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Condensing Wood Stove

Cool exhaust below dew point
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There exists a growing category of condensing gas-fired heat producing appliances (furnaces, hot water heaters, even fireplaces) which extract so much heat from the combustion byproducts, that some of the H2O produced by the combustion process condenses, and is drained away. The remaining gaseous exhaust (which is generally merely warm, not hot) is then vented from the building in a PVC pipe.

Modern designs of wood-burning stoves (and fireplaces) have very efficient combustion processes (compared to older designs) because they have either a secondary combustion zone, or a catalytic converter, to turn carbon monoxide, and creosote, into useful heat.

However, all wood-burning appliances are designed to produce exhaust temperatures high enough that none of the water vapor in the exhaust condenses. This is done to avoid corroding either the (brick) flue or the (metal) flue liner.

This idea is to use eco-friendly, renewable, wood (or wood pellets), as the fuel, together with the high efficiency, corrosion-resistant heat exchangers, warm (not hot) exhaust, condensate drainage, and PVC exhaust pipe typical of a condensing gas-fired appliance.

The idea may *optionally* be improved by making the stove sealed from the room it is heating, and giving it it's own intake, instead of sucking warm air from the room and forcing cold air to leak into the house elsewhere. Since there do exist sealed wood stoves (though not condensing sealed wood stoves), this part of the idea is baked.

The idea may *also* be optionally improved (if it has been made a sealed system) by preheating the cold fresh air that's about to be burned, by using a heat exchanger (counter-current by preference) and the exhaust which has already been used to heat the room.

As long as the air outside the building is colder than the air inside the building, the preheater will be useful -- it will extract more heat from the exhaust, and cause more of the exhaust's water vapor to condense.

The downside is that it can potentially produce an exhaust stream that's colder than the room being heated. It will still be above the temperature of the intake air, of course, but potentially cold -- even freezing. To prevent exhaust passing through the preheater from freezing, we would want a thermostat controlled bypass on the intake -- whenever the exhaust temperature coming out of the preheater is at or below 33F, the intake air bypasses the preheater instead of going through it.

Regardless of whether or not the above optimizations are done, it may be desirable to use the heat of our condensing wood stove (or condensing wood fireplace) to heat our home's domestic hot water, or an absorption chiller, or perhaps even a Vuilleumier cycle chiller. Either way, this idea makes a wood fire more useful (and more efficient) all year round.

goldbb, Aug 19 2012

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