Sweep's Library: Heating Water With Woodstoves

Q: Years ago, my Dad had a water coil system running through our woodstove that preheated the water going into our electric tank whenever we had a fire going. Do any of the new, EPA approved "super stoves" offer this option?

A: No, I'm afraid not. Years ago, we did many such installations with fairly good results except for a marked increase in emissions (and creosote formation in the chimney) caused by the cooling effect on the exhaust gases due to heat absorption by the water. With today's high-tech woodstoves, this cooling effect can totally eliminate the secondary burn that makes the "super stoves" so super, and turn an environmentally clean woodstove into a smoke-belching monster.

Here's what the US Department of Energy has to say:

"Although it may be physically possible to retrofit some woodstoves with a water heat exchanger, it is not advisable for certain types of stoves. EPA-approved wood-burning appliances are sophisticated in design, and a "do-it-yourself" retrofit may negatively affect their performance. Woodstoves with catalytic converters ("cats") require high temperatures to protect the catalyst from sooting up and loosing (SIC) their ability to work effectively. Non-catalytic woodstoves rely on a high temperature in the firebox for efficient and clean combustion. A hot water heat exchanger placed inside the stove can lower the temperature so far that it might reduce combustion efficiency and thus increase the amount of smoke produced. In any case, all of the manufacturers of woodstoves will void your warranty if you make such changes or alterations."

Note: Wood cookstoves are exempt from EPA regulations, and many cookstove manufacturers offer water heat exchangers.

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