The most common type of heating system, a forced air furnace draws air from different parts of the house into ductwork, through a filter, and into the furnace, where it is then heated and blown back out through ductwork and into the room. The heated air can be 115-120 degrees.
Older furnaces simply use the principle of convection instead of actually blowing the air. ’Gravity furnaces’ such as these let the hot air rise and then heat the cool air that flows beneath it.
Fuels types and efficiency:
All furnaces create heat by using a fuel. One of the most popular fuels is gas. Other fuels for a forced air furnace are oil and propane. Some dual-fuel heaters can use either of a couple types of fuel. You can convert it to either type by flipping a switch.
Furnaces or heating systems fueled by natural gas are the most economical, with an efficiency of up to 97 percent (all US furnaces have to have a minimum of 78 percent efficiency, measured by an AFUE rating- Annual Fuel Utilization Efficiency).
How it works:
In a gas-powered furnace, natural gas enters a combustion chamber, is mixed with air, and is ignited by a pilot light or spark. The flame is then used to heat the heat exchanger, metal tubing through which hot gasses pass, heating the air as it comes in contact with the hot metal.
Exhaust gasses, called combustion air, are vented through the roof or through the wall by a metal or plastic pipe. Only newer, more efficient furnaces vent through the wall, similar in fashion to a clothes dryer vent.
There are, in fact, newer and extremely efficient gas heaters that are actually ’ventless’. If all of the fuel is burned, there should be very little CO (carbon monoxide) and CO2 (carbon dioxide) produced as a by-product, at least not enough to be dangerous if you have normal air flow in your home. If the flame doesn't have enough oxygen for complete combustion, that is when CO is produced.
Ventless heaters do have a sensor that will shut it down if oxygen is low or if the CO levels get too high. Sometimes this type of furnace will be banned by local law or not allowed in sleeping areas, so check to be sure before you get one. You shouldn't have a ventless furnace and a very airtight home.
When the temperature reaches a certain low, there is a thermostat that triggers the furnace to open a gas valve and allow fuel to make contact with the spark or pilot light, igniting to heat the heat exchanger, and thus creating more hot air.
Older models use a pilot light, whereas a more modern method is a spark ignition system that saves energy.
Disposable air filters cost less, but must be replaced frequently and don't let air flow as efficiently as washable filters.
Washable air filters are more expensive and more labor intensive (take time to clean), but they last a long time and allow for more efficient air flow.