British thermal unit explained

The British thermal unit (Btu) is a measure of heat, which is a form of energy. It was originally defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. It is also part of the United States customary units.^[1] The SI unit for energy is the joule (J); one Btu equals about 1,055 J (varying within the range of 1,054–1,060 J depending on the specific definition of BTU; see below).

While units of heat are often supplanted by energy units in scientific work, they are still used in some fields. For example, in the United States the price of natural gas is quoted in dollars per the amount of natural gas that would give 1 million Btu (1 "MMBtu") of heat energy if burned.^[2]

Definitions

A Btu was originally defined as the amount of heat required to raise the temperature of one pound of liquid water by one degree Fahrenheit at a constant pressure of one atmospheric unit.^[3] There are several different definitions of the Btu that differ slightly. This reflects the fact that the temperature change of a mass of water due to the addition of a specific amount of heat (calculated in energy units, usually joules) depends slightly upon the water's initial temperature. As seen in the table below, definitions of the Btu based on different water temperatures vary by up to 0.5%.

Variant	Energy (J)	Notes
Thermochemical	≈1,054.35	Originally, the thermochemical Btu was defined as the heat required to raise the temperature of one pound of water from its freezing point to its boiling point, divided by 180 (the temperature change being 180 °F). The basis for its modern definition in terms of SI units is the conceptually similar thermochemical calorie, originally defined as the heat required to raise the temperature of one gram of water from freezing to boiling divided by 100 (the temperature change being 100 °C). The thermochemical calorie is exactly 4.184 J by definition of the International Organization for Standardization (ISO). The thermochemical Btu is calculated by converting from grams to pounds and from Celsius to Fahrenheit.^[4]
59°F	≈1,054.80^[5]	Used for American natural gas pricing.^[6]
60°F	≈1,054.68^[7]	Mainly Canadian.
39°F	≈1,059.67	Uses the calorie value of water at its maximum density (4°C).
IT	≈1,055.06	An early effort to define heat units directly in terms of energy units, and hence to remove the direct association with the properties of water, was made by the International Steam Table Conferences. These conferences originally adopted the simplified definition that 860 "IT" calories corresponded to exactly 1 international watt-hour (not the same as a modern watt-hour). This definition ultimately became the statement that 1 IT calorie is exactly 4.1868 J.^[8] The Btu is then calculated from the calorie as is done for the thermochemical definitions of the BTU and the calorie, as in International standard ISO 31-4 Quantities and units—Part 4: Heat and British Standard BS 350:Part 1:1974 Conversion factors and tables.^[9] ^[10]

Prefixes

Units of kBtu are used in building energy use tracking and heating system sizing. Energy Use Index (EUI) represents kBtu per square foot of conditioned floor area. "k" stands for 1,000.^[11]

The unit Mbtu is used in natural gas and other industries to indicate 1,000 Btu.^[12] However, there is an ambiguity in that the metric system (SI) uses the prefix "M" to indicate 'Mega-', one million (1,000,000). Even so, "MMbtu" is often used to indicate one million Btu particularly in the oil and gas industry.^[13]

Energy analysts accustomed to the metric "k" ('kilo-') for 1,000 are more likely to use MBtu to represent one million, especially in documents where M represents one million in other energy or cost units, such as MW, MWh and $.^[14]

The unit 'therm' is used to represent 100,000 Btu.^[15] A decatherm is 10 therms or one MMBtu (million Btu). The unit quad is commonly used to represent one quadrillion (10¹⁵) Btu.^[13]

Conversions

One Btu is approximately:

• (kilojoules)
• (watt hours)
• (calories)
• (kilocalories)
• 25,031 to 25,160 ft⋅pdl (foot-poundal)
• (foot-pounds-force)
• 5.40395 (lbf/in²)⋅ft³

A Btu can be approximated as the heat produced by burning a single wooden kitchen match or as the amount of energy it takes to lift a 1lb weight 778feet.^[16]

For natural gas

See main article: Natural gas prices.

• In natural gas pricing, the Canadian definition is that ≡ .^[17]
• The energy content (high or low heating value) of a volume of natural gas varies with the composition of the natural gas, which means there is no universal conversion factor for energy to volume. 1ft3 of average natural gas yields ≈ 1,030 Btu (between 1,010 Btu and 1,070 Btu, depending on quality, when burned)
• As a coarse approximation, 1000ft3 of natural gas yields ≈ ≈ .
• For natural gas price conversion ≈ 36.9 million Btu and ≈

BTU/h

The SI unit of power for heating and cooling systems is the watt. Btu per hour (Btu/h) is sometimes used in North America and the United Kingdom - the latter for air conditioning mainly, though "Btu/h" is sometimes abbreviated to just "Btu".^[18] MBH—thousands of Btu per hour—is also common.^[19]

• 1 W is approximately 1disp=outNaNdisp=out^[20]
• 1,000 Btu/h is approximately 1000disp=outNaNdisp=out
• 1 hp is approximately 1disp=outNaNdisp=out

Associated units

• 1 ton of cooling, a common unit in North American refrigeration and air conditioning applications, is 120002NaN2. It is the rate of heat transfer needed to freeze 1ST of water into ice in 24 hours.
• In the United States and Canada, the R-value that describes the performance of thermal insulation is typically quoted in square foot degree Fahrenheit hours per British thermal unit (ft²⋅°F⋅h/Btu). For one square foot of the insulation, one Btu per hour of heat flows across the insulator for each degree of temperature difference across it.
• 1 therm is defined in the United States and European Union as 100,000 Btu—but the U.S. uses the while the EU uses the BTU_IT. United Kingdom regulations were amended to replace therms with joules with effect from 1 January 2000.^[21] the therm is still used in natural gas pricing in the United Kingdom.^[22]
• 1 quad (short for quadrillion Btu) is 10¹⁵ Btu, which is about 1 exajoule . Quads are used in the United States for representing the annual energy consumption of large economies: for example, the U.S. economy used 99.75 quads in 2005.^[23] One quad/year is about 33.43 gigawatts.

The Btu should not be confused with the Board of Trade Unit (BTU), an obsolete UK synonym for kilowatt hour (1kW.h).

The Btu is often used to express the conversion-efficiency of heat into electrical energy in power plants. Figures are quoted in terms of the quantity of heat in Btu required to generate 1 kW⋅h of electrical energy. A typical coal-fired power plant works at 10500Btu/kWh, an efficiency of 32–33%.^[24]

The centigrade heat unit (CHU) is the amount of heat required to raise the temperature of one pound of water by one Celsius degree. It is equal to 1.8 Btu or 1,899 joules.^[25] In 1974, this unit was "still sometimes used" in the United Kingdom as an alternative to Btu.^[26]

Another legacy unit for energy in the metric system is the calorie, which is defined as the amount of heat required to raise the temperature of one gram of water by one degree Celsius.^[27]

See also

• Conversion of units
• Latent heat
• Metrication
• Ton of refrigeration

External links

• Web site: The Units of Measurement Regulations 1995 . 13 July 1995 . www.legislation.gov.uk . . 23 February 2018 .
• Web site: Natural Gas: A Primer . 27 November 2015 . www.nrcan.gc.ca . . 23 February 2018 . https://web.archive.org/web/20180224052827/http://www.nrcan.gc.ca/energy/natural-gas/5641?destination=node%2F1233 . 24 February 2018. dead .

Notes and References

1. In a short note, Woledge notes that the actual technical term "British thermal unit" apparently originated in the United States, and was subsequently adopted in Great Britain. See History of the British Thermal chicken (bhuUnit. G.. Woledge. Nature. 149. 30 May 1942. 10.1038/149613c0. 613. 1942Natur.149..613W. 149. 4104904. free.
2. Web site: Henry Hub Natural Gas Spot Price. U.S. Energy Information Administration. live. https://web.archive.org/web/20170801191040/https://www.eia.gov/dnav/ng/hist/rngwhhdm.htm. 1 August 2017.
3. Book: Introduction to Chemical Engineering Thermodynamics . 6 . Smith . J. M. . Van Ness . H. C. . Abbott . M. M. . B. I. Bhatt (adaptation) . 0-07-049486-X . Tata McGraw-Hill Education . 2003 . 15 .
4. The pound is 453.59237 grams by definition; see Web site: Appendix C of NIST Handbook 44, Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices, General Tables of Units of Measurement . United States National Bureau of Standards . C-12 . dead . https://web.archive.org/web/20061126120208/http://ts.nist.gov/WeightsAndMeasures/Publications/upload/h4402_appenc.pdf . 26 November 2006. One degree Fahrenheit is exactly of a degree Celsius by definition.
5. Web site: Guide for the Use of the International System of Units (SI) 2008 Edition . Ambler . Thompson . Barry N. . Taylor . National Institute of Standards and Technology (NIST) . 58 . live . https://web.archive.org/web/20160603203340/http://physics.nist.gov/cuu/pdf/sp811.pdf . 3 June 2016. NIST Special Publication 811.
6. The Btu used in American natural gas pricing is "the amount of heat required to raise the temperature of 1 avoirdupois pound of pure water from 58.5to at a constant pressure of 14.73 pounds per square inch." See Web site: NYMex Rulebook. Chapter 220: Henry Hub Natural Gas Futures. New York Mercantile Exchange (NYMex). 6 January 2017. live. https://web.archive.org/web/20161110004509/http://www.cmegroup.com/rulebook/NYMEX/2/220.pdf. 10 November 2016. dmy-all.
7. Book: Sørensen , Bent . Renewable Energy Focus e-Mega Handbook . Academic Press . 2008 . 9780123747068 . 5 .
8. Book: Koch , Werner . VDI Steam Tables . Springer . 4 . 2013 . 8 . 9783642529412 . Published under the auspices of the German: Verein Deutscher Ingenieure (VDI).
9. Book: Cardarelli, Francois . Scientific Unit Conversion: A Practical Guide to Metrication . M.J. Shields (translation) . 2 . Springer . 2012 . 19 . 9781447108054 .
10. Book: BS 350:Part 1:1974 Conversion factors and tables, Part 1. Basis of tables. Conversion factors . 1974 . British Standards Institution . 59.
11. Web site: VOLUNTARY BUILDING ENERGY PERFORMANCE SCORE SYSTEMS. Oregon Secretary of State Administrative Rules.
12. Book: Price, Gary D. . Power Systems and Renewable Energy: Design, Operation, and Systems Analysis . Momentum Press . 2014 . 98 . 9781606505717 .
13. Web site: Energy Units . . 26 December 2016 . live . https://web.archive.org/web/20161231123621/http://www.aps.org/policy/reports/popa-reports/energy/units.cfm . 31 December 2016.
14. Cook, Warren C (2018): https://betterbuildingssolutioncenter.energy.gov/sites/default/files/attachments/Home%20Energy%20Score%20Report%20Example.pdf https://www.oregon.gov/energy/Data-and-Reports/Documents/BER-Chapter-1-Energy-Numbers.pdf
15. Web site: What are Mcf, Btu, and therms? How do I convert prices in Mcf to Btus and therms?. U.S. Energy Information Administration. 6 April 2016. 30 December 2016. live. https://web.archive.org/web/20161225220005/http://www.eia.gov/tools/faqs/faq.cfm?id=45&t=8. 25 December 2016.
16. Book: Ristinen. Robert A. . Kraushaar. Jack J. . Energy and the Environment. registration. 2006. John Wiley & Sons. 978-0-471-73989-0. 13–14.
17. Web site: Energy Measurements . 7 January 2017 . Government of Alberta Province . dead . https://web.archive.org/web/20170120191331/http://www.energy.gov.ab.ca/About_Us/1132.asp . 20 January 2017 . dmy-all .
18. Book: Masonry Heaters: Designing, Building, and Living with a Piece of the Sun. Ken Matesz. 148. Chelsea Green Publishing. 2010.
19. Book: Arimes, Tom . 32314774 . HVAC and chemical resistance handbook for the engineer and architect : a compilation . BCT . 1994 . 0-9640967-0-6 . Lexington, Ky. . 11-12 .
20. Web site: 2009 ASHRAE Handbook – Fundamentals (I-P Edition). live. https://web.archive.org/web/20151017172515/http://app.knovel.com/web/toc.v/cid:kpASHRAE22/viewerType:toc/root_slug:ashrae-handbook-fundamentals/url_slug:ashrae-handbook-fundamentals/. 17 October 2015. 21 September 2015. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.
21. Web site: The Units of Measurement Regulations 1995. 7 November 2019.
22. Web site: The GB gas wholesale market. Office of Gas and Electricity Markets. The wholesale gas market in Britain has one price for gas irrespective of where the gas comes from. This is called the National Balancing Point (NBP) price of gas and is usually quoted in price per therm of gas.. 13 January 2013. live. https://web.archive.org/web/20130930030713/https://www.ofgem.gov.uk/gas/wholesale-market/gb-gas-wholesale-market. 30 September 2013.
23. Book: Husher . John Durbin . Crises of the 21st Century: Start Drilling-The Year 2020 Is Coming Fast . iUniverse . 2009 . 376 . 610004375 . 9781440140549. .
24. Web site: Electric Generation Efficiency . National Petroleum Council (NPC) . David K. . Bellman . 18 July 2007 . dead . https://web.archive.org/web/20081120094040/http://www.npc.org/Study_Topic_Papers/4-DTG-ElectricEfficiency.pdf . 20 November 2008 . 30 March 2012 . Working Document of the NPC Global Oil & Gas Study.
25. Web site: Centigrade Heat Unit . 10 February 2017 . https://web.archive.org/web/20161225090911/http://www.diracdelta.co.uk/science/source/c/e/centigrade%20heat%20unit/source.html . 25 December 2016 . dead .
26. Book: BS 350: Part 1:1974 - Conversion factors and tables; Part 1. Basis of tables, Conversion factors . 1974 . British Standards Institution . 59.
27. Does the history of food energy units suggest a solution to 'Calorie confusion'?. Hargrove. James L.. 2007. Nutrition Journal. 10.1186/1475-2891-6-44. 6. 44. 2238749. 18086303 . free .