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This article gives definitions of BTU or British Thermal Unit, BTUs, BTUH, and related terms.
We explain how to express BTUs in other measurements, and how BTUs are used in describing the operation of heating or air conditioning equipment and their capacities.
We include a table showing how to translate BTUs into other measurements such as raising the temperature of ice or water, calories, joules, and tons of air conditioner capacity or heating system capacity. Sketch courtesy of Carson Dunlop Associates.
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Definition of BTU - British Thermal Unit
A BTU is a measure of heat energy, or the amount of heat given off when a unit of fuel is consumed. One BTU is the amount of heat energy we need to raise the temperature of one pound of water by one degree Fahrenheit. One BTU also is defined as 252 heat calories (this is not the same as food calories).
When talking about air conditioners or heaters, we talk about the A/C unit's BTUh capacity - the number of BTUs of cooling (lowering rather than raising temperature) it can produce in an hour of running.
When we are heating a building BTUs describe heat given off by consuming fuel or energy from some source (electricity, natural gas, LP gas, oil, etc.) of which some portion is delivered to the building occupied space (see AFUE and HSPF).
When we are cooling a building, or when we are describing an air conditioner or heat pump's rated capacity (in BTUs), we are describing the removal of a quantity of heat from the building - or really from the building's air.
Also see DEFINITION of HEATING & COOLING TERMS where we further discuss and define BTUs, Calories, and other energy measures.
Table of BTUs translated into other measurements
(1) How many BTUs are required to convert one pound of water at 212 °F to one pound of steam vapor at 212 °F? This figure is the latent heat of vaporization, the number of BTUs of energy used to raise one pound of water at 212 °F to one pound of steam vapor at the same temperature; in other words, the temperature is unchanged but the state of matter is changed from liquid to vapor. - Refrigeration License Examinations.
See BLEVE explosionsor boiling liquid vapor expansion explosions. We discuss the role of pressure/temperature relief valves in protecting against these hazards
Definition of BTUH - British Thermal Units per Hour
Technical note: Heating equipment such as boilers and furnaces often will show two different BTUH numbers on the heating appliance label. Boilers, steam boilers or furnaces include a data tag that may indicate BTU data as BTUH or MBTUH, referring to thousands of BTUs per hour.
Input BTUH = the energy consumed by the heating appliance measured in thousands of BTUs per hour.
Output BTUH = the heat output from the heating appliance, measured in thousands of BTUs per hour. This is the theoretical maximum heat output that the appliance could deliver to the building. The actual heat delivered into the building will be this amount or less - as there are also losses in the heat distribution system as well.
The input BTUH will always be greater than the output BTUH because the heating appliance will not operate at 100% efficiency. (And for the output BTUH to exceed the input BTUH the heating appliance would have to be operating at greater than 100% efficiency - defying the laws of physics.)
A calorie is defined as the quantity of heat needed to raise the temperature of one gram of water by one degree Centigrade
So what's a calorie? (Definition of Calories)
Definitions for & explanations of latent heat, super heat, latent heat of vaporization, latent heat of condensation, sensible heat & specific heat
Latent heat is defined as the amount of heat absorbed by a substance with no change in a temperature - such as when a substance changes state (from water to steam, for example)
In other words, heat that is absorbed by a substance with no change in temperature is latent heat. For example when a substance changes state (liquid to gas) latent heat is involved.
Definition of Superheat:
The latent heat of vaporization is defined as the number of BTUs to raise one pound of liquid to a pound of vapor (to a varying degree per BTU depending on the type of vapor - this is "superheat"). Our Sketch explaining latent heat of vaporization shown at left is provided courtesy of Carson Dunlop Associates.
The latent heat of condensation is defined as the number of BTUs necessary to change a state back from a vapor to a liquid
The latent heat of solidification is defined as the amount of energy (or number of BTUs) needed to change a liquid to a solid (such as water to ice) while the temperature remains unchanged (at sea level, 32 °F).
Sensible heat is defined as the amount of heat that we can sense or feel or measure.
When an air conditioner system is working, the larger diameter tubing on the low-side of the system combined with the effects of the refrigerant metering device (cap tube or thermostatic expansion valve) results in a reduced pressure on the low side (compared with high side pressure). The reduced pressure causes vaporization of the liquid refrigerant inside the cooling coil, which in turn means that sensible heat is absorbed by the cooling coil.
When the same air conditioner system is working, the smaller diameter tubing on the high side reduces available volume so that (along with the effect of the compressor itself) we increase the pressure and temperature of the refrigerant so that sensible heat can be transferred to ambient outdoor air.
Specific heat is defined as the amount of heat required to raise the temperature of a given substance by one unit of temperature (in our examples by one °F.) Specific heat is also defined as the amount of heat (in calories) to increase the temperature of one gram of a substance by one deg C (Celsius).
In which direction does heat flow: heat energy always flows from the warmer substance to the cooler substance, down to -460 °F where all molecular movement stops.
A neat fact is that the heat flows more rapidly (efficiently) between two substances when there is a greater temperature difference between them. That's why the thermal conductivity of finned copper tubing heating baseboard is exponentially greater at higher degrees of heating water temperature, and that's why we like to run our heating boiler at a higher rather than a lower upper limit temperature.
Definnition of a Joule: BTUs translated into SI units of energy: relationship of Watts, Joules & Newton Meters
Outside of the U.S. in some countries, BTUs as a measure of energy are being replaced with the SI unit of energy, the Joule. (J).
The English have beaten out the Scots by James Prescott Joule (an Englishman) who defined the Joule.
Since there are 3600 seconds in an hour the following formulas equating Watts, Joules, and Newton meters can be written:
We can think of an air conditioner's "efficiency" as expressed either in the total operating cost for a season of use, or you may prefer to just express the air conditioner's efficiency as its operating cost to run the system for one hour.
The equation shown at page top is designed to reduce all of the parameters describing air conditioning efficiency to a single efficiency number, SEER. SEER numbers are useful when we're comparing one air conditioner with another. But suppose we want to know the actual air conditioning cost per season, or air conditioning cost per operating hour to operate our air conditioner?
To translate an air conditioner or heat pumps SEER rating into actual air conditioning operating costs we need to know these measurements - as they allow us to translate BTUs into Watts or other electrical measurements.
One ton of air conditioning capacity produces the same cooling ability as melting one ton of ice in 24 hours. Sketch courtesy of Carson Dunlop Associates.
288,000 BTUs / 24 hours = 1 Ton of cooling
12,000 BTUs / hour = a 1-ton air conditioning system
A one-ton air conditioner claims to remove 12,000 BTUs of heat from the building air per hour of operation.
Or if we know the total number of BTUs at which an air conditioning system is rated, since this number is usually given in BTUH or BTUs / hour, we just divide that number by 12,000 to get the number of tons of cooling capacity.
A 36,000 BTUh air conditioner is providing 36,000 / 12,000 or 3 Tons of cooling capability per hour.
If we know the number of tons of cooling capacity that an air conditioning system is rated for, we just multiply the number of air conditioning capacity in Tons by 12,000 to get the number of BTUs of cooling capacity of the system.
To assist in choosing the right sized air conditioner, we provide a typical air conditioner chart at AIR CONDITIONER BTU CHART.
Watch out: more is not always better. Don't buy an air conditioner that is too big: if you install a system that is too powerful (too many tons of cooling capacity) the building will be less comfortable than if you install a properly-sized air conditioner. Too many tons of air conditioning mean the system will shut off on short cycles and won't run long enough to reduce the indoor humidity to a comfortable level.
Details are at DEHUMIDIFICATION PROBLEMS or select a topic from the More Reading links shown below.
Continue reading at DEFINITION of HEATING & COOLING TERMS or select a topic from the More Reading links shown below.
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