Forced air heating

Forced air heating is a form of central heating in which air – not water – is used to heat the home. It does this via a system of ducts and plenums integrated into the building, and one or more vents in each room. The heat itself originates from a furnace which heats the air via a heat exchanger.

Forced air heating is the most common form of central heating in North American homes. This is contrast to the hydronic or “wet” systems that are typically found in European homes, where a boiler heats water for radiators.

Forced air heating systems have a number of advantages over wet central heating. For example, there is no need for bulky radiators which would otherwise take up wall space and restrict interior design and furniture choices. The vents and ducts also lend themselves to air conditioning systems, allowing full control of the temperature in the home via a single integrated system.

How does forced air heating work?

When the temperature of the home falls below the desired pre-set level, the thermostat will activate the furnace. A fan or blower distributes the warm air generated by the furnace throughout the system and into each room. Once the target room temperature is reached, the thermostat will shut off the system until the room temperature drops low enough to activate it again. Special covers and grills for the vents direct the flow of warm air into the room. Baffles inside the ducts reduce the speed of the air flow, preventing turbulence which would generate unwanted noise.

Forced air heating furnaces

The furnace is the key component of a forced air heating system.  Common gas fuels include propane or kerosene, or methane (natural gas). Electric forced air furnaces are also available, as well as solid fuel ones which typically burn wood. Hydronic coils are also available. These consist of coils of copper pipe which carry hot water from the property’s water heater. The pipes emit heat, consequently heating the surrounding air for distribution by the blower.

Forced air furnace types

From around 1900, the most popular type of furnace was the gravity warm air furnace. Such heating systems still worked the same way as the modern variety using ducts and vents. However, the furnace did not have a blower, relying instead on natural convection. Warm air would heat the home by simply rising up through the ducts. The cool air (or return air to use its technical name) would fall and be reheated by the furnace.

Modern forced air heating furnaces superseded the gravity kind in the 1970s. There are now three different types of furnace: upflow, downflow, and horizontal. The difference is exactly as the name implies and refers to the direction of the air flow from the furnace.

In an upflow furnace, cool air is drawn in from the side or at the bottom, and the hot air is discharged at the top. For basements or rooms with a low ceiling, low profile furnaces discharge the hot air from the side.

Downflow furnaces draw cool air in at the top and expel the warm air out from the bottom. This makes them ideal for installation in the attic, because the air can be expelled down into the plenum from above. Horizontal furnaces work from side to side, drawing the cool air in at one side and expelling it out the other.

The most modern gas-burning furnaces have condensing technology. This makes them much more energy efficient than standard gas furnaces. They achieve this by drawing so much heat out of the exhaust gas, the water vapour inside it condenses back into a liquid. On the other hand, standard furnaces simply expel the gases into the atmosphere without any further treatment, wasting the heat it contains.

Advantages & disadvantages

One of the biggest advantages of forced air heating is that other home environment and climate controls can be incorporated. An obvious example is that an air conditioning system can use the same vents and ducts. Plus, there is no need for bulky radiators on the wall.

Filtration systems can remove allergens for asthma and allergy sufferers. Humidifier and dehumidifier systems can control moisture levels.

However, forced air heating does have its drawbacks:

  • Without a filtration system, forced air heating will be very effective at spreading airborne allergens.
  • Obstruction of the vents by furniture may cause the room to heat up slowly or unevenly.
  • Air is much less efficient at carrying heat than water, and consequently the ducts in the wall must be wide. As a result of the wider ducts, this means less space for insulation in the wall cavity. The heat lost this way can have a 10-30% impact on your energy bill.