For the Curious Consumer: Components Defined
(keywords are in bold/italics)

THERMOSTAT
The thermostat (Stat) is the "brain" of the entire system. It turns everything on and off. New thermostats are now giving more and more information about the comfort levels in a home. Thermostats come in programmable and nonprogrammable types. Both stats are becoming more accurate because digital inputs and outputs have narrowed the on/off setting to be approximately +/- 1 degree. Programmable stats have the ability to “setback” while you are at work or asleep, then return to the desired setting before you arrive home or waken.

AIR CONDITIONING
The compressor is the “heart” of the system. It pumps the refrigerant through the outdoor coils, up the line set, through the evaporator, and back to the compressor. A metering device controls the flow of refrigerant into the evaporator coil to maximize the coil’s efficiency.

An evaporator coil, A.K.A. an “A-Coil” because of its “A” shape, is usually mounted on top of, next to, or below your furnace (indoors.) The evaporator coil evaporates the refrigerant, picks up heat from the house and moves it to the condenser (outside unit.) It goes from a high-pressure liquid to a low-pressure liquid, changing to vapor. A secondary, equally important, feature is that it dehumidifies. The dehumidification process requires a drain line to safely drain the condensation from the A-Coil. The blower moves the air through ductwork in either heating or cooling mode. Refrigerant runs through the condenser coil to be “condensed”, giving up its heat and condensing back into a liquid to be recycled into the cooling process.

A condenser fan motor pulls air from outside, across the condenser coil that is holding heat absorbed by the refrigerant, then exhausts the heat that needed to be removed so the refrigerant is cooled enough to return to liquid form. The lineset carries refrigerant between the evaporator and condenser. An insulated line goes to the condenser coil carrying vapor. (After the vapor goes through condenser coil process, the liquid is returned indoors through a copper line.)

Refrigerant is the high pressure liquid that changes to a vapor, then back again in the cooling process. Older units use R22, and newer, more efficient units utilize environmentally friendly R410-A. Refrigerant should continuously go through the liquid, vapor, liquid, vapor cycle in your home’s cooling process. Ideally it should be a completely closed system; never leaking.

HEATING
Forced Air Heating:
Fuel (natural gas, propane or fuel oil) is brought into the home through a fuel line then it is heated. In many locales there are very specific codes regarding how this fuel line must be installed. A gas valve is operated by the thermostat. It controls the release of fuel into your furnace. Ignition is achieved through a standing pilot, electronic ignition, or hot surface ignition.

A standing pilot was used in heating for many years, and is still used on water heaters. It is being replaced with more efficient means of ignition in newly produced furnaces. Electronic ignition occurs when an electrical spark is used to ignite the fuel for the burners. It’s used on some gas stove, BBQ grills, and some gas furnaces. Oil furnaces use electronic ignition almost exclusively. Hot Surface Ignition (HSI) uses electricity to heat up a surface until it becomes hot enough to light the burners.

The HSI has become the predominant form of ignition. A combustion chamber contains the burner(s) used to burn your fuel and create heat. The actual heating of air takes place here. The blower moves the air through ductwork in either heating (or cooling) mode. The venting of fuel burning appliances is necessary for the same reason we need the exhaust pipe to extend past the back of the car. If the exhaust gasses of a furnace are mixed in the house many problems could occur, with carbon monoxide poisoning being only one of them. 90%+ furnaces vent through a home’s side wall with plastic PVC pipe. Less efficient furnaces vent through the roof, through a metal pipe.

Consumer Tip: You can determine a rough estimate of a furnace’s efficiency by checking what type of vent it has. A metal vent out the roof means it is 80% or less. A PVC pipe out the side of the house means it is 90% or more.

Consumer Tip: Maintenance habits affect the long-term efficiency rating of heating and cooling systems.