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.
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 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 or Btu / Hr - British Thermal Units per Hour
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.
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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Frequently Asked Questions (FAQs)
13 June 2015 Kjell Gjære said:
Why write BTUH when meaning BTU/h? I find it confusing. Kjell Gjære said:
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?
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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