A gas fired warm air furnace uses natural (city) gas or propane burned in an open or sealed chamber to heat an exchanger, and with the help of a blower or fan, move air into ductwork to heat the space. It’s basic components are: a gas valve, burner assembly, heat exchanger, blower, and cabinet.
There are a number of unique combustion designs for gas furnaces, but they are copyrighted or patented by different manufactures and will not be described in detail.
Efficiency of a gas furnace corresponds to the type of burner. Some open burners use a standing pilot or electronic ignition, and draft naturally into a hood and smoke pipe, and are usually 80% efficient or less. Other open burners use electronic ignition and a ventor motor to meter the exhaust gases through the heat exchanger and into a chimney, and are 80-90% efficient. Condensing gas furnaces use a sealed chamber, a condensing coil to extract as much heat from the exhaust gases as possible, and a ventor motor to meter the flow through the exchanger, then vent into PVC pipe. This furnace has an efficiency rating over 90%.
Accessories that can be installed inside or outside the gas furnace are: a fan center to turn the blower on independent of the heat cycle, an air filter, electronic air cleaner, high performance media air filter, humidifier, and an evaporator coil for central air conditioning, Sometimes a condensate pump will be used to dispose of the water generated in the dehumidification process of the air conditioning, or the condensing of exhaust gases from a high efficiency furnace.
A Zone control panel and motor actuated dampers will be attached to the furnace or ductwork if the system is zoned.
Routine maintenance of a gas furnace include: monthly filter replacement or washing the electronic air cleaner grids when in constant use, and cleaning the humidifier if so equipped. A properly functioning gas burner will not need cleaning. Black soot is evidence of an incorrect air mixture or other malfunction, and should be checked by a qualified serviceman.
There are basically four gas furnace styles; choosing a specific style of gas furnace depends on the direction of the airflow and design of duct system.
A counterflow gas furnace moves air from top to bottom. This type is used to blow air into ducts that are installed under the slab.
A high boy gas furnace blows air upwards. The blower is at the base of the furnace and the return duct is mounted into the side.
A low boy gas furnace also blows upwards but has a shorter profile with the blower monted next to the combustion chamber.
A horizontal gas furnace blows air sideways. This type of gas furnace is used "lying down" in crawl spaces or attics.