Know Your HVAC Terminology

In order to converse with air conditioning experts, it is essential to be familiar with basic terms used in the description of the various types of cooling equipment. The following questions illustrate some of the key terms that are more commonly used – and confused.

1. What is a BTU?
2. What is the difference between the terms EER and SEER?
3. What is a "ton" and how is it used to define air conditioning capacity?
4. What is COP?
5. What is relative humidity?

1. What is a BTU?
A British Thermal Unit (BTU) is defined as the amount of heat required to raise the temperature of one pound of water one degree Fahrenheit. It is approximately the amount of heat that a lit match will generate.

2. What is the Difference Between EER and SEER?
EER (energy efficiency ratio) is a measure of how efficiently a cooling system will operate when the outdoor temperature is at a specific level (usually 95° F). A higher EER means the system is more efficient. You can figure out kW if you know EER and tons.

EER = BTUs of Cooling @ 95° F / Watts used @ 95° F

In the case of a 10 EER, 2 ton air conditioner:

        10 EER = 24,000 BTUs Out / 2,400 Watts In

For the same size unit, but rated at 12 EER:

         12 EER = 24,000 BTUs Out / 2,000 Watts In or 20% more efficient.

If you want to calculate kWh, just multiply the "Watts In" by the number of hours that the air conditioning is running. If  you'd like to convert watts to kWh, simply divide by 1000. 

SEER (seasonal energy efficiency ratio) is a measure of efficiency over an entire cooling season, as opposed to a single outdoor temperature. Residential units are almost always rated in SEER. SEER came into use as a more practical measure, since the temperature outside is not always 95º F. Also, the denominator is in watt-hours, not in watts as is the case for EER. The same relationship holds ... a higher SEER means the system is more efficient. SEER is the total amount of cooling the air conditioner will provide over the entire cooling season divided by the total number of watt-hours it will consume or:

SEER = Seasonal BTUs of cooling / Seasonal watt-hours used

3. What is a "Ton" and How is it Used to Define Air Conditioning Capacity?
A "ton" has come to be defined as the cooling capacity of an air conditioning system. One ton is equal to the BTU's required to melt one ton of ice in a 24 hour period. A one ton air conditioner is rated at 12,000 BTU's, a 2 ton unit at 24,000 BTU's, a 3 ton unit at 36,000 BTU's and so on. It takes 144 BTU's of heat to melt 1 pound of ice in 24 hours, or 288,000 BTU to melt a ton (2,000 pounds) in 24 hours.

Typically residential central heating systems provide from 2 to 5 tons of cooling. Commercial rooftop units are typically 3 to 20 tons each. Chillers can range from 15 tons up to 1,500 tons.

4. What is COP?
COP (coefficient of performance) ratings are more typically found in chiller ratings and on gas cooling equipment, as well as in heat pumps. It is a measure of how efficiently a heating or cooling system will operate at a single outdoor temperature condition. As an example the commonly used outdoor temperature condition for a heat pump calculation in the heating mode is 47º F. As is also the case for EER and SEER, higher COP's mean higher efficiency.

For a heat pump:

COP = BTU of heat produced at 47º F divided by the BTU equivalent of electricity to produce that same amount of heat. The COP rating for a heat pump typically ranges from 3.0 to 5.0.

Where 1 kW = 3,413 BTU/hr

For a chiller:

COP = 3.516 divided by the kW/ton rating of the chiller

Where the kW/ton rating of a chiller is typically in the 0.6 to 1.0kW/ton range. Therefore, the COP rating for an electric chiller will generally fall between 3.0 and 5.0. Gas absorption chillers have COPs from 1.5 to 2.0.

5. What is Relative Humidity?
Relative humidity is a measure of the amount of water in the air compared with the amount of water the air can hold at a particular temperature. Warmer air has more capacity to "hold" water vapor than colder air. Dew point is a measure of how much water vapor is actually in the air.

Take the Next Step
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