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HVAC Measures of Input and Output

more HVAC definitions

The HVAC industry measures the capacity of heating and cooling systems with many different terms. This page presents and defines the most commonly used terms.

BTU (British Thermal Unit)

A BTU is the quantity of heat required to raise 1 pound of water by 1 degree Fahrenheit.

BTUH (British Thermal Units per Hour)

Air conditioner cooling capacity is often measured in either BTUH or tons. 1 ton = 12,000 BTUH.

Indoor Blower Capacity

A furnace's output can be measured in several ways. One of those measures is the volume of air that passes out of the furnace and into the ductwork over a predefined span of time. This is a furnace's indoor blower capacity and it is measured in cubic feet per minute (cfm).

You may see documentation that refers to the "maximum" capacity as compared to the "default" capacity. The maximum indoor blower capacity represents the furnace's highest setting. The default capacity is the factory setting at the time that the furnace was manufactured.

A second and related measure is the amount of heat created by the furnace, measured as British Thermal Units (BTUs) or BTUH.

Input BTUH

The number of British Thermal Units per hour that the furnace or boiler consumes when running at a steady state. (That is, when it has finished startup and is running at full power.)

Boilers with an input BTUh greater than 350,000 are typically considered commercial boilers.

Ton (as a unit of cooling capacity)

A ton of cooling capacity is the amount of cooling that would be provided by melting a ton of ice. Thus, a central air conditioning system that is rated as a 2-ton system would provide the same cooling as melting two tons of ice per day (24-hour period).

Ton vs BTU

A ton is equal to 12,000 BTUs. Thus, a 2-ton air conditioner is equivalent to a 24,000 BTU air conditioner.


Technically speaking, voltage is the rate at which energy is drawn from an electricity source. A simple analogy is that voltage can be likened to the pressure of water in a pipe. Voltage is measured in volts.

Motors are designed to operate with electrical input within a certain range of voltage. If the electrical input source provides electricity at a voltage outside the specified range, performance is likely to be sub-optimal.

Watt (w)

The unit of electrical power equal to 1 ampere (amp) under a pressure of 1 volt. Equal to 1/746 horsepower.

Watt-hour (Wh)

An electrical energy unit of measure equal to 1 watt of power supplied to, or taken from, an electric circuit steadily for 1 hour.

Kilowatt-hour (kWh)

A unit of work or energy, measured as 1,000 watts (1 kilowatt) of power expended for 1 hour. Once generated, one kWh is equivalent to 3,412 Btu.

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