What is a BTU or British Thermal Unit?

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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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.
Terminology note: in these articles we use BTUs and BTUS as synonyms in which the "s" denotes the plural of the term or British Thermal Units.
Also see DEFINITION of HEATING, COOLING & INSULATION TERMS where we further discuss and define BTUs, Calories, and other energy measures.
Table of British Thermal Units BTU's Translated into Other Measurements  
1 BTU =  One BTU = the amount of energy needed to raise 1 pound of water by one degree Fahrenheit or 1^{o}F One BTU is equal to 252 calories, so by the definition of calorie, 1 BTU will raise 252 grams of water by one degree Centigrade or 1^{o}C. One BTU is also described by some as about the amount of energy given by burning one wooden kitchen match. 
1/2 BTU =  the amount of energy to raise one pound of ice by one deg Fahrenheit. 
16 BTUs =  the amount of energy to raise 1 pound of ice from 0 °F to 32 deg F as ice 
144 BTUs =  the amount of energy to raise 1 pound of ice at 32 deg F to 1 pound of water at 32 deg F 
180 BTUs =  the amount of energy to raise 1 pound of water at 32 deg F to 1 pound of water at 212 deg F. 
970 BTUs =  the amount of energy to raise 1 pound of water at 212 deg F to 1 pound of steam vapor at 212 deg F (1) 
12,000 BTUH =  one ton of heating or cooling capacity per hour 
NOTE: you can see by these entries that a state change, from ice to water or from water to steam vapor requires much more energy than simply raising a material in temperature by one °F. Whether we are adding heat or removing heat, these BTU amounts are the same: it doesn't matter which direction we're going: heating up or cooling down. 
(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
at RELIEF VALVE, TP VALVE, BOILER
and
at RELIEF VALVE, WATER HEATER.
Based on the definition of BTUs above, BTUH describes the number of BTUs of energy produced (as heat) or removed (by air conditioning) in one hour.
One BTU is also equal to 252 calories.
[Click to enlarge any image]
Technical note: HVAC quipment such as boilers and furnaces often will show one or sometimes two different BTU capacity numbers on the heating or cooling appliance label:
Input BTUH = the energy consumed by the cooling or heating appliance measured in thousands of BTUs per hour.
Output BTUH = the cooling capacity or the heat output from the cooling or heating appliance, measured in thousands of BTUs per hour may be written also as MBTUH. This is the theoretical maximum cooling capacity or maximum heat output that the appliance could deliver to the building. The actual cooling capacity or heat delivered into the building will be this amount or less  as there are also losses in the cooling or heating 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.)
Terminology note: Synonyms used on heating or cooling appliance data tags will include BTUH or BTUS/hour or Btu / Hr. You'll see an example of BTU / Hr in the data tag shown at aboveleft.
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)
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.
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 lowside 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).
The specific heat of water is defined as a constant and = 1
The specific heat of ice is is 5
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.
Also see DEFINITION of HEATING, COOLING & INSULATION TERMS
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:
1 Watt second (Ws) = 1 joule (J) = 1 newton meter 1 Watt hour (Wh) = 3600 Joules
1 kilowatt hour = 3.6 x 10^{6} Joules, since there are 1000 watts in a kilowatt.
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.
See Definition of SEER RATINGS
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 1ton air conditioning system
A oneton 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.
A 3ton air conditioner is providing 3 x 12,0000 or 36,000 BTUs of cooling capability per hour.
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 properlysized 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 or topic ARTICLE INDEX shown below.
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Continue reading at DEFINITION of HEATING, COOLING & INSULATION TERMS or select a topic from the More Reading links or topic ARTICLE INDEX shown below.
Or see BTU MONITORS & HEATING COST APPORTIONMENT
Or see COOLING CAPACITY and also COOLING LOAD
Or see HEAT LOSS RATE and also perfect or STOICHIOMETRIC COMBUSTION
Or see PASCAL CALCULATIONS
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13 June 2015 Kjell Gjære said:
You write about BTU and BTUH . I'm sorry, I do not understand.
Normally we differ between kW/h kilowatt per hour, and kWh kilowatthour. Here you say BTUH is equal to BTU per hour? BTUs do not seem to be BTU per second?
Why write BTUH when meaning BTU/h? I find it confusing. Kjell Gjære said:
You write about BTU and BTUH . I'm sorry, I do not understand.
Normally we differ between kW/h kilowatt per hour, and kWh kilowatthour. Here you say BTUH is equal to BTU per hour? BTUs do not seem to be BTU per second?
Why write BTUH when meaning BTU/h? I find it confusing.
Thanks for the question Kjell. Because of their firing rates and applications, the fuel consumption rate and heating range for heating equipment or cooling equipment are quoted in input BTUH  typically thousands of btus per hour; some data tags may also give output BTUH. If you want to convert from BTUH to BTUs/second of course you can but you won't find that in building HVAC discussions.
We deliberately use synonyums BTUH, BTUh, Btu / Hr, and BTUs per hour in this article series to improve the chances that a reader who searches on those various terms will find the information she or he seeks.
14 June 2016 Kjell Gjære wrote:
From my perspective "BTU per hour" can only be abbrevated to BTU/h. The "/" is indicating a division, or "per". You also write: "the fuel consumptin rate" i.e. energy per time = say Joule/s or BTU/h, not BTUH = energy multiplied with time?
I find the "unit" BTUH inconsistent and confusing.
Now, if BTUH means BTU per hour, then BTUs seems to mean BTU per second. When using defined symbols for unit, s means seconds, not plural of BTU?
I'm confused.
Reply:
Thank you for your comments Kjell. As you find BTUH and BTUh and BTUs/h confusing we'll consider that in reediting the article. Many readers understand that / in most languages can mean "per" while it might also mean "division by" which is also indicated by the obelus (÷) that has been used as a division symbol world wide since first proposed by Johann Rahn in 1659 in Teutsche Algebra (Cajori vol. 2, page 211).
Using the term BTU (singular) or BTUs (plural) doesn't have much useful meaning in heating or cooling applications for buildings if we don't describe the time period over which the BTU energy consumption is used or the rate (exressed in BTUs) over time. Generally people use BTUH or BTUs per hour or Btus / Hr for that discussion.
Since we are interested in the BTUs consumed (input BTUs) or delivered in heating or cooling capacity (output BTUs) over time and since we usually use hours as the time frame, a number such as 30,000 Input BTUH can be taken to mean 30,000 BTUS of energy input IN ONE HOUR or PER HOUR or 30,000 BTUS / hour where "hour" is set to 1. So you can if you find it more clear, think of "30,000 BTUH" as the same as "30,000 BTUS / 1 hour" as the same as "30,000 BTUS / hour"
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