Common air conditioning and heating terms defined in plain language


A | B | C | D | E| F | G | H | I | J | K| L | M | N | O | P | Q | R | S| T | U | V | W | X | Y | Z


Air filter - Material that is put in the return air flow to catch particles before they get to the equipment. The most common type of filter is a 5% filter. This filter is one inch thick and you can see through them. This filter will only stop the largest particles. A three month filter is the same width but much denser. It can still allow the same airflow through it due to its accordion design giving it more surface area. The next step up in filtration are referred to as air cleaners. They are specially installed in their own cases. Space-Gard filters made by Aprilaire, are six inches thick and have 27 square feet of surface area, allowing it to stop up to 95% of the particles, including mold spores, bacteria, pollen and pet dander.  These filters require no electrical connections and are changed on average of once a year. Electronic air cleaners use a washable pre-filter to catch the large particles, the air cleaner then gives all the rest of the particles a positive charge through ionizing wires. The particles then stick to negatively charged plates. These can be annoying as the larger particles will cause a spark when sticking to the negative plates. This causes unwanted noise and ozone to be generated. These cell must also be pulled and soaked at least twice a year to clean them. They are also much more expensive to install and maintain.


Blower motor- This is the motor that moves the air around your house. It moves the air across the heat exchanger (for heat) and the evaporator coil (for cooling). Most need to be oiled once a year by a serviceman. There is a blower wheel attached to the shaft of the motor and this is what moves the air.

Top of page


Capacitor - There are two types of capacitors, the start capacitor and the run capacitor. Both types of capacitors store electricity and release it to give a motor more power. The start capacitor is used for starting your compressor, it basically gives the compressor a short burst of power when it starts up. Most residential compressors do not need a start capacitor, but as the compressor gets older it may need one. This is usually a sign that the compressor is at the end of its life. A run capacitor is used for the compressor, the condenser fan motor, and the blower motor. They provide extra power for the motors when they are running. Capacitors are rated in microfarads and need to be checked every year. If a capacitor is rated a 40 microfarads is only showing 20, it will put extra stress on the motor causing it to run hotter and less efficiently. A capacitor that is not operating normally will run your electric bills up and shorten the life span of the motor.

CFM - Cubic feet per minute. This is a measurement of how much air is flowing through an area. If a furnace is rated a 1200 CFM, it will move 1200 cubic feet of air every minute. When matching a furnace to a condensing unit, the furnace must be rated at 400 CFM for every ton the condensing unit is rated at. Ex. Four ton condensing unit needs a 1600 CFM furnace. When trying to get the proper CFM needed for a particular house, the rule is you need six air changes per hour or one every 10 minutes. A house that has 2000 square feet of conditioned space and 10 foot ceilings would have 20,000 cubic feet of air, to get one air change every 10 minutes you would need a furnace that can move 2000 CFM of air, which is five tons of air-conditioning. Many other factors have to be taken into consideration to properly size an an air-conditioning system, such as insulation, windows, climate, etc. but this is a quick way to guess the size needed.

Circuit board - The circuit board simply controls the sequence of events needed for proper operation of the equipment. Most new equipment now use circuit boards instead of a series of relays and timers. Circuit boards are generally very reliable, which is good because they are usually very expensive.

Compressor – The heart of the system. It takes the refrigerant and turns it from a cool, low pressure gas, and turns it into a hot, high pressure gas through some type of compression. Most residential compressors use either a piston or a scroll to achieve this. A piston traps gas in a cylinder then the piston compresses the gas and releases it. A scroll uses two seashell type plates rotating against each other, forcing the gas to travel through a space that is getting ever smaller.

Condenser coils- Located in the outside unit, it’s the part that the refrigerant flows through, and is condensed from a hot gas into a liquid. It is made of copper or aluminum tubing and some type of fins for air to flow through.

Compressor Condenser fan motor Circuit board Run capacitor Start capacitor Condenser coils Contactor

Condenser fan motor- Located in the outside unit, it’s function is to move the air across the condenser coil to cool the refrigerant, and change it from a hot gas into a liquid. Some motors require lubrication once a year while others are sealed and never need lubrication. It’s always best to have a service technician do the lubrication for you, so the right type of oil is used.

Condensing Unit - It is the large piece of equipment that is outside your house. It contains a compressor, condenser coils, a fan motor, and various controls. A Carrier condensing unit is pictured here. Mouse over the unit to see the names of all the different parts, click to go to the definition.

Contactor - Part of your outside unit that turns the unit on and off. It receives a low voltage signal from your thermostat causing the contacts to close sending power to the compressor and fan motor. The most common cause of failure for this part is bugs. Ants are particularly fond of crawling between the contacts. When the contacts close you get a smashed ant that can cause the connection to fail and your unit will not come on. Always keep ant mounds away from the outside unit.

Top of page


Disconnect - A component where all power can be shut off for a particular piece of equipment. Usually found mounted on a outside wall next to the condensing unit. This is just to turn the power off, it offers no circuit protection, that is done by the circuit breakers. It is now required for all installations of condensing units to have a disconnect within sight. Its a recommended to turn the power to the unit off at the disconnect whenever you are washing the condenser coils.

Top of page


Evaporator coil - This is the equipment that is installed next to or on top of your furnace. In applications that use electric heat instead of gas, the evaporator coil is usually built in with the blower motor and the heat strips. The evaporator coil is made of copper tubing, a metering device, a drain pan, and fins. When the refrigerant enters the coil it is in a liquid form, it flows through a metering device that regulates how much refrigerant can pass through, like a hose nozzle. When the refrigerant goes through through the metering device its pressure is reduced and sent through a series of copper pipes which have fins attached to them. As the warm air from your house is blown across these pipes and fins, the refrigerant pulls the heat from the air, causing the refrigerant to boil from a liquid to a gas, and the air to cool. When the air is cooled it looses some of the water it holds, the water accumulates on the fins and drips down into the drain pan, then it flows down a pipe into your sewage. A Carrier horizontal coil is pictured here.

Top of Page


Inducer motor Blower motor Circuit board Heat exchanger Gas valve Hot surface ignitor Pressure switch Run capacitor

Gas furnace - This is the equipment that is installed in a closet or in the attic. It contains a gas valve, blower motor, blower wheel, heat exchanger, burners, either a standing pilot, electronic ignition, or hot surface ignition, several limits and safeties, and electronic controls. A Carrier gas furnace is pictured here. Mouse over the unit to see the names of all the different parts, click to go to the definition.

Gas valve - This controls the flow of gas to the burners. Located in the furnace, it takes the gas that is piped into your house and sends it to the burners at a lower pressure.

Gas flex line - This is a flexible tube that allows an easy connection from the gas line to the furnace. The proper connection should never be made inside the furnace, there should be black pipe coming out of the furnace. The flex line should only be used as a transition between the black pipe from the house to the black pipe coming out of the furnace.

Top of Page



Heat exchanger - Inside your gas furnace you have two separate areas. On one side there is the fire, and on the other side is the air that carries the heat through the house. When the gas burns, it gives off carbon monoxide and obviously you do not want this to get into the air that goes into your house. This is where the heat exchanger comes in. It is a metal chamber that heats up from the fire and gives the heat to the air being blown across it, always keeping the air and the fire apart. On a heat check, the most important thing that a serviceman checks for are holes and cracks in the heat exchanger, these can allow air to be blown on to the flame, or carbon monoxide to get into the air flow. These cracks are usually formed due to lack of maintenance. When the heat exchanger heats up and cools down it expands and contracts. When the heat exchanger is dirty or not heating evenly, the expansion and contraction is uneven. This stresses the metal and will eventually cause it to crack.

Heat pump - Just like a condensing unit except it has the ability to reverse the flow of refrigerant to provide heating in the winter. A reversing valve is what routes the refrigerant for heating or cooling. Heat pumps are usually used when natural gas is not available to the home. It is cheaper to run a heat pump than straight electric heat, however at lower temperatures a heat pump cannot put enough heat into the air and is forced to use electric heat to supplement it. The air from a heat pump is much cooler than from a gas heater, causing complaints such as the house feels drafty. This is why a gas furnace should be used instead of a heat pump wherever possible.

Hot surface ignitor – Part of a gas furnace, it is what lights the gas on fire. Made of a ceramic composite that gets extremely hot when electricity is applied to it. When the furnace gets a call for heat ,it energizes the circuit to the hot surface ignitor. When the ignitor is hot enough, the gas is blown across it causing the gas to ignite, and the fire to go into the heat exchanger. The ignitor is then turned off until the next call for heat. They are very delicate and can shatter easily from uneven heating or any physical blow. You must be careful to never touch it because even a tiny amount of oil from you hands can cause it to heat unevenly and shatter.

Top of Page


Inducer motor - Small motor located inside a gas furnace. It has a blower attached on each end. One blower is for cooling the motor itself, it is usually plastic and can be seen when the furnace door is open. The other blower is for pulling the heat from the burners, through the heat exchanger, and out a vent pipe.

Top of Page


Pressure switch - Used to verify the inducer motor is operating and that the heat exchanger is under pressure. It has a small hose that goes to the inducer chamber. When the inducer motor comes on a diaphragm pulls a switch closed due to the suction from the hose.

Top of Page


Refrigerant - Chemical that transfers heat. Most residential systems use chlorodifluoromethane which is commonly referred to as Freon or R-22. The new environmentally sound refrigerant is a mixture of difluoromethane and pentafluoromethane which is commonly referred to as R-410A or Puron. This is what runs through your system allowing it to transfer heat from your house to the outside. It travels from the compressor through the condenser coils, where it is changed from a hot gas into a liquid, then through the liquid line, past a metering device and into the evaporator coil, where it is changed back into a gas, through the suction line, and back to the compressor to start all over again.

Top of Page


SEER – SEER stands for Seasonal Energy Efficiency Rating. Electricity is purchased in a unit/measurement called Watts. Whereas the amount of air conditioning is expressed in BTUs(British Thermal Units). The SEER lets you convert watts to BTUs. For example, if you had a 14.0 SEER unit, you would get 14 BTUs for every watt of power, so the higher SEER the better. Currently the lowest SEER is 13.0 and the highest is 21.0. The higher the SEER, the higher the cost will be to initially install the better equipment. You should take into account how long it will take you to start getting paid back on your initial investment before you decide what SEER you should buy.

Top of Page


Thermostat - This is where you interface with the air-conditioning system. It should be mounted at eye level, near a return air grill, and away from any outside doors, heat sources, or windows. This will make the thermostat read the overall temperature of the house more accurately. Thermostats range from simplistic to extremely complicated with the price rising with the complexity. Some of the optional features on the newest thermostats include: outdoor temperature sensors, automatic changeover from heat to cool, humidity controls, dirty filter warnings, seven day programming schedules, extra large displays and buttons, backlights, and PC connections. Pictured is one of Carriers better thermostats, it has most of the optional features described above.

Thermostatic expansion valve - (TXV) A type of metering device. It controls the flow of refrigerant going into the evaporator coils. Unlike a fixed metering device that allows a set amount of refrigerant to flow through it, the TXV uses a sensing bulb to detect the temperature of the refrigerant leaving the coil and adjusts the flow accordingly. This allows for optimal efficiency and should be used when a condensing unit with a SEER rating over 13.0 is used.

Top of Page


Vent pipe - The pipe that carries the carbon monoxide out of your house. This is another part that needs to be checked every year to ensure that it has not come loose. Another common problem are birds. They will make a nest in the top of the vent pipe and this will keep the carbon monoxide from getting out of your house. Your serviceman will be able to tell if there is any blockage through simple tests.

Variable-speed blower motor - The term variable-speed blower motor refers to the furnace’s indoor blower motor, which moves at different speeds to control constant air  flow. ECM motor, DC motor, and variable speed motor are all different names for the same type of the furnace motor. Better airflow control results in energy efficiency.

Top of Page