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, a Toronto home inspection, education & report writing tool company [ carsondunlop.com ].

Definition of BTU - British Thermal Unit

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.

Notes to the BTU table:

(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.

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.

Definition of BTUH or Btu / Hr - British Thermal Units per Hour

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 above-left.

How do We Translate BTUs to Tons of Air Conditioning or Cooling Capacity?

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, a Toronto home inspection, education & report writing tool company [ carsondunlop.com ].

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.

A 3-ton 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 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 closely-related articles below, or see the complete ARTICLE INDEX.

Typical Cooling BTUh requirements

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

Typical Heating BTUh requirements

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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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

Question:

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.

Reply:

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 re-editing 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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Continue reading at TONS of COOLING CAPACITYor select a topic from closely-related articles below, or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• AIR FLOW MEASUREMENT CFM
• BTU MONITORS & HEATING COST APPORTIONMENT
• COOLING CAPACITY and also COOLING LOAD
• CONVERT BTUs to TONS of COOLING CAPACITY
• DEFINITION of HEATING, COOLING & INSULATION TERMS - home
• HEAT LOSS RATE and also perfect or STOICHIOMETRIC COMBUSTION
• PASCAL CALCULATIONS

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BTUs DEFINITIONS OF at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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