A wood furnace or coal fired furnace burns wood or coal in a sealed firebox to heat an exchanger. With the help of a blower air is forced over the exchanger and ducted into the space. It’s basic components include the cabinet or jacket, firebox, blower, access doors for stoking and cleaning, and a small blower to fan the fire when more heat is needed.
Accessories for a wood furnace can include: electronic air cleaner, high performance media filter, humidifier, evaporator coil for central air, and a coil to make domestic hot water. Sometimes a condensate pump will be used to dispose of the water produced by the dehumidifying action of the air conditioning.
Due to high temperatures in the supply plenum, extra precautions must be taken with regards to air conditioning, evaporator coils, as well as ductwork clearance from combustibles. Be sure to check the manufacturer's recommendations.
A combination wood furnace will include an oil or gas burner, or electric elements as a backup.
Proper sizing of a wood furnace is very important for comfort and peformance. Btu output and blower size should not be guessed at but should be professionally calculated. Air distribution is also critical and just as involved as any central heating system, if not more so because of building code requirements. If the system isn't correct, the accessories won't work properly either. The air filter, humidifier, air purifier and central cooling should match the system and will not accomplish their task if the furnace is too big.