come to the question of whether to use metal strips on the back side to increase the tightness between the plywood and the 2 x 4 frame. Or should we trust large washers under the bolts? There's no limit to how many bolts one can insert through the plywood so that a successful seal without a metal strip should be possible eventually. The only question is whether the bolts will loosen over time, and this is where plastic (non-organic) beats wood (organic), I think. I tend to think that bolts along the frame, every four inches, with large washers of about 1.25", is plenty for the plywood. The "Average Screw Pull Out" value for plastic lumber as per a data sheet from American Plastic Lumber is 646 pounds.
But if you wish, you can even use angle-iron...both on top and bottom of the frame perimeter. Angle-iron is a very tough "washer" for the entire perimeter. Moreover, it comes galvanized or even in aluminum from your local supply store, making for a nicer-looking border all around the tank. However, after writing much of this chapter with metal angle-iron in mind, I came to the conclusion that a piece of 7/8 plastic lumber would be by far the best choice for expansion purposes. It will be explained near the end of this chapter, but in the meantime, whenever I speak on angle-iron, you may wish to perceive a piece of easily-worked plastic instead.
Let's assume 3/16" angle-iron top and bottom. The angle-iron will now rest on the platform joists, thus elevating the plywood 3/16" above the joists. We can't have that, so in such a case you need to add 3/16" strips between the joists and the plywood. It's a simple fix, but it requires that the dividers are not directly over each joist because the bolt heads would then be on the joists. No problem, just shift the tank a couple of inches one way or the other so that the bolts clear the joists.
The platform can be insulated between joists, by the way, and moreover if you love raccoons, they will come sleep there. Otherwise, cover nicely all around the platform with siding, and tell them to find another place to bunker down in. Bonus, your piping can be covered up under there too.
The Great Seal Hunt: Where the Rubber Meets the Plexiglas
There's no use going on to the plexiglas until we talk rubber gaskets. And while we're at it, we may as well talk rubber type and rubber quality. There's nothing like life rubbing against you the right way because you know what you're doing. But who in tarnation knows anything about rubber? What's there to know, you ask. Isn't it just that black stuff? I wish.
What if the rubber you choose darkens the water after a period of no water use? What if the rubber leaches into the water slowly and stains the plexiglas over time? Who has access to a chemist that will tell which leached rubber type is repelled or attracted by the plexiglas that we end up using? I mean, every material has some electromagnetic charge that either takes to, or veers from, another material. What if plexiglas atoms love rubber atoms? Should we use an imitation rubber or rubber alternative? I suppose, so long as it makes for a long-lasting seal as well as being water proof for the plywood.
The rubber website below shows: "Jesus is the way, the Truth and the Life." It must lose a lot of business by making that courageous claim. Each webpage has links to various other rubber and non-rubber products in rolls or sheets. See, for example, the SBR rubber page for a low-priced material. Skirtboard rubber (good elasticity, may work well at the pin-point area) is rated to 180 F. The recycled rubber rolls/sheets are very low cost and have operating temperatures of 150 F. If these temperatures are too borderline, enquire on nitrile rubber (good elasticity) with a service temperature of over 200 F. EPDM is rated 280 F, at a good price and including good elasticity. High elasticity may mean that we should tighten the nuts after a couple of years, or when we see a leak. Your tank is only as good as the rubber seals. Let's go with longevity rubber if we're building now.
Here's a bit from the black SBR page: "As with most higher tensile strength SBR rubber materials, the higher the tensile strength, the lower the elongation (or stretch)." "Tensile strength" refers to resistance from being pulled apart in opposite directions. Therefore, if elasticity is important for gaskets, rubbers with high tensile strength may best be avoided. Yet, we find: "This [black type] is a higher strength SBR Rubber material that is durable...SBR rubber in specification grade material is used in conveyor belts, chute liners and low pressure gaskets." The durability part sounds excellent, but "low pressure gasket" may be for metal parts unlike our wood and plastic combination. In other words, SBR (temperature rating is -20 to 180 F) may not be the best choice, as it seems like a harder material in the incompressibility department.
Most pages above do not offer rolls in 4' widths, meaning that if our tank is 4' x 8', we may need to piece together the plywood protector...meaning that we now have reason to seal under each divider. It's risky. The only rolls I've seen in 4' widths was the general-purpose rubber page. The page (below) on large sheets of general purpose starts at 10' x 5' sheets (under $76 for 1/16" thick in January 2013), and shows a temperature rating of -40 to 225 F. It sounds good. If we decide not to use that rubber, then consider that the black SBR comes in 36" x 48" sheets, good where your dividers are spaced 18 inches or less.
See rubber flooring if that can help. The page below has some floor mats 4' wide and up to 60 feet long, some of it in 100%vinyl. Can we use vinyl? At their marbleized floor-runner page (100% rubber), their 4-footers are about $6 per square foot. They offer samples so that the texture on both sides can be checked out. I'm assuming that flat-as-possible is best.
Perhaps vinyl is not suitable for our gasket purposes at the ends of the 2 x 4s, but perhaps it's just find to cover the plywood. It's a lot cheaper. At the page below, Chinese-made black vinyl at 1.5 meters (almost 5') wide goes for under $20 per running meter. It's suggested for car bodies meaning that it can take heat. The thickness is 19c, but I don't know what that is unless it's .19 of a centimeter. A heat gun can be used on it. As the English is atrocious, make sure this is a legitimate business and not some foreign thief pretending to be:
...Service life: for outdoor 6 years, indoor 12 years...3D carbon fiber vinyl film is not just a cheap imitation vinyl, it is a high quality...3D carbon fiber vinyl film is also water resistant and UV stable, so applying it to exterior surfaces such as a hood, spoiler, or the whole car is not an issue.
The product is an adhesive back one, wherefore the service life above may be referring to the adhesive, not the waterproof performance of the material. I'm assuming it's fully waterproof.
I wonder whether simply laying rubber, or squeezing it as gasket material, in the cold spoils it. Probably not, but you had best ask about it if you're at the US-Canada border area. Some of these rubbers are rated for cold up to -40 F, but nitrile only to -25. Some pages show no temperature ratings. It could be that the ratings are for workability of the rubber, but irrelevant for its destruction due to cold.
None of the pages speak on rubber leaching into water, however, which can either mean that they all do to a point but no one wants to tell, or that none do as it's a non-issue. But regardless of your choice, the plywood needs 100% waterproof material ongoing.
Also, you can check out Wikipedia's article on synthetic rubbers for a list near the article's end. There's a link there for neoprene:
Neoprene rubber is an all purpose elastomer and an extremely versatile synthetic rubber used in thousands of applications. Neoprene rubber resists degradation from sun, ozone and weather...Neoprene rubber maintains its strength, flexing, twisting and elongation very well over a wide temperature range while having outstanding physical toughness...
Email us your drawings if you need rubber gaskets or rubber parts made.
There is also a page for rubber gaskets.
There's No Such Thing as Inexpensive Plexiglas
Finally, we come to the last step in building the heater tank. The plexiglas top. It's perhaps best to make the tank top a multi-piece one rather than a full plexiglas sheet due to its expansion and contraction under tight joints at the bolts. Then again, if the full sheet is the final choice after much deliberation with all the wise men of the community, one may be fortunate where it simply bows a little between dividers, without cracking. It's a little nerve-rattling just to think about it. Certainly, I would rather use the entire sheet...in which case it would need to be drilled out when putting the bolt holes into the 2 x 4s. Are you game?Everything I'm about to say in this indented section may be eliminate-able, but it's here in case you need / want it. If we don't drill the plexiglas at all, it's because we're using small sections instead, with a plexiglas joint over dividers (the eight-foot stretch of tank needs only three plexiglas sections). True, we would then increase the locations for potential leakage, but assuming that leakage is not going to be a problem, it's a viable option. Having plexiglas joints above the dividers suggests that dividers should be more than 1 1/2" wide. There are plastic-lumber lengths available in the dimensions of 2.5" x 3.5" from American Plastic Lumber, with which we have the room to separate the edges of two plexiglas sections by as much as a 1/2", allowing 1/4" bolts to pass through without need of drilling.
It amounts to a precaution because thin plexiglas is prone to cracking when drilling it wrongly, and you may have no experience in drilling it. I do, ask my many cracks using regular wood bits. There is a special drill bit that one can purchase to drill plexiglas. Do some practicing first on some scrap.
Also, when using sections, the leak potential (and how to deal with it) at the pin-point locations of all four outer corners applies also to the inside corners formed by dividers, meaning that some decent inside corner brackets on all dividers would be a good idea. However, below, you may feel that plexiglas sections are not necessary.
Each section must sit only partially on the frame top too if we don't wish to drill there, with the other part of the frame top used for bolt holes. The result is metal strips / angle-iron pressing down on the plexiglas from a small distance rather than right overtop of it, a good reason not to skimp on metal thickness.
The entire discussion in this section may be made to vanish if we opt for a plastic perimeter and plastic bottom sheet rather than using steel and plywood respectively. See end of chapter for 7/8" x 2 1/8" plastic lumber strips that can replace the angle iron. If the plexiglas expands roughly at the rate of this plastic strip of the 2 x 4 frame. I don't see a problem using a full plexiglas sheets. Wonderful. If you see a problem, you can keep to the steel and plywood. I'll continue to speak below as though the plexiglas sections may be a way to go, but keep in mind at those points an all-plastic tank possibility.
Nuts or bolt heads should not be tightened directly on the plexiglas surface. Very large washers on the order of two inches in diameter may do the trick, but if the extra work in using metal strips across an entire line of bolts doesn't faze you, it's the way to go. It's important that the "cool" strips under the metal are not compressible, or, if the material is somewhat compressible, it compresses only once...when the nuts are tightened.
I suggest the use of cool material due to not knowing the effect on plexiglas of high heat flow into it, from metal in the sun. High heat predicts a softening of the plexiglas right at the area of the seal, which might be beneficial as it results in some stretch capability. However, the high heat flow may also produce brittle plexiglas over time (ask the manufacturer), maybe even over little time, which may not be a problem in a window installation, but in our case we're using plexiglas tightened under pressure that will want to bow slightly under pressures of expansion. We could be asking too much of it, so that any compensation we can offer is a good move. "Cool" material is anything with a low heat-transfer rate, and may even be strips of the plexiglas itself.
It would probably be more than you'd like to spend to get a single gasket 4' x 8' square. Piecing it all together would be far less money, but the material, be it rubber or not, must have good elasticity so that when tightened down, the seams will close tightly. Gasket material of 1/8" is better than 1/16" for that purpose. To cut seams, overlap two ends of the material and cut them together with a sharp knife. If you cut at an angle, the water will need to penetrate more seam, slowing any slow leaks. I would strongly suggest no seams at the very corners and at the ends of dividers. Make L-shaped gaskets at the corners and T-shaped ones over the dividers. Have the seams directly over nuts.
If that doesn't work, you've not wasted much money. If you don't glue the gasket material down, it'll be a piece of cake to get off and try something else. Another idea is cork-gasket strips. There is the question of cork deterioration in water over the long haul: "Temperature and age stability: cork retains its properties at both high and low extremes of temperature and usually lasts 20 years without deterioration." Another webpage gives it ten years for wine bottles, while others flat out say that cork resists deterioration in water:
...cork is practically impermeable to liquids and gases. Its resistance to moisture enables it to age without deteriorating.
Compression, elasticity, resilience - cork can be compressed to around half its thickness without losing any flexibility, and recovers its shape and volume as soon as it is released. It is the only solid which, when compressed on one side, does not increase in volume on another; and as a result of its elasticity it is able to adapt, for example, to variations in temperature and pressure without suffering alterations. When it is compressed, the air inside the cell is squeezed to a smaller space...
I'm thinking that it's a toss up between cork and spongy rubber. I'm not meaning to get over-technical here because there is a reason for it. A more-rigid material in conjunction with flexibility seems best, and a cork could beat spongy rubber in this department. For example, a gummy rubber in the shape and size of a wine-bottle cork could bend easily by hand whereas a cork does not bend much by hand. The cork has both rigidity and ability to expand under pressure, thus suggesting the ability for making a better seal at a seam, especially when the water is under some pressure as would be the case when the heater tank is below, and connected to, a storage tank.
On the other hand, the ability of a soft / gummy rubber to expand more than cork, under equal pressure levels, may be more important. There are varied degrees of soft rubber products which may include one or more types equalling cork in all our requirements, in which case the object is to choose which product requires less bolt tightening to get the desired seal. That is, for the sake of the plexiglas, it would be best to attain a seal with the least amount of bolt pressure, for if the plexiglas has ability to expand and contract acceptably under the bolt pressure, it would be ideal for allowing us to use a large sheet. All that talk above on plexiglas sections would be welcomed wasted breath.
Cork under pressure may offer less friction to the shifting plexiglas than rubber, yet ultimately the gummy rubber may be best due to less bolt pressure required to keep the seal. Lowe's sells thinner cork sheets that can be cut into strips (I'm assuming cork is cork is cork). Here is a New York webpage with some cork sheets for cutting into strips:
The ultimate question may be how cork performs or deteriorates when saturated with water. There is "cork rubber" that you may want to look into to. The cork-rubber webpage below (California-based location with several locations worldwide) has a plexiglas link showing you pricing information. My choice thus far for plexiglas can be found at the "Cast Acrylic Sheets" link at the Plexiglas page.
Another gasket idea is a powder of some sort to be mixed with waterproof glue to form a paste. The idea here is to eliminate seams in the gasket (in case we can't get a seal using seams). For example, use cork powder or cork granules. With granules, it's not so much a paste as it's a mass of sticky granules, but let's call it paste for lack of a better word. There are two options after applying the cork-paste to the frame about a 1/4" thick: 1) tighten the plexiglas very loosely until paste is about 1/8" thick and let it harden, then tighten down hard after the glue has cured sufficiently; or, 2) tighten the plexiglas down hard from the start and let paste cure under pressure. With the last option, there could be some cork material falling into the tank; dried cork pieces would then spread into the water, obstructing sunlight. It may take a few draining attempts to get the cork out, if you have a drain at that time. Do some cork-and-glue tests before any attempts at the tank to assure success. Once the cork is under pressure, it shouldn't spread into the tank.
The glue will need to dry somewhat slowly (i.e. contact cement and wood glue may not be slow enough) so that one can apply the cork-paste as a whole to every rectangular section being sealed at once. Waterproof wood glue would adhere cork granules well and meanwhile, hopefully, not stick well to plastic should we need to open the tank, or find another gasket method. I'm reading online: "I've seen wine cork instructables using wood glue, Gorilla (R) glue and rubber cement. I haven't worked with the first two." Another page: "Cut cork with scissors to fit size and shape of plywood. 2. Apply wood glue to the back of the cork and spread evenly..."
Actually, the seal should be successful even without glue in the cork, but glue is preferred to get the cork to sit on the frame top while applying the plexiglas. But then, wet cork might work (with just a little sugar thrown in to give it more stick?).
If seals at the plexiglas can be accomplished with relatively little bolt pressure, one might be able to use a full sheet of plexiglas rather than smaller sections. Here's a plan: start off by drilling bolt holes in a full sheet; if one gasket type doesn't work under a pressure mild enough to your senses (for allowing expansion/contraction), try another. If no gasket material seems to be acceptable, you can then succumb to cutting the sheet into smaller sections (use the correct saw and cutting technique or the plexiglas can be destroyed as fast as the speed of a tooth on the saw blade). You can find people lamenting their plexiglas cuts online.
If you own a torque wrench, record how much bolt pressure is needed just to make the seal. Then call the plexiglas company's technical department to ask whether that particular pressure between your gasket material and your cool-strip material will allow the plexiglas to expand/contract. This is one good reason for using plexiglas (or other smooth surface) as the cool strip.
The thicker the plexiglas, the safer to cut and drill it. The thicker the plexiglas, the better it can handle the weight of snow without transferring pressure to the joints (one should cover the tank with plywood in heavy-snow regions). The thicker the plexiglas, the more it can handle heat and operating pressures. The thicker the plexiglas, the faster your joy for the benefits above will vanish due to the extra cost. I've been praying that plexiglas manufactures be jailed for thievery. But if it's not the fault of manufacturers, then jail the distributors. Somebody deserves to be put away. Al Gore must have stock in glass and plexiglas, rubbing his hands at the prospect of the whole world being forced to go solar. It's no ultimate bad thing if that happens because it will give us all a great trib benefit. But in the meantime, the globalists have made the price of gas cost $Ouch.
To keep your solar heater hotter, make a thin-plexiglas cover for it that traps hot air in. Otherwise, you'll lose a lot of heat from the water when wind blows, and when the sun don't shine past clouds. Heavy snow on such a cover would be an issue to resolve.
Beware the Ruinous Expansion Phantom
If we don't bolt plexiglas to the frame, plexiglas expansion versus the rate of plastic-lumber expansion woudn't matter near as much. The thermal-expansion coefficient of plastic lumber, according to a data sheet from American Plastic Lumber, is 0.000033 inches per inch per degree. That is, if your tank is 48" wide and 96" long, you need to multiply .000033 by 48 and 96 respectively, and then multiply each result by about 70 to find the maximum expansion at 140 F if you bolt the plexiglas to the frame when the air is 70 F. The final number (I have lost my calculator) is how much the tank dimensions will grow in both width and length. Without using a calculator, I can figure that the 96" length will grow by about .25" over that temperature difference.
Can the plexiglas stretch that far without cracking? I doubt it very much. It would be ideal if the plexiglas would expand by exactly the same amount. At the data sheet below, cast acrylic has a value of .00005 inch / inch / degree, meaning that it expands even larger than the plastic wood above (see other plastic wood below with .000055).
The thermal coefficient for steel is .00000645, about 5 times less than for plastic wood. As the expansion distance for an eight-foot piece of steel (at 140 F) is therefore in the order of .05", the plastic is going to expand by about .2" (= 3/15") more than the metal, suggesting that the bolt holes through the plastic lumber should at bare minimum be 3/16" larger than the bolt-thread diameter. But I have in mind to do away from the metal altogether.
I have this from online: "...the coefficient of thermal expansion of rubber is ten times that of steel." Apparently, rubber, though not all rubbers are equally, expands up to twice as much as plastic lumber. Is it an issue? I don't know. It would be much more an issue where long pieces of rubber are in contact with wood, though I wouldn't be concerned with the rubber on the plywood. The long rubber gaskets, however, may pose a slight problem. On the other hand, if rubber expansion is restricted in the long direction due to bolt pressure, it, being a soft material, can expand sideways instead. It sounds very hopeful. In fact, the tightening of the seal due to expansion of rubber in proportion to increased temperature seems ideal. When purchasing rubber, inquire what could be expected of its expansion stains under bolt pressure.
I'm reading online that the thermal expansion coefficient of wood is one to two millionths of an inch per degree. That is: .000002 at most, or three or four times less than steel. The way I see it, drilling 1/2" holes in the plastic wood eliminates the threat from both the expansion factors of plywood and steel. That is, It allows the plastic lumber to expand between the plywood and steel without pressing against bolts. The only question is how the gasket will take to repeated expansion and contraction? Shouldn't we get a rubber with the smallest friction levels? Shouldn't the humidity in saturated cork be a natural lubricant?
Below is a data page for plastic lumber of the brand name, Selectforce, where the expansion is listed at .000055, significantly more than the plastic lumber above (i.e. it may mean that 1/2" holes are not large enough if this brand is used). Selectforce is not to be used for structural purposes, which may mean that it can be had by us for less money and be satisfactory for our purposes. Selectforce weighs slightly less than the plastic lumber from American Plastic Lumber, but has an equal value for the screw pull out factor. In fact, the page reads, "Screws must be pre-drilled," meaning that if holes are needed first, the plastic is very hard to screw into. The page also reads: "A 10' (3.048m) length will expand and contract up to 3/8" (1cm)."
If you intend to rip cut your plastic lumber to a certain width, see "Rip Cutting" at the page above.
Theoretically, another method of solving expansion threats aside from large-diameter holes in the plastic lumber is a plastic sheet instead of plywood (I'm trying my best to avoid it due to the high cost), and plastic "angle-iron" instead of steel. It's then an all-plastic tank. But in my most-final analysis thus far, we can do with the plywood base. It, with sectioned off plexiglas, seems very safe. The plywood base with a full sheet is a possibility; the plastic-sheet base with full sheet is a certainty. It's your money, you can buy the plastic-sheet base and be more sure. If you give it some thought, you can figure out how to use a very thing plastic sheet (can't hold the weight on its own) that sits on plywood (can handle the weight on its own) and yet is not attached to the plywood.
Plastic angle may be less likely to provide a long-lasting seal after the sun attacks it repeatedly. It may even fail to hold the seal as the sun softens it by day. On the other hand, if it's thick enough and specialized for sunlight, plastic angle (if it's not merely a decorative mold) could retain more than enough rigidity to keep the seal. We could eliminate the cool strips as well as the metal by using it. And it would look better than rusted or galvanized angle-iron.
If the plastic is thick enough, we shouldn't need angle at all, which allows us to use the plastic lumber, a seemingly perfect choice for expansion considerations. Plus, as plastic lumber comes in structural grades, we can use thinner strips as compared to other general plastics. I'm guessing that, as of January 2013, the 7/8" x 2 1/8" strips at American Plastic Lumber are $1.50 per running foot. Compare with the price of metal strips and then think of how much easier it is to drill the plastic. It may be worth the risk just to try it. In fact, I see no risk because we will get a seal eventually; the only question is how close the bolts need to go.
But wait, the beauty is that we can even screw through the top plastic strip only as deep as into the plastic 2 x 4s, without need to go right through the base. It will therefore be easy to add extra seal insurance between bolts. Aren't you glad you this far?
Remember, Selectforce plastic lumber has a thermal expansion rate equal to that of cast acrylic so that we can bolt an entire sheet down without cutting sections. This use of the plastic strip in place of the metal seems to offer yet another benefit: the plexiglas will not need to be re-drilled. That is, the hole size through the plastic strip can be the same size as though the plexiglas/acrylic, and for good measure make that hole size a good amount larger than the bolt threads. You will always want the holes through the plastic 2 x 4s to be 1/2".
American Plastic Lumber shows the 7/8" thick strips not only in 2 1/8" widths (which I would use over the dividers), but in 2 7/8" wide which I would use around the frame (it provides a little overhand). Then, just a decorative mold all around to hide the edge of the plexiglas and the rubber. I think we're done.
The makers of Selectforce have a structural line, Fiberforce, all made by Bedford Technology / plasticboards.com.
The next chapter will be largely on making our own storage tanks, and will show how to pipe to the tank in this chapter in order to make it flow without water pressure in the tank. If there is room, I'll also add a water-heater design on the ground (and up against the house) for those whose roofs are too undesirable / steep to work on. I'm going to arrange an automatic shut-off valve outside the hearer tank, meaning that the tank in this chapter will be water-filled completely. Let's say you opt for a shut-off valve inside the tank so that it doesn't fill up completely. Some of your rubber may then be in sun and air, allowing the rubber to dry out inside that sweat box that's half the temperature of the solar surface. How important would it be to use the rubber (or non-rubber) best suited to that situation?
Finally, for those who would like to spend as little money as possible, and especially if it's known that the tribulation is at hand, one can make the frame out of wood 2 x 4s by wrapping them a few times in plastic poly, taping the ends a few times over for extra seal insurance with waterproof tape, and for good measure wrapping their faces with rubber. Now that the base, frame and dividers are wood, the top strips above the plexiglas should be metal angle iron so as to create no expansion issues. The plexiglas can yet be a full sheet if it's holes are large enough to permit expansion, but only if the bolt pressure under the metal is not so tight as to inhibit plexiglas expansion. If unsure, cut the sheet into sections to drastically reduce expansion distances. Over a four-foot section, expansion is 3/16" total, or 3/32" in one direction and 3/32" in the other.
Probably, the warmer the plexiglas, the better for drilling. Try laying out the test pieces in the sun for a few hours versus those not in the sun. Drilling down into plexiglas as it lies on a surface reduces chances of damage. If thin plexiglas is stabilized by some decent weight upon it, or by clamps, all the better.