It is one of life's cruel ironies that the most common heating system in the USA is also one of the least environmentally friendly. Forced-air heating installations are present in over 60% of homes in the country. Of course, some are better than others – these systems have heat generated by solar thermal systems, heat pumps, and furnaces of all types (oil-fired, gas-fired, electric, or wood).
Forced-air systems work by taking the heat and circulating it through extensive ductwork built into wall, floor, and ceiling cavities. The air escapes through "registers" into each room, where it cools and is brought back to the furnace by means of a second set of ductwork (the cold-air return). Forced-air systems require a large, powerful fan to push the air around.
- Heating is immediate.
- The ductwork can be designed for multi-purpose function, transporting cold air from an air conditioning system in the hot season, and tying in to the building's ventilation.
- Air filters in the ducts can remove pollen, dust, and other pollutants.
- In a passive solar home, the fan can be used to circulate passively-heated air as well.
- The systems are so common that installation and maintenance is cheap and easy to source.
- Forced-air heating is almost always the least efficient heating method, usually meaning the thermostat has to be set higher to achieve similar comfort to an alternative system.
- The pumped air increases the pressure inside the building, encouraging warm air to push its way out through cracks and crevices.
- Ducts can leak, reducing efficiency.
- The drafts coming out of registers are uncomfortable, especially if you sit right underneath one for a while.
- Even if you shut individual registers, forced-air system don't do zone heating very well.
- The constantly-moving air in the home can stir up dust, pet dander, and spores. This can be a serious problem for people with allergies.
- The noise level is significant when the fans are running.