Annual Fuel Utilization Efficiency. Indicated as a percentage, your furnace’s AFUE tells you how much energy is being converted to heat. For example, an AFUE of 90 means that 90% of the fuel is being used to warm your home, while the other 10% escapes as exhaust and is vented to the atmosphere.
Atmospheric burner
This refers to a burner that has no combustion blower. All modern furnaces use a combustion blower most boilers do as well. However all standard water heaters us an atmospheric burner. Cast iron boilers still use them because they can transfer heat to the water without a convoluted heat exchanger. In most cases it takes a combustion blower and a multipass heat exchanger to acheave the efficiency of modern equipment.
Hot water boilers heat water for space heating, domestic water heating, or for a process like milk pasteurizing.
Steam boilers heat water into steam for the same purpose steam however does not need to be pumped and is of course hotter.
British Thermal Unit. Used for both heating and cooling, BTU is a measure of the heat given off when fuel is combusted. Or for cooling, it’s a measure of heat extracted from your home. One BTU is equal to the amount of heat it takes to heat one pound of water one degree.
The ability of a heating or cooling system to heat or cool a given amount of space. For heating, this is usually expressed in BTUs. For cooling, it is usually given in tons. One ton of cooling= 12,000 BTU and usually needs 400 CFM of moving air. Originally this referred to the amount of heat transfer it takes to make 1 ton of ice in a day.
Condenser Coil
Part of the outdoor portion of a split-system air conditioner. By converting refrigerant that is in a gas form back to a liquid, the coil sends heat carried by the refrigerant to the outside.

In a heat pump the condenser coil switches between inside and outside depending on its cycle.

A type of furnace that takes cool air from the top and blows warm air to the bottom. Also referred to as counter flow.
Electronic Air Cleaner (EAC)
There are many types of electronic air cleaners. Usually this refers to the type that uses very high voltage charging plates, pins, or wires to place a positive charge on small airborne particles that are then attracted to a negatively charged collection cell.

There are also Ultraviolet light electronic air cleaners

See also: Filter , Ultraviolet air purifier

Evaporator Coil
The evaporator coil cools and dehumidifies the air by converting liquid refrigerant into a gas, which absorbs the heat from the air. The warmed refrigerant is then carried through a tube to the outdoor unit (condenser coil). They are usually constructed of aluminum and copper. This is a important maintenance point. Because they are wet while in operation they tend to collect dust and become dirty. They need to be cleaned periodically.
Fan Coil
An indoor component of a heat pump system, this houses the blower and the indoor coil. It will also often have electronic resistance heaters for additional heat when necessary.
The air filter is part of any Forced air system. Its primary purpose is to keep the equipment clean. Of course a major benefit is it also cleans the air as the system runs. With a high quality air filter you can remove mold spores, pet dander, pollen, and all types of small air born particles. Combined this with a Ultra violet air purifier and you have clean sterile air recirculating from the vents.

See also: Electronic Air Cleaner (EAC)

A central heating gas or oil fired appliance that circulates warm air throughout the building through a duct system. This is the most common form of heating in a residential home.