Terminology Primer - Heating, Ventilation and Cooling (HVAC) – Drummond House Plans Blog

Have you ever noticed that when it comes to heating, ventilation and cooling discussions, you feel like there is another language being spoken? As with most areas of specialty, this is the case! The following list of terms is a primer on the common terms bantered about by HVAC specialists.

Programmable thermostat - image courtesy of PlanetForward

Glossary of HVAC Terms


ACH stands for Air Changes per Hour. It is the number of times in one hour that the air in a house is completely replaced with outside air.


The AFUE, or Annualized Fuel Utilization Efficiency, is the ratio of the total useful heat the gas furnace delivers to the house to the heat value of the fuel it consumes.


Btu, short for British Thermal Unit is a unit of heat energy. One Btu is the amount of heat needed to raise the temperature of one pound of water 1°F. To get a rough idea of how much heat energy this is, the heat given off by burning one wooden kitchen match is approximately one Btu.


The transfer of heat through a solid material.


The transfer of heat by air flow.


Coefficient of Performance, COP, is the ratio of energy input to heating capacity. This is the instantaneous measurement of the heating performance of the heat pump. It is comparable to knowing how many miles per gallon of gasoline a car gets when it is going full speed. You can find the COP on the nameplate of the heat pump.


Energy Efficiency Ratio, EER, is the instantaneous measurement of the cooling efficiency of an air conditioner or heat pump. The higher the EER for an air conditioner or heat pump, the more efficient it is.

Heat Pump

A heat pump is basically an air conditioner with a reversible valve that allows it to operate in reverse, removing heat from the house and shunting it outdoors in the summer, and removing heat from outdoor air and shunting it into the house in the winter. Because heat pumps do not actually create heat—they just move it from one place to another—heat pumps are more efficient than other forms of heating.


The Heating Seasonal Performance Factor, HSPF, is an efficiency rating for heat pumps. It is a measure of the average number of Btu of heat delivered for every Watt-hour of electricity used by the heat pump over the heating season. It takes into account variations due to weather conditions over a season. HSPF is comparable to knowing how many miles per gallon of gasoline a car got, averaged over the entire year.

Programmable Thermostat

A thermostat with the ability to record different temperature settings for different times for heating and/or cooling equipment. Programmable thermostats can be electronic, or mechanical.


The transfer of heat directly from one surface to another (whithout the intermediate air acting as a transfer mechanism).


The Seasonal Energy Efficiency Ratio, SEER, measures the average cooling efficiency, over the entire cooling season for an air conditioner or heat pump.

System Capacity

System capacity is a measurement of the total amount of heat or cooling the furnace, heat pump or air conditioner can produce in one hour. This amount is reported in Btu/hr on the nameplate of the equipment. 

(Source of Terminology: Home Energy Saver)


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