Heating and Cooling Glossary

Explore heating, water heating, and cooling terms.

Expand Your Knowledge of Heating and Cooling

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Air duct:
A metal or insulated flexible passageway (rectangular, square or round shape) moving air from one place to another.

Air source heat pump (ASHP):
Air-source heat pumps don’t generate heat, but simply move it from one place to another using a refrigerant. Because of this, they can deliver up to three times more heating or cooling energy to a home for the electricity consumed. Air-source heat pumps can be either central systems with ducts, or mini-split systems with air handlers in the rooms they’re conditioning.

Air supply and return:
Heating and cooling systems that move air through ducts have both supply and return portions of the duct system. The supply and return ducts are connected to the heating and cooling equipment, which contains a heat exchanger and a blower. The supply air is pushed across the heat exchanger, and it is either heated or cooled before it is distributed throughout the house to deliver the tempered air. The air is then drawn back into the return ducts where it is returned to the heat exchanger, and the cycle starts over again.

Air vent:
Valve, either manual or automatic, to remove air from the highest point of a coil or piping assembly.

Annual fuel utilization efficiency (AFUE):
This is ratio of annual output of useful energy or heat to the annual energy input to the furnace or boiler. The higher the AFUE, the more efficient the equipment. The higher the efficiency, the more savings on fuel bills.

This is process of adjusting the flow of air in duct systems or water flow in hot-water heating systems. Proper balancing is performed using accurate instrumentation to deliver the right amount of heating or cooling to each area or room of the house.

Baseboard radiator:
Most modern boilers work by heating up water and then circulating it throughout the house through copper tubing that is connected to baseboard radiators. Much like a car radiator, baseboard radiators contain many “fins” that remove the energy from the hot water and dissipate it into the house. Regardless of what fuel is used to heat the water (natural gas, oil, propane, or other), these are “closed” systems, meaning that the water is returned to the boiler after its energy is removed so that it can be reheated and sent back through the piping system to the radiators. (see “steam radiator” for a description of the other common type of “hydronic” heating).

British thermal unit (BTU):
This is quantity of heat required to raise the temperature of a pound of water one degree from 59 degrees to 60 degrees. BTUH stands for British Thermal Unit Per Hour. This establishes a time reference to BTU input or output rates.

Condensing boiler:
This is a Gas-fired boiler that achieves efficiency by condensing water vapor in exhaust gases and recovering latent heat that would normally be lost up the flue. By doing so, it can achieve efficiency of 90 percent or higher. Condensing boilers require drainage and materials that won’t be corroded by the slightly acidic condensate.

Coefficient of performance (COP):
COP is a measure of performance for electrically-powered heating and cooling equipment, and is normally used to describe heat pumps. It is the ratio of energy output per unit of energy input, so a higher COP number means more efficient operation. For example, a heat pump that delivers three kWh worth of energy for each one kWh in power that it consumes would have a COP of three.

Electronically commutated motor (ECM): 
An ECM is an energy-efficient furnace or air handler blower motor that can also: 1) move air more efficiently against restricted duct systems, 2) offer low power constant air flow for better indoor air filtration use, and 3) provide two-speed or variable-speed air flow for multistage heating and cooling systems.

This is the rate at which a system maximizes fuel use. This rate is numerically described as a ratio called AFUE for heating systems, EF for water heating systems, SEER for cooling systems, and HSPF for heat pump (heating and cooling) systems.

Energy efficiency ratio (EER):
A measure of how efficiently a cooling system will operate when the outdoor temperature is 95 degrees Fahrenheit. It is calculated by dividing the rated cooling output at 95 degrees Fahrenheit by the watts used by the AC/HP system. A higher EER means the system is more efficient. It is an instantaneous measure of electrical efficiency, unlike SEER (Seasonal Energy Efficiency Rating), which is an averaged value of efficiency. This is a term applied to air conditioning equipment.

Energy factor (EF):
EF is based on the amount of hot water produced per unit of fuel consumed over a typical day. A higher EF means the system is more efficient.

Heat exchanger:
This transfers heat from combustion gases to the air blowing through the ductwork. It is vital that none of the combustion gas itself gets into the air stream. The primary heat exchanger handles the hottest gases.

In high-efficiency furnaces, secondary heat exchangers recover heat that used to be vented up the chimney with the exhaust gases. By recovering this heat, the furnace becomes more efficient. Part of the heat recovered causes the water and acid to condense out of the exhaust gas. Because this liquid is corrosive, secondary heat exchangers must be designed to prevent deterioration. Typically, these heat exchangers are made of stainless steel or some derivative.

Heat loss calculation:
This is the Calculation used to determine house BTU heat loss. Factors include conduction through construction materials, air infiltration losses, and the difference between actual or projected outside temperatures, and desired temperatures inside the house. It is used for determining necessary heat output from the heating appliance. When replacing HVAC equipment, it is important that the contractor consider all past and future insulation and efficiency measures when performing the load calculation before selecting the new equipment.

Heat pump water heater (HPWH):
Like air-source heat pumps, heat pump water heaters don’t generate heat, but simply move it from one place to another in the same way that a refrigerator does. Because of this, they can be two to three times more efficient than an electric resistance water heater.

Heating seasonal performance factor (HSPF):
This is a ratio of heat output over the entire heating season to electricity used (similar to SEER for cooling). A higher HSPF means the system is more efficient. This is a term applied to air-source heat pumps.

Indirect water heater:
This circulates boiler water through a heat exchanger in the tank. Some direct water heaters use a flat plate heat exchanger rather than an internal coil. The domestic hot water is contained in an insulated storage tank. Because the boiler does not need to operate frequently, this system is more efficient than the tankless coil. In fact, when an indirect water heater is used with a highly efficient boiler, the combination may provide one of the least expensive methods of water heating.

Infrared radiant heating:
A gas-fired infrared heating system emulates the heat transfer of the sun by generating radiant energy that is converted into heat when absorbed by objects in its path. Once the floors, machinery, stock, and people absorb the infrared energy, it is then re-radiated to warm the surrounding air. 
This method of heating, as opposed to filling a room with warm air (such as a forced air unit), allows the source of heat to be suspended at ceiling height. Infrared radiant heating is the most efficient and effective method in which to heat under the diverse conditions present in warehouses, storerooms, and even the most immense structures imaginable.

Mini-split heat pump (MSHP):
These systems, also called ductless heat pumps or air-source heat pumps, have an outdoor condenser unit connected to one or more indoor units. These units provide heating and cooling for homes. The system pulls air from the outdoors through a refrigerant line, to heat the space during the winter, and does the reverse in the summer by pulling heat from the home to the outside to cool the space. Because of this, they can deliver up to three times more heating or cooling energy to a home for the electricity consumed.


National fuel gas code (ANSI Z23.1, NFPA 54):A standard for the installation of gas appliances, piping, and venting.

Outdoor reset controls:
Outdoor reset controls reduce the temperature in a boiler during moderate outside temperatures when high-temperature supply water isn’t necessary to maintain indoor comfort. This reduces standby losses and, in the case of condensing boilers, allows return water temperature to be low enough to condense.

This is a heat that moves out in waves from a central point and heats objects in its path. The level of heat that is felt is determined by the proximity to the heat source.

Seasonal energy-efficiency ratio (SEER):
This is a measure of efficiency over an entire cooling season as opposed to a single outdoor temperature. SEER is an indicator of the total amount of cooling the air conditioner will provide over the entire cooling season, divided by the total number of electricity it will consume. A higher SEER means the system is more efficient. This is a term applied to air conditioning equipment, including air-source heat pumps.

This is the procedure a heating contractor goes through to determine how large a system is needed to heat a house efficiently. Too small a system will not deliver enough heating, while too large a system increases energy costs and can have an adverse effect on comfort. Sizing depends on the square footage of a home, the amount of ceiling and wall insulation, the window area, and use of storm doors and storm windows.

Steam radiator:
Some hot water boilers, typically older ones, heat a house by delivering steam instead of hot water. While hot water systems circulate heated water through a closed system of radiators (often baseboard style), steam radiators allow the pressure of the steam to vent out, thereby dropping the temperature so that the steam converts to water and runs back to the boiler to be reheated into steam.

Tankless water heater:
A tankless water heater (also called an on-demand water heater) heats water as it’s requested, rather than preheating and storing it. Because of this, they don’t have the standby efficiency losses of storage water heaters, and never run out of hot water. However, their flow rate is limited, so one tankless water heater may not be able to handle multiple simultaneous uses.

This is another measurement of heat. One therm equals approximately 100,000 BTUH.

This is a system in which living areas are divided into separate spaces, with each space's heating and air conditioning controlled independently. This can be accomplished by using either multiple independent systems, or a single system using electronic controls or motorized dampers.