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This article gives definitions of BTU or British Thermal Unit, BTUs, BTUH, and related terms.
Discussed here: Definitions of BTU & BTUH: What is a BTU or British Thermal Unit? Definition of BTU  British Thermal Units per Hour. Table of BTUs translated into other measurements.
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, joules, and tons of air conditioner capacity or heating system capacity. Sketch courtesy of Carson Dunlop Associates.
We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.
Definition of BTUH  British Thermal Units per Hour: 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).
One calorie is defined as the quantity of heat energy needed to raise the temperature of one gram of water by one degree Centigrade
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 Unts.
A BTUH is also defined as the number of BTU's lost (if we're talking about heat loss or air conditioning), or provided (if we're talking about providing heat for a building) in one hour. You'll often see BTUH as a number on data plates on air conditioners and on heating systems.
Also see DEFINTION of JOULE for details about BTUs and various examples of BTU and BTUh calculations.
At DEFINITION of HEATING, COOLING & INSULATION TERMS we give definitions of related terms such as latent heat, superheat, latent heat of condensation, sensible heat, and specific heat.
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 EXPLOSIONS or 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.
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 closelyrelated articles below, or see our complete INDEX to RELATED ARTICLES below.
See COOLING ZONE BTU REQUIREMENTS where we give cooling BTUs per square foot for climate zones and also illustrate and explain CLIMATE ZONES for the U.S
See HEATING ZONE BTU REQUIREMENTS for heating BTU requirements by climate zone.
Also see DEGREE DAY HEATING DEGREE DAYS or COOLING DEGREE DAYS for cooling BTU requirements by climate zone.
Also see GAS BTUH, CUBIC FEET & ENERGY  One gallon of propane contains about 91,500 BTUs and One cubic foot (0.028 cubic meters) of natural gas contains about 1,050 BTUs
Also see HEATING OIL TYPES & PROPERTIES for the BTUs in heating oil  One gallon of No. 2 home heating oil will provide about 138,500 BTUs per gallon.
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Continue reading at TONS of COOLING CAPACITYor select a topic from closelyrelated articles below, or see our complete INDEX to RELATED ARTICLES below.
Or see AIR FLOW MEASUREMENT CFM
Or see BTU MONITORS & HEATING COST APPORTIONMENT
Or see COOLING CAPACITY and also COOLING LOAD
Or see CONVERT BTUs to TONS of COOLING CAPACITY
Or see HEAT LOSS RATE and also perfect or STOICHIOMETRIC COMBUSTION
Or see PASCAL CALCULATIONS
Or see DEFINITION of HEATING, COOLING & INSULATION TERMS  home
BTUs DEFINITIONS OF at InspectApedia.com  online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.
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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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