Posted by Harold Kestenholz - Hydronic Network on February 16, 2000 at 20:43:57:
In Reply to: Multi fuel heaters posted by Scott on February 16, 2000 at 19:36:57:
Wood or gas in the same unit is not efficient. A wood heater is about 50% efficient in converting fuel to heat because the passages must be large enough to pass soot and flyash without clogging.
When using or converted to gas the heater becomes less efficient - about 30% - as gas has no radiant component from the flame and all the work must be done by the wide open passsages meant for wood. A gas heater needs tight passages with bumps to take all the heat from the combustion gases.
You do better to put a separate wood and gas heater in parallel so you can use one without heating the other and gain the maximum efficiency. In parallel, you will also be able to repair or clean one while the other is in use.
They can not use the same chimney. This is not much of a problem as many fine gas heaters use direct venting today. A wood fire requires a high-quality multi-wall stainless steel lined chimney or a masonry chimney. A high efficiency gas heater would condense water in a masonry chimney and flood or weaken it.