Forced-air Totally Explained

Everything about Forced-air totally explained

A Forced-air or Warm air heating system is one which uses air as its heat transfer medium. These systems use ductwork and vents as a means of air distribution. The return plenum carries the air from several large return grills (vents) to a central air handler for re-heating. The supply plenum directs heated 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 system, thermostats are used to control forced air heating systems. Forced air heating is probably the type most commonly installed in North America. It is less common in Europe, particularly in the United Kingdom (where it's typically referred to a "warm air" heating).

Types

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, standing pilot, or hot surface igniter
Safety devices ensure that combustion gases and/or unburned fuel don't accumulate in the event of an ignition failure or venting problem

Electric

A simple electric heating element warms the air

When the thermostat call for heat, blower and element come on at the same time

When thermostat is "satisfied", blower and element shut off

Requires very little maintenance

Extremely expensive to operate

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 pumps

Hydronic coil

Combines hydronic (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:

Thermostat calls for heat

Source of ignition is provided at the boiler

Circulator initiates water flow to the hydronic coil (heat exchanger)

Once the heat exchanger warms up, the main blower is activated

When call for heat ceases, the boiler and circulator turn off

Blower shuts off after period of time (depending on the particular equipment involved this may be a fixed or programmable amount of time)

Advantages and disadvantages of forced air systems

Disadvantages

When improperly installed, they're prone to leaky ducts and other problems with lead to air infiltration. Air infiltration typically lowers humidity in the heated space as well as requiring extra heating energy.

More noisy than hydronic systems

Can distribute allergens and cooking odors throughout heated space

Requires a filter, which must be replaced periodically

Advantages

Can accommodate central air conditioning, humidifiers, HRVs, and whole-house air cleaners

Excellent for setback thermostats - 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

Two stage and modulating forced air systems can match the comfort of hydronic systems with none of the disadavantages (Water leaks, can't accommodate a/c, not suitable for use with programmable thermostats, expensive to install) Further Information

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