A Backwoods Home Anthology
How to build a safe, effective wood-fired hot water heater
“lead-free” solder for joining copper pipes. (See BHM #8 on the danger of lead in water.)
If all this seems scary, it’s meant to. It’s really not difficult to build a safe, reliable, cheap, hot water system. But you need to know what you’re doing.
(A couple of Issues ago I touched on this subject, but it was a surface effort and con- tained the questionable suggestion that you could use a car radiator as a woodstove’s hot water storage tank This is a more in-depth, more correct attempt by Don Fallick of Davenport, WA.—Dave)
By Don Fallick
little such a system can do to you. This is not to say it’s completely safe; at 212 degrees Fahrenheit, it is plenty hot enough for serious burns.
But the real danger comes when you enclose the system, which can allow temperature and pressure to rise above normal boiling.
Hot water systems come in three basic types, depending on how heat gets into the tank.
Heated directly by fire
Figure 2 shows a water tank heated directly by fire. I built such a system
h e v e r y s i m p l e s t w o o d - f i r e d h o t T water “system” is a pot of water on the back of the stove. It’s cheap, sim- ple, and safe. It’s also hard to heat up a lot of water at once that way. It’s labor intensive. And the water cools off fast when the fire goes out.
If you’ve ever used a pressure cooker, you know that increasing the pressure even a little allows much higher temperatures to develop, high enough to cook food in a fraction of the normal time.
My family lived this way for three years. Now we have a closed-loop system that provides plenty of hot water from normal cooking and heating fires.
The trade-off is in greater attention to safety. In an open system, like a pot on a stove or the hand-filled tank in figure 1, water temperature is limited by its boiling point. Barring clogged pipes or boiling the tank dry, there’s
High temperature steam can also cook you in a hurry. And as the tem- perature increases, so does pressure. Left unchecked, it can blow ordinary water pipes apart. To prevent this, water heaters are built with a temper- ature and pressure relief valve.
This device automatically opens if the temperature or pressure exceed safe limits. For around $10 it’s cheap insurance. I have frequently seen home-made hot-water systems without safety valves. This is folly.
Figure 2. Water tank heated directly by fire.
as an experiment. It didn’t work very well. My neighbor-Peter Hammil built a much better one. A comparison might be instructive.
Plastic, iron, & copper pipe
Preventing explosions is not the only safety consideration. Hot water is a much better solvent than cold water. That’s why it cleans better. It also dis- solves chemicals in pipes better. That’s one reason for using “bone” colored plastic CPVC pipes for hot water, instead of the cheaper, pure- white, PVC pipes. There’s also special glue for CPVC pipes that won’t dis- solve in hot water.
Iron pipes will rust fast in hot water if not galvanized, but use plain iron where pipes are exposed to flame.
Hot water quickly dissolves lead out of solder, too, so it pays to use only
We both began with recycled gas water heaters, which are designed for direct flame on the bottom. I used mine pretty much as it was after removing the gas burner. Natural gas doesn’t produce much smoke, so the 4-inch diameter stovepipe in the cen- ter of the tank works fine…for gas.
For a hot wood fire, there’s just not enough room for the smoke to get out. The choice is to build small fires or let the smoke choke the fire. Some choice! Also, the tank I used had a lot of lime and sand in the bottom, which slowed down the water heating even more.
Peter was smart. He used a torch to cut out the original smokestack, cleaned out the tank thoroughly, and