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.