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Explanation of a ton of cooling capacity (C) Carson Dunlop AssociatesDefinitions of BTUs & Related Terms
BTU, BTUH & how to 5translate BTUs into other measures.

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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 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.

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 1oF

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 1oC.

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.

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.

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

Boiler data tag (C) Daniel FriedmanBased 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?

Explanation of a ton of cooling capacity (C) Carson Dunlop AssociatesOne 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.

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 closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

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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Continue reading at TONS of COOLING CAPACITYor select a topic from closely-related 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

Suggested citation for this web page

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

INDEX to RELATED ARTICLES: ARTICLE INDEX to BUILDING ENERGY SAVINGS

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