somewhere, we must insert one in the lowest part of the system. So we put in a tee, and screw a hose bib (a faucet) into it. Between the tee and the stove is another union.
Theoretically, all these pipes except the stove supply are cold water pipes and could be made of any standard pipe material, even plastic PVC, but I don’t recommend it. All pipes beyond the first union should be made of heat- tolerant materials only—metal or plastic CPVC. There is a rare condi- tion that can occur in which the ther- mosyphon works backwards. If this happens, you’ll be glad that all the pipes in your water heating system can take the heat. I have seen ordinary plastic pipe fittings deformed by hot water until the pipes burst apart. It’s not funny.
Within the stove itself are two plain iron pipes (not galvanized) joined with a 180 degree bend. Avoid using gal- vanized pipes in the firebox. The fire can burn off the galvanizing. The upper of the pipes in the stove is joined with an elbow to a vertical pipe, the hot water riser. This is joined with another elbow to a union, then to the tank with a tee. When the fire is lit, but no hot water is being used, the hot water leaving the stove through the riser reenters the tank, sucking relatively cooler water from the bottom of the tank into the stove supply, and repeating the cycle until the whole tank is full of hot water. When hot water is being used, it is drawn from the top of the tank, through the tee and another elbow to the hot water outlet pipe. This con- nects to the hot water distribution pipes in your house by a union and another valve. This time the valve goes on the distribution side of the union.
Will your floor support the water weight?
A typical, 30-gallon, water heater tank weighs about 300 pounds, filled, so it’s imperative that it be mounted on a sturdy support, and that the floor under it be strong enough to take the weight. If there’s any doubt about the
floor’s strength, place the support legs on boards (2 x 4 or larger), long enough to span at least two floor joists. It’s not a good idea to use ply- wood or particle board, as they deteri- orate when wet.
Pressure relief valve
Finally, note the temperature/pres- sure relief valve in the center of the tank top. This is more than just a “temperature” or a “pressure” relief valve. You want both. If it ever does release, steam and hot water are going to spray out of the valve. These valves are threaded so they can be connected to pipes, and the spray directed some- where safe. In commercial installa- tions, they are usually piped to a floor drain. For a home-brew system like this, a plastic bucket will do…if you are there to turn off the water! Moral: don’t leave a home-brew system run- ning unattended.
The system just outlined is not, by any means, the only way to hook up a water-heating stove. In fact, each installation is unique.
My friend, Jeff Moore, mounted his tank on the roof, in a box insulated with four inches of rigid foam. An openable south side reveals the black- painted tank behind a layer of glass. Behind the tank is a curved reflector made from plywood covered with alu- minum foil. Voila! A cheap and easy solar water heater when it’s too hot to use the stove, and an outdoor, insulat- ed tank for his woodstove to heat in the winter. It works, too.
All water has some air dissolved in it (or fish couldn’t live). Heating water frees some of the air. If there is no way for air to get out of the pipes, it will rise to the highest point and stay there, blocking the pipes. Pipes arranged in the shape of an upside- down “U” (see figure 5) will not allow hot water to enter the tank. It will just keep getting hotter and hotter, until it turns to super-heated steam. Eventually, either the relief valve will blow, or the pipes will.
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Tom Weinert, who built my hot water system, got tired of air in the pipes and added a hose bib at the highest point of the system. It did wonders for eliminating air in the lines. This can be caused by running the stove a lot, but not using much hot water. I bought a $10, high-tempera- ture, bleeder valve to replace it when the rubber faucet washers died.
Authors of a book titled Blazing Showers (now out of print) say the horizontal distance between the stove and the tank should never be greater than twice the vertical length of the hot water riser. I guess if you put your tank in the attic, you could have a hor- izontal distance of 15 feet or more! I have never seen an installation which came anywhere near this ratio, but it’s nice to know. Most folks instinctively
Figure 5. Trapped air in shaded area of pipe will prevent water from flowing.
try to minimize the distance from stove to tank, if only because pipe is expen- sive. Vertical distance doesn’t seem to matter, as long as there is some.
You may wish to use copper pipe in the firebox. I can’t recommend it. Even at room temperature, copper is easily deformed. When heated, it bends very easily. If you bump it with a piece of firewood, it could crimp, restricting the flow. Copper “work- hardens” very rapidly, so it would be