A forced-air system is one which uses air as its heat

Heat

In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...

 transfer medium. These systems rely on ductwork

Duct (HVAC)

Ducts are used in heating, ventilation, and air conditioning to deliver and remove air. These needed airflows include, for example, supply air, return air, and exhaust air. Ducts also deliver, most commonly as part of the supply air, ventilation air...

, vents

Diffuser (thermodynamics)

A diffuser is the mechanical device that is designed to control the characteristics of a fluid at the entrance to a thermodynamic open system. Diffusers are used to slow the fluid's velocity and to enhance its mixing into the surrounding fluid...

, and plenums as means of air distribution, separate from the actual heating and air conditioning systems. The return plenum carries the air from several large return grills (vents) to a central air handler

Air handler

An air handler, or air handling unit , is a device used to condition and circulate air as part of a heating, ventilating, and air-conditioning system. An air handler is usually a large metal box containing a blower, heating or cooling elements, filter racks or chambers, sound attenuators, and...

 for re-heating. The supply plenum directs air from the central unit to the rooms which the system is designed to heat. Regardless of type, all air handlers consist of an air filter, blower, heat exchanger/element/coil, and various controls. Like any other kind of central heating

Central heating

A central heating system provides warmth to the whole interior of a building from one point to multiple rooms. When combined with other systems in order to control the building climate, the whole system may be a HVAC system.Central heating differs from local heating in that the heat generation...

 system, thermostats are used to control forced air heating systems.

Forced air heating is probably the type most commonly installed in North America

North America

North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...

. It is much less common in Europe

Europe

Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

, where hydronic heating predominates, especially in the form of hot-water radiators.

Natural gas/propane/oil

• Heat is produced via combustion of fuel.
• A heat exchanger keeps the combustion byproducts from entering the air stream.
• A ribbon style (long with holes), inshot (torch-like), or oil type burner is located in the heat exchanger.
• Ignition is provided by an electric spark
  Electric spark
  An electric spark is a type of electrostatic discharge that occurs when an electric field creates an ionized electrically conductive channel in air producing a brief emission of light and sound. A spark is formed when the electric field strength exceeds the dielectric field strength of air...
  
  , standing pilot, or hot surface igniter.
• Safety devices ensure that combustion gases and/or unburned fuel do not accumulate in the event of an ignition failure or venting problem.

Electric

• A simple electric heating element warms the air.
• When the thermostat calls for heat, blower and element come on at the same time.
• When thermostat is "satisfied", blower and element shut off.
• Requires very little maintenance.
• Usually more expensive to operate than a natural gas furnace.
• Safer operation—a home with an electric furnace does not have to worry about being poisoned by carbon monoxide (CO) or the possibility of a natural gas leak which could cause an explosion.
• Very quiet operation.
• Does not produce on-site emissions. (The electric power company will generally produce emissions especially if burning coal, oil, or gas).
• Many cities or states offer incentive programs to home owners who install electric furnaces, in an attempt to reduce fugitive emissions. Many utility companies also will have a secondary, cheaper electricity rate offered to those who run their home with electric heat. A separate meter must be installed and hooked to the heater, this will cost the consumer at first, but the discount will pay itself off in less than a year.
• Electric rates vary greatly across the US, so it is important to do research on local KWH prices with different utilities.

Heat pump

• Extracts heat from outdoor air via the refrigeration cycle
• More efficient than fossil fuel fired furnaces (gas/oil) and electric resistance heating
• Not suitable for cold climates unless used with backup (secondary) source of heat. Newer air/air heatpumps can provide heat well below 0 degrees Celsius (32 °F).
• A refrigerant coil is located in the air handler instead of a heat exchanger or element. System can also be used in cooling, just as any central air-conditioning system.
• See Heat pump
  Heat pump
  A heat pump is a machine or device that effectively "moves" thermal energy from one location called the "source," which is at a lower temperature, to another location called the "sink" or "heat sink", which is at a higher temperature. An air conditioner is a particular type of heat pump, but the...
  
  s

Hydronic coil

• Combines hydronic
  Hydronics
  Hydronics is the use of water as the heat-transfer medium in heating and cooling systems.Some of the oldest and most common examples are steam and hot-water radiators. Historically, in large-scale commercial buildings such as high-rise and campus facilities, a hydronic system may include both a...
  
   (hot water) heating with a forced air delivery
• Heat is produced via combustion of fuel (gas/propane/oil) in a boiler
• A heat exchanger (hydronic coil) is placed in the air handler similar to the refrigerant coil in a Heat Pump system or a Central AC
• Heated water is pumped through the heat exchanger then back to the boiler to be reheated

Sequence of operation

1. Thermostat calls for heat
2. Source of ignition is provided at the boiler
3. Circulator initiates water flow to the hydronic coil (heat exchanger)
4. Once the heat exchanger warms up, the main blower is activated
5. When call for heat ceases, the boiler and circulator turn off
6. Blower shuts off after period of time (depending on the particular equipment involved this may be a fixed or programmable amount of time)

Advantages

• Can accommodate central air conditioning, humidifiers, HRVs, and whole-house air cleaners
• Excellent for programmable thermostat
  Programmable thermostat
  A programmable thermostat is a thermostat which is designed to adjust the temperature according to a series of programmed settings that take effect at different times of the day...
  
  s - takes a short time to recover
• Less expensive than hydronic systems
• No risk of structural damage due to water leaks
• Heatpumps and 90%+ AFUE (Annual Fuel Utilization Efficiency) systems are energy efficient
• High efficiency furnaces are more readily available than high efficiency boilers

Disadvantages

• When improperly installed, they are prone to air infiltration; especially if the ductwork is located in an un-conditioned space. Air infiltration typically lowers humidity
  Humidity
  Humidity is a term for the amount of water vapor in the air, and can refer to any one of several measurements of humidity. Formally, humid air is not "moist air" but a mixture of water vapor and other constituents of air, and humidity is defined in terms of the water content of this mixture,...
  
   in the heated space and increases energy consumption.
• Stratified (uneven) temperature differences when compared to e.g. hydronic heating
• More noisy than hydronic systems
  Hydronics
  Hydronics is the use of water as the heat-transfer medium in heating and cooling systems.Some of the oldest and most common examples are steam and hot-water radiators. Historically, in large-scale commercial buildings such as high-rise and campus facilities, a hydronic system may include both a...
  
  
• Can distribute allergens and cooking odors throughout heated space.
• Requires a filter, which must be replaced periodically.
• Can be difficult to retrofit a property with the required ducting.