Forced-air
From Wikipedia, the free encyclopedia
A Forced-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 system, thermostats are used to control forced air heating systems.
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[edit] Types
[edit] 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 do not accumulate in the event of an ignition failure or venting problem
- A draft hood or induction fan "pushes" combustion gases up the stack/chimney.
- Efficiency ranges from 55-65% (standing pilot + draft hood), 78%-80%, and 90-96% (secondary heat exchanger)
[edit] Sequence of operation:
- Thermostat calls for heat
- Source of ignition is provided
- Gas valve opens (for standing pilot models, this does not occur unless pilot is lit)
- A flame sensor proves ignition. (electronic ignition models only)
- If ignition is successful, electric spark ceases/hot surface igniter de-energizes (electronic ignition models only)
- If ignition fails, gas valve closes and ignition control locks out. (Note: Some systems retry a number of times)
- Once the heat exchanger warms up, the main blower comes on
- When call for heat ceases, gas valve closes
- Blower shuts off after heat exchanger cools down (1-3 minutes)
[edit] 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
[edit] 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
- A refrigerant coil is located in the air handler instead of a heat exchanger or element
- See Heat pumps
[edit] 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
[edit] 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)
[edit] Advantages and Disadvantages
[edit] Disadvantages
- Distributes cooking odors throughout dwelling
- Requires a filter which must be replace periodically
- More noisy than hydronic systems
- Provides less even heat ==> Hot spots and cold spots
[edit] 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
