Q. A friend of mine says I can get free hot water if I connect my hot-water piping to my woodstove. Does this really work?
A. Some woodstoves are manufactured with water heater connections, and some are designed only for space heat. This is an important distinction, because it can be dangerous to convert a space-heat stove into one that also heats water.
I’m all for do-it-yourself jobs, but retrofitting a space-heat woodstove isn’t one of them. The system must be engineered, installed, and inspected by a professional. Extreme care must be given to the installation of any kind of woodstove water-heating system to prevent overheated water, which poses an explosion hazard. In order to be safe, any such system must have its own expansion tank and pressure relief valve, as well as circulating pumps and a way to dissipate the water heat if the hot-water tank’s aquastat has been satisfied. Another risk is that improperly diverted heat can mean that toxic gases in the stove may become cool enough to be unable to rise up the chimney under certain conditions. Those trapped gases enter your house when you open the stove door.
If you’ve got a stove that’s designed by the manufacturer to provide both space heat and water heat, yes, it will work. But it’s a bit misleading to call the hot water free; it costs you, just as the hot air from the stove costs you, in wood. You see, there’s a fixed amount of heat produced by any heating system. If you redirect some of it for a different use (hot water), you’ll need to produce more total heat to make up for the loss to the first use (hot air). I hope this answers your question.
Li Ling for The Home Team