Dreaming about a new kind of solar water heating device

One of the biggest wastes of electricity, natural gas, coal, oil or what have you is home heating.

I was sitting here pontificating on a few things, one of them being that. The other being that, right now, I'm working with someone trying to get the temperature of his attic down. The guy's got black shingles on his roof in North Carolina, which is a very WARM climate, and his attic is hitting 175 degrees. We put a ridge vent in there, we put about four power vents, we've got gable vents...I've actually suggested putting a second roof about three feet above the first one and cutting big holes in the first one. The only reason he didn't do it was the expense.

Now let me see...we've got a roof system that's capable of heating his attic to 175°F, which means we've got a lot of waste heat we'd just LOVE to throw away. Can we turn that "waste" which we got COMPLETELY FREE OF CHARGE OR IMPACT TO THE ENVIRONMENT into usable heat? I think we can.

We'll make a shitload of small oblong black-anodized aluminum tanks...let's make them three feet long, 3.5 inches wide and 3/4" thick. Each one has a little nipple sticking out one end, and a gasketed hole in the other for a nipple to fit into. They'll have a nailing fin along one edge. The system also comes with various tubing to connect it together, a water pump, a heat exchanger and an overflow tank.

I didn't come up with those dimensions by accident. A standard asphalt-fiberglass composition roof shingle has five inches of exposure, it's three feet long and a 1x2 is 1-1/2" wide by 3/4" thick. The final piece is a Micronized Copper Quaternary-treated 1x3 starter strip--they're 2-1/2" wide x 3/4" thick--which offsets the tanks enough so that there's wood, not tanks, where you need to drive nails into the shingles.

When you install this system on the southern side of the roof--I don't see getting MUCH of a gain out of also having it on the north side--you first lay EPDM membrane on your roof deck, which should be 23/32" plywood. (Standard roof decking is 7/16" OSB.) Next, nail the starter strip to the edge of your roof and put the strips of doorskin above it on the roof deck. Then start alternating--a row of tanks, a row of 1x2, a row of tanks, a row of 1x2 all the way up until you get to the top of the roof. For short gaps at the ends of the rows of tanks, you just cut hunks of 1x4 and stick them in there.

Next, you connect the provided tubing to the end of each row of tanks. You drill a hole in your roof deck to feed the tubes into your attic, and run them to the heat exchanger and to the water pump. You add a relief valve to protect the system in case of freezing. And finally, you nail standard shingles--either standard-size three-tab shingles or architectural shingles--to the roof over the tanks and 1x2s.

The heat exchanger is just a water tank with a radiator in it; the water tank is plumbed into your hot water lines in front of a standard 80-gallon water heater. (Routing: water comes in from the mains, goes through the solar heater, into the main heater and out to the rest of your home.) The system is charged with nontoxic antifreeze. There's a device to monitor the fluid level in the system so you know if it's leaking.

Caveats: Obviously you won't get any hot water out of it at night. Just as obviously you won't get MUCH hot water out of it in the winter, especially if you live in Alaska or someplace like that. But...y'know, last I checked homes in Florida, in Alabama, in Texas, in all these really fucking hot states have water heaters. People don't install standard solar collectors for water heating, even if they can afford them, for two reasons--they're ugly, and many homeowners' associations bar them. These collectors are different: You can't see them. They're UNDER the roof surface, just working away and drawing heat away from your attic.

I am wondering, though, about the weight. Water is pretty heavy; would most roofs be able to support the water tanks without reinforcement?

sort of like circulating it through a solar water heater system. Use it to PREHEAT the water before it goes into the actual heater.

These "tanks" aren't very large.

The calculations I did:

I assumed we'd do both sides of the roof--if a house's ridge runs north-south, this would let you pick up both morning and afternoon sun. I also assumed the roof was 2400 square feet.

You've got 30 percent of the deck covered with 1x3s and the other 70 percent covered with tanks. Square footage of the tanks is 1680.

The tanks are only 3/4" thick, so let's multiply that 1680 by .75 to get 1260 board feet. Divide by twelve to get 105 cubic feet.

Water weighs 62 pounds per cubic foot, so the water itself weighs 6510 lbs. If the tanks weigh 1 lb/square foot--probably high--and the EPDM weighs a quarter-pound per square foot, you're adding another ton. Plus maybe 200 pounds of lumber.

Compare that to 24 squares of GAF Timberline 40 shingles at 284 lb/square--one layer of shingles weighs6816 lbs.

So...we're basically looking at the equivalent of having two layers of shingles on the roof, and that's legal in every code jurisdiction I can think of on the trusses everyone uses these days. You could do it on 15/32" CDX plywood--you wouldn't even need the 23/32" CDX I was originally thinking of.

in the attic to be used as a supplemental water heater...I think it was from sometime in the late 70's.

The notion of using the attic heat is a great one. Even if it didn't heat the water to "bath" temperatures, the pre-heating it could provide to water moving to a water heater would save energy.

please define:

EPDM
doorskin
OSB
starter strip

A drawing would help.

Are you saying that you have a row of water tank, alternating with a row of 1 x 2; and then, you
shingle over this alternating assembly. And that this starter strip is some kind of spacer that gives
you something to nail the tank into?

arendt

EPDM: Ethylene Propylene Diene Monomer. It's a synthetic rubber used in many applications. A lot of commercial roofs are made from it, as are liners for koi ponds. Basically, you want a nice thick rubbery sheet for this system to sit on that doesn't cost an arm and a leg, and EPDM is a very good choice for that application.

Doorskin is 1/8" thick plywood. Actually, it shouldn't be in there--I was GOING to elevate the first row of tanks 1/8" and forego the starter strip of shingles, but then I remembered: there are two gaps in each three-tab shingle. To fill in the gaps, you gotta put a shingle under there.

OSB is Oriented Strand Board. Also known as Waferboard. Big wood chips glued together in a calculated pattern. It's cheap and it works usually, but when you've got water running around under the shingles, I really don't want to take the chance of water breaking down the OSB.

Starter strip: In this case, a word with two meanings.

Meaning 1: To make a "starter strip" shingle you cut a shingle in half lengthwise and nail the part that would normally be covered up by the next course of shingles to the edge of the roof. It's there for two reasons--to fill in the gaps in three-tab shingles, and to lift up the first course (row) of shingles so the roof looks uniform.

Meaning 2: A shingle is 12" high x 36" long. The first five inches are "exposure"--the part you see when you look at the roof. The next inch and a half contains the "self sealing" strip of roofing cement, which melts after the shingle is installed and welds the roof together. Then, about half an inch above that, is where you drive the four to six nails you'll use to hold the shingle on. We have 3-1/2" of tank and 1-1/2" of 1x2, and we're nailing to the 1x2 because, for reasons that are quite obvious, we don't want to nail into a tank. To get the 1x2 pushed up so it's where the nails go, we need to push it up 2-1/2" from the edge of the roof...and a 1x3 does exactly what we want.

And you guessed it exactly: row of 2x3, row of tanks, row of 1x3, row of tanks, row of 1x3s all the way up the roof deck, and you nail into the 1x3s.

New construction (truss) are designed to be fast, cheap, and carry only the load weight of the shingles, also very limited access. (a 2x2 hatch inside a bedroom closet).

When you order trusses you have to specify the live load and the dead load you want. North Carolina code calls for a minimum of 10 pounds/square foot dead load; if you order them with 40psf or 60psf dead load, you'll have plenty of strength in the roof system to support this. You'd want to run your fluid lines down into a utility closet, but that's not a huge problem.

I just thought of another application: In the north you have to shovel roofs, and people fall off. Assume you had a big rock bed to store heat. You spent all summer charging this up. In the winter, you could conceivably circulate heated water through the tanks, which would keep the roof warm enough that snow wouldn't stick to it.

They are essentially "heat pumps" that take heat from one place and move it to another.

Your refrigerator and/or freezer takes heat out of the chamber where your food is stored, and dumps it into your house.

And then your air conditioner takes heat from your house and dumps it outside.

Why not dump that heat into your hot water heater?

Just use all these heat sources to PREHEAT the water......

Absolutely nothing wrong with sinking the heat you're paying your AC, refrigerator and so on to get rid of, into your water heater. Or better...dig a huge-ass hole, fill it with rocks and PEX pipe to store this heat, and install two heat exchangers--one in the water heating system, one in your air handler.

OTOH, roof heat is not only free, people pay BIG BUCKS to get rid of it. I'm thinkin' that if you were to steal this heat off at least the south side of the house and stick it in your water heater, you might wind up with a cooler attic.

I know my poorly vented attic gets hot as an oven.

All you need is a good tank, painted flat black, mounted on a tower or on the roof. The sun and gravity does the rest.

My system has two huge attractions over just painting an oil barrel flat black and sticking it on a tower.

The major thing I'm trying to do is to pull heat out of the roofing system. Hot attics increase energy bills and negatively impact the environment--you have to generate power to run all the air conditioners.

The OTHER thing that's nice here, is once you put the shingles on the roof you can't see the tanks any more. No homeowner's association is going to let you put an oil barrel on your roof. None of them would object to tanks under the shingles.