Glossary of Heating and Air Conditioning Parts

NOTE: Any text in RED should be closely read as it either pertains to safety or saving a service call as in the case of fuses as an example.


To add heat or remove temperature and humidity in the form of condensation and promote clean air in a building.


British Thermal Unit, the quantity of heat required to raise one pound of water one degree in Fahrenheit.

BOILER: A piece of heating equipment which is used to heat water to a hot water condition or a steam condition, depending on the type of system. This hot water or steam then circulates throughout a building into baseboards or radiators.


A part of the flue in an oil furnace used to control the "STACK" temperature of the flue. It is usually a weighted device which allows air into the flue as the need is required.


A part in the furnace where gas is injected into the heat exchanger along with the correct amount of combustion air for proper ignition and burning of the gases. Yearly service of this part should be done by a professional service technician.


An electric device consisting of two or more conducting plates separated from one another by an insulating material and used to store an electrical charge. There are RUN capacitors and START capacitors.

CIRCUIT BOARD: Shown are (3) typical circuit boards. There are scores of different boards used by as many manufacturers. To learn more Circuit Boards

Various control boards

A laminated electrical board, usually fitted with electronic components used in furnaces, boilers, air handlers and condensers to control most of the functions of the equipment. Some of these boards also have relays on them.

COMBUSTION AIR: Learn more here Combustion Air and Flue Liners

The air that must be fed to the burners of a gas or oil fired furnace or package heat unit for safe combustion of the fuel. As an example: most basements will need combustion air "PIPES" to bring and return the correct amount of outside air into the furnace. Direct vent furnaces usually don't require this however.

COMPRESSOR: Shown below are cut a ways of (2) Trane compressors. The one on the left is a Scroll which is the newest type of compressor, and on the right is the old reliable Reciprocating compressor which has been used for years.

Trane Compressors

A "PUMP" used in air conditioning systems and usually located in the CONDENSER to move a refrigerant through an EVAPORATOR and back to a CONDENSER. This compressor changes the temperature and state of the refrigerant if it is operating correctly. Located in the CONDENSER.This is the most expensive part of an air conditioning system to replace and should be "checked" yearly by a professional service technician.


The piece of equipment used to remove the heat that has already been removed from a building. Usually located at ground level, but can be mounted on a roof, but is almost always outdoors. There are some mostly older systems that have the condenser indoors and "DUCT" the unwanted heat outdoors.


A series of aluminum or copper tubes with aluminum plates or spinney fins attached to them used to dissipate heat which the COMPRESSOR has removed from the building. Located in the CONDENSER.


The motor which turns the condenser fan blade or wheel in the CONDENSER which removes heat from the CONDENSER COIL.


A small pump used to pump the condensate water which was removed by the EVAPORATOR COIL.


An electrical relay which controls the flow of electricity to different components in the system.


The controls below, are used to melt the ice that builds up on all Heat Pumps during the winter operation. The board controls the outdoor fan operation while in defrost. The thermostat starts and terminates the defrost cycle if ice is present ( depends on the manufacturer ).

Defrost boardDefrost termination nomenclature


Disconnect Box

An electrical box used to shut off equipment for servicing. Usually used outside by the Condenser or Heat Pump ( as shown ), may also be used by some indoor units such as Air Handlers and Electric Furnaces. Some of these have fuses or circuit breakers in them.


A fiberglass sheet, usually used to to build plenums, trunks drops and sometimes ducts themselves in a typical building duct system.


Tape used to seal any air leaks in a system. They can be fabric or metal depending on the application.


An insulating fiberglass blanket wrapped around ductwork used to prevent the loss or gain of heat from or into the duct.


A system used to move heated or cooled , filtered air through a building consisting of plenums, metal or flexible pipes. These are connected to such things as grilles, registers, boots and return drops. Plenums, trunks and drops may be constructed of sheet metal and or fiberglass duct board material.


A series of aluminum or copper tubes usually covered with aluminum plates, used to remove heat and humidity as condensation in a cooling system. This part is usually mounted above or below the blower in a furnace or air handler and is an integral part of a package unit. There is a condensate drain attached to the coil and run to a floor drain, directly outdoors or to a condensate pump.

FAN CENTER:Fan center

An electrical control box consisting of a transformer and a relay used to be a connection point of the thermostat wires and to the furnace and condenser. It has the function of telling the various components within the system to open or close their operation. Primarily used on older furnaces as this function is now controlled from the CIRCUIT BOARD in newer furnaces.


Flexible gas line connector

This flexible gas line connector should be installed between your main gas line and your furnace or water heater according to the new codes. It may NOT pass through the furnace cabinet or any enclosure.


A pipe or series of pipes used on a gas or oil furnace, fireplace or any other fossil fuel burning piece of heating equipment. This flue is connected to the heat exchanger and must be kept clean and in good condition.


An electro/mechanical device used in a gas furnace or package heating unit. This device opens and closes on command of the building thermostat to inject gas into the heat exchanger through the burners.

IGNITION CONTROL:White Rodgers ignition module

The control in a gas or oil fired piece of equipment used to start the actual ignition within the gas furnace heat exchanger or oil furnace fire box.


A motor driven wheel used to circulate or move air through a duct system.


A motor driven small blower wheel assembly, used to move air and gases through a heat exchanger. Shown below is a typical inducer, there are many different styles.

Furnace draft inducer


The metal enclosure in a gas furnace or oil furnace or package heating system where the gas or oil is burned. The air from the building is circulated around this enclosure and picks up heat to be sent to the occupied space through ductwork. In NO WAY can there be a connection between these gases that are burned and the air being delivered to the occupied space as carbon monoxide poisoning and death can be the result. A yearly check of your heating system should be done by a professional service technician.


To raise the temperature in a building by burning fossil fuels such as liquefied petroleum, natural gas, refrigeration, or with electricity using gas furnaces, boilers, heat pumps, and package air conditioning.


A heating and air conditioning piece of equipment used to both HEAT and COOL a building. This really is a "Reverse Acting Air Conditioner". It is capable of reversing the flow of refrigerant in a cooling system, therefore, extracting heat from outdoors which is then brought to the indoor coil in the AIR HANDLER and distributed to the building through ductwork. Additional ELECTRIC HEAT STRIPS are added to the AIR HANDLER to help heat the building if the outdoor temperature is extremely low.


Hot surface igniter

This is a ceramic/carbon element which glows red and is used to ignite the gas and air mixture coming out of the burners in a gas furnace. This device replaces the old "PILOT" which had to continually be burning gas to operate. Much more efficient!


To raise the temperature of domestic hot water usually through the use of a gas water heater, an electric water heater or a boiler.


Pictured below are (2) of the various fan/limit switches. These not only shut the furnace down if it overheats, but also start and stop the blower.

Fan-Limit switch Cam stat

There are also high temperature controls used in newer furnaces called Plenum Thermostats. They only shut the furnace down temporarily if it over heats, but do not start or stop the blower at all. One of these is pictured below.

Typical plenum thermostat

The safety control used in a gas or oil furnace or an air handler to limit the "HIGH" temperature.


The copper tubing connecting the EVAPORATOR COIL to the CONDENSER. Also called the refrigerant lines.


A self contained piece of equipment used to heat a building usually mounted on a roof of a commercial building or at ground level to which ductwork is connected and run to various parts of the building for heating purposes.


A switch usually used within a gas or oil furnace which "Senses" low combustion or improper drafting of the flue gases and will shut the furnace down on safety.


A part in a gas furnace which has a small orifice and burns a small amount of gas used to ignite the burners in a gas furnace on a call for heat.


Circuit boards ICM control board ICM control board

An electronic control which sends signals to all operations of a furnace, air handler, oil burner or air conditioning condenser. There are scores of different boards used in as many furnaces.

The relay in an Electric Furnace or Heat Pump Air Handler that starts the heat strips and stops them in a " sequence " so as to NOT put too much of a load on the system as would be the case if they all came on at the same time. The sequencer in the picture will also start and stop the indoor blower.


Also called an ELECTRONIC THERMOSTAT: A device used to monitor and adjust the building temperature and has set points which can be "PROGRAMMED" by the building owner to lower or raise the building temperature at given times. A great energy saver.


Equipment which uses water sprayed over "pads" which then brings outside air into the building by means of ductwork or simply blown in through one central location. As the hot outside air passes over the wet pads the temperature of the air is reduced. No dehumidification takes place however.


A small tube filled with a material which generates a small amount of electricity and acts as a safety control within a gas furnace , gas water heater, gas boiler or a gas clothes dryer. This device is placed in the path of a burning pilot flame to sense that there is actually a pilot burning before the gas valve will allow gas to be sent to the burners.


An electric or electronic device used to regulate a control through the measurement of temperature.


Due to the nature of the products we sell directly to homeowners we must have you read and understand our terms of use page. If for any reason you can't understand or agree to these terms we suggest you call a reputable Heating and Air Conditioning Contractor in your area for assistance. Your safety and the manufacturers warranty also requires it. Please click this link.

Terms of Use and
Privacy Policy

Denver Web Development and Denver eCommerce by Denver Web Success