Hubbry Logo
British thermal unitBritish thermal unitMain
Open search
British thermal unit
Community hub
British thermal unit
logo
8 pages, 0 posts
0 subscribers
Be the first to start a discussion here.
Be the first to start a discussion here.
British thermal unit
British thermal unit
British thermal unit
View on Wikipedia
from Wikipedia

British thermal unit
Graph of total energy consumption in the US (divided into energy sources) in quadrillion Btu
General information
Unit systemimperial/US units
Unit ofEnergy (heat)
SymbolBtu
Conversions
1 Btu in ...... is equal to ...
   SI units   ≈ 1055 J

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][3]

Definitions

[edit]

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.[4] 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[a] 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).[4] The thermochemical Btu is calculated by converting from grams to pounds and from Celsius to Fahrenheit.[5]
59 °F (15.0 °C) ≈1,054.80[6] Used for American natural gas pricing.[3]
60 °F (15.6 °C) ≈1,054.68[7] Mainly Canadian.[citation needed]
39 °F (3.9 °C) ≈1,059.67[7] Uses the calorie value of water at its maximum density (4 °C, 39.2 °F).[citation needed]
IT ≈1,055.06[b] 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.[4][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.[5][9][10]

Prefixes

[edit]

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][13] 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.[14]

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 $.[15]

The unit 'therm' is used to represent 100,000 Btu.[12] A decatherm is 10 therms or one million Btu. The unit quad is commonly used to represent one quadrillion (1015) Btu.[14]

Conversions

[edit]

One Btu is approximately:

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 one-pound (0.45 kg) weight 778 feet (237 m).[16]

Btu conversion factors

[edit]

For purposes of site energy[clarify], the following conversion factors apply:[17]

  • Electricity: 3,412 Btu/kilowatthour
  • Natural gas: 1,031 Btu/cubic foot
  • Fuel oil No. 1: 135,000 Btu/gallon
  • Kerosene: 135,000 Btu/gallon
  • Fuel oil No. 2: 138,690 Btu/gallon
  • LPG (propane): 91,330 Btu/gallon
  • Wood: 20 million Btu/cord

For natural gas

[edit]
Main article: Natural gas prices
  • In natural gas pricing, the Canadian definition is that 1,000,000 Btu ≡ 1.054615 GJ.[18]
  • 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. 1 cubic foot (28 litres) 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, 1,000 cubic feet (28 m3) of natural gas yields ≈ 1,000,000 Btu ≈ 1 GJ.
  • For natural gas price conversion 1,000 m3 ≈ 36.9 million Btu and 1,000,000 Btu ≈ 27.1 m3

Btu/h

[edit]

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".[19] MBH—thousands of Btu per hour—is also common.[20]

  • 1 W is approximately 3.412142 Btu/h[21]
  • 1,000 Btu/h is approximately 0.2931 kW
  • 1 hp is approximately 2,544 Btu/h

Associated units

[edit]
  • 1 ton of cooling, a common unit in North American refrigeration and air conditioning applications, is 12,000 Btu/h (3.52 kW). It is the rate of heat transfer needed to freeze 1 short ton (907 kg) 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 as 100,000 Btu using the Btu59 °F definition. In the EU it was listed in 1979 with the BTUIT definition and planned to be discarded as a legal unit of trade by 1994.[22] United Kingdom regulations were amended to replace therms with joules with effect from 1 January 2000.[23] As of 2013 the therm was still used in natural gas pricing in the United Kingdom.[24]
  • 1 quad (short for quadrillion Btu) is 1015 Btu, which is about 1 exajoule (1.055×1018 J). 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.[25] 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 (1 kW⋅h or 3,412 Btu).

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 10,500 Btu/kWh (3.1 kWh/kWh), an efficiency of 32–33%.[26]

The centigrade heat unit (CHU) is the amount of heat required to raise the temperature of one pound (0.45 kg) of water by one Celsius degree. It is equal to 1.8 Btu or 1,899 joules.[27] In 1974, this unit was "still sometimes used" in the United Kingdom as an alternative to Btu.[28]

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.[29]

See also

[edit]

Notes

[edit]

References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

British thermal unit

View on Grokipedia
from Grokipedia
The British thermal unit (BTU or Btu) is a unit of energy traditionally used in , particularly in the United States, defined as the amount of heat required to raise the temperature of one avoirdupois pound (approximately 453.6 grams) of liquid by one degree at the temperature at which water has its greatest (approximately 39 °F or ) and at a specified . This definition stems from early 19th-century efforts to quantify the heating value of fuels, making it a key measure in industries like power generation, steam systems, and (HVAC). Several variations of the BTU exist due to differences in the reference temperature for the water's , leading to slight discrepancies in value. The most common modern standard is the international table BTU (IT BTU), equivalent to exactly 1 055.056 joules (J). Other variants include the thermochemical BTU (1,054.35 J), the 59°F BTU (1,054.80 J), and the mean BTU, which averages the over the range from 32°F to 212°F and equals 1,055.87 J. In practice, the IT BTU is the one adopted for most international standards and energy calculations, while the mean BTU appears in some historical or specific U.S. contexts. The BTU remains integral to and industry, especially for expressing the heat content of fossil fuels, where, for example, one of typically contains around 1,000–1,050 BTUs. It also rates the output of appliances, such as furnaces in BTU per hour (BTU/h), and features prominently in U.S. energy statistics from agencies like the , which report national consumption in quadrillion BTUs (quads). Although the (SI) joule has largely supplanted it globally, the BTU persists in North American HVAC design, fuel pricing, and building codes for its practical familiarity in imperial measurements.

Fundamentals

Definition

The British thermal unit (BTU or Btu) is a unit of energy defined in the imperial and customary systems as the quantity of required to raise the temperature of one pound () of liquid by one degree Fahrenheit at a constant of one atmosphere and a specified . Alternatively, the mean BTU can be expressed as one-one hundred eightieth of the total needed to raise one pound of from 32°F to 212°F at standard . In thermodynamics, the BTU specifically measures thermal energy in the form of , which involves the transfer of due to differences, distinguishing it from mechanical work units like the foot-pound that quantify from applied over . Although both represent , the BTU's focus on makes it suitable for contexts involving changes in substances, particularly as the reference medium. As an imperial unit, the BTU is primarily used in the United States and, to a lesser extent, the for heating, ventilation, air conditioning systems, and assessing the energy content of fuels. The "British" in its name reflects its origins in the Anglo-Saxon traditions of the , despite its predominant modern application in American industries.

Variations

The variations in the definition of the British thermal unit (BTU) stem from differences in the specific heat capacity of water, which is not constant but varies slightly with temperature and pressure conditions, leading to distinct energy values for raising one pound of water by one degree Fahrenheit at different reference points. These discrepancies arose historically as engineers and scientists refined measurements based on experimental data for specific applications, such as steam tables or chemical processes. The International Table BTU (IT BTU), established through international agreements in the , is defined as exactly 1055.05585262 joules and serves as a standardized value for general and scientific calculations. In contrast, the thermochemical BTU, valued at 1054.350 joules, derives from the thermochemical and is primarily employed in contexts where precise thermodynamic reactions are analyzed. The mean BTU represents an average value over the temperature range from 32°F to 212°F, equating to approximately 1055.87 joules, and was used in early tables to approximate across and freezing points. Other variants include the 59°F BTU, defined at 1054.80 joules, which appears in older references for room-temperature applications like HVAC design, reflecting the specific heat at that ambient condition. In modern usage, the IT BTU is preferred as the standard by the National Institute of Standards and Technology (NIST) for consistency with international metric conversions and contemporary energy measurements.

Historical Development

Origins

The conceptual origins of the British thermal unit (BTU) lie in the 1840s experiments on the mechanical equivalent of conducted by and contemporaries, which demonstrated the convertibility of mechanical work into and laid the groundwork for quantifying as energy in engineering contexts. The earliest known definition of a unit akin to the BTU was provided by British engineer Thomas Tredgold in 1824, as the heat required to raise the temperature of one cubic foot of water by one degree Fahrenheit. The term "British thermal unit" appeared in 1859 in the work of Scottish engineer William John Macquorn Rankine, A Manual of the Steam Engine and Other Prime Movers, where he defined it as the quantity of heat required to raise the temperature of one pound of water by one degree Fahrenheit for use in steam engine efficiency calculations. The BTU initially found application in British for evaluating coal combustion and steam power generation, reflecting the era's focus on industrial machinery, well before its broader uptake in American practices. Early implementations revealed ambiguities in the definition, notably the absence of a precise reference for the , which caused variations across 19th-century engineering tables and computations.

Standardization

In the early , efforts to standardize the British thermal unit (BTU) focused on selecting a specific reference to minimize variations in of . The American Society of Refrigerating Engineers (ASRE) adopted the 59°F BTU in 1929 for and cooling calculations, defining it as the heat required to raise one pound of from 59°F to 60°F at standard . This choice aligned with practical needs in temperature-controlled environments. The merger of ASRE and the American Society of Heating and Air-Conditioning Engineers (ASHAE) to form the American Society of Heating, Refrigerating and Air-Conditioning Engineers () in 1959 accelerated unification. In 1961, ASHRAE incorporated the International Table BTU (IT BTU) into its data books and standards, defining it as the mean heat to raise one pound of by one degree over the range from 32°F to 212°F, equivalent to the energy for a 1°F rise at 39.37°F (4°C). The (ISO) supported this in 1978 through ISO 31-4, which standardized related thermal units and conversions, promoting the IT BTU for international consistency in engineering tables. The National Institute of Standards and Technology (NIST), formerly the National Bureau of Standards (NBS), played a pivotal role in calibrating and fixing the IT BTU value during the 1940s and 1960s. Through precise thermochemical measurements and steam table validations, NBS established the IT BTU as exactly 1055.05585262 joules, providing a definitive for calibrations in and HVAC applications. During the U.S. debates, initiated by the of 1975 and overseen by the U.S. Metric Board, the BTU resisted full replacement by SI units like the joule due to its deep integration in the , HVAC, and building sectors. Despite recommendations for SI adoption, industries lobbied for BTU retention, citing cost and familiarity. NIST refined the BTU-to-joule conversion in the 1990s, confirming the exact factor in Special Publication 811 (1995 edition) to support hybrid metric-imperial workflows. Post-2000 updates have been minimal, with no redefinition of the BTU, but the SI revision reaffirmed legacy units' role via fixed exact conversions in the SI Brochure, ensuring ongoing compatibility without altering values. NIST continues digital efforts, including updated references for computational tools in the . Standardization faced challenges from regional preferences pre-1980s, such as the UK's reliance on the mean BTU from imperial steam tables (approximately 1055.06 J), differing slightly from the U.S. IT BTU, which complicated transatlantic and collaborations until ISO reduced discrepancies.

Conversions and Equivalents

To SI Units

The International Table British thermal unit (BTU), the most commonly used variant, is defined as exactly equal to 1055.05585262 joules (J). This exact value stems from international agreements on energy unit conversions to facilitate precise scientific and calculations. The general conversion equation for energy from BTU to joules is: EJ=EBTU×1055.05585262E_\text{J} = E_\text{BTU} \times 1055.05585262 where EJE_\text{J} is the in joules and EBTUE_\text{BTU} is the in BTU. For the thermochemical BTU, which is based on the thermochemical definition, the conversion factor is 1 thermochemical BTU = 1054.3503 J. Common equivalents between BTU and other units are summarized in the following table, using the International Table BTU for consistency:
BTUJoules (J)Calories (cal, international)Watt-hours (W·h)
11055.05585262 (exact)≈ 251.9958≈ 0.293071
These approximations for calories and watt-hours derive directly from the primary joule conversion, with 1 international calorie = 4.1868 J exactly and 1 watt-hour = 3600 J. BTU often employs prefixes for scaling in large-scale applications, such as thousand BTU (kBtu = 1000 BTU = 1.05505585262 megajoules, or MJ) and million BTU (MBtu = 1,000,000 BTU = 1.05505585262 gigajoules, or GJ). These scaled units maintain the same base conversion factor, enabling straightforward multiplication by 10^3 or 10^6, respectively. For power conversions, which represent energy rates, 1 BTU per hour (BTU/h) equals 0.293071 (). The corresponding equation is: PW=PBTU/h×0.293071P_\text{W} = P_\text{BTU/h} \times 0.293071 where PWP_\text{W} is power in watts and PBTU/hP_\text{BTU/h} is power in BTU per hour; this factor results from dividing the energy conversion by 3600 seconds per hour. For the thermochemical variant, the power conversion adjusts proportionally to 1 thermochemical BTU/h ≈ 0.2929 W using its energy factor of 1054.3503 J.

For Natural Gas

The British thermal unit (BTU) serves as a key measure for the heating value of , quantifying the energy released when a volume of gas is combusted under standard conditions of 60°F and 1 atmosphere (14.73 psia). In the United States, the typical heating value of ranges from 1,000 to 1,050 BTU per (scf), with the 2023 annual average delivered to end-use sectors at approximately 1,038 BTU/scf. This value reflects the gross (higher heating value, or HHV), which accounts for the total heat produced, including the of from formed during . In contrast, the net (lower heating value, or LHV) excludes this , assuming water remains as vapor, resulting in about 11% less energy than the HHV for . For example, a typical HHV of 1,090 BTU/scf corresponds to an LHV of 983 BTU/scf, depending on the gas's composition. The choice between HHV and LHV depends on the application; utilities commonly use HHV for billing and reporting, as it aligns with standard assumptions in heating systems. To determine the total energy content, the conversion equation is straightforward: total energy in BTU equals the volume in scf multiplied by the specific BTU/scf rating of the gas. This rating varies due to differences in gas composition, primarily the content, which typically ranges from 85% to 95% by volume in pipeline-quality , with smaller amounts of , , and inert gases like influencing the overall heat value. Composition is analyzed using for precise measurement, and utilities adjust BTU ratings annually based on sampled data to reflect regional variations. In U.S. residential billing, natural gas meters record usage in cubic feet, which is then multiplied by the local BTU/scf factor to yield total BTU consumed before conversion to therms (where 1 therm equals 100,000 BTU) for pricing. While the BTU remains the standard for U.S. (LNG) trade—often priced per million BTU (MMBtu)—BTU equivalents are still referenced in transatlantic contracts.

Power Units

The British thermal unit per hour (BTU/h or Btu/h) serves as a unit of power, quantifying the rate at which heat energy is transferred or produced, defined as the delivery of one BTU over one hour. This unit is widely applied in contexts to denote the capacity of heating, cooling, and power-generating systems, distinguishing it from the static energy measurement of the BTU itself. For instance, 1 BTU/h is approximately equivalent to 0.293 watts in the (SI). The fundamental equation for power in this unit is P=EtP = \frac{E}{t}, where PP is power in BTU/h, EE is energy in BTU, and tt is time in hours; this expresses the rate of thermal energy flow, analogous to how the watt represents joules per second. Unlike the BTU, which captures total heat quantity (similar to the joule for ), BTU/h measures dynamic heat transfer rates, essential for specifying operational performance in rate-based systems. For larger-scale applications, such as in (HVAC) design, multiples like kBTU/h (thousands of BTU per hour) and MBH (also denoting 1,000 BTU/h) are standard, facilitating concise notation for capacities exceeding thousands of BTU/h. Historically, early 20th-century engineering specifications for engines and equipment often employed BTU per minute (BTU/min) to describe power outputs, as seen in calculations for where rates like 42.42 BTU/min equated to one horsepower. Over time, a shift occurred toward BTU/h in modern standards, driven by the convenience of aligning with hourly metrics and simplifying HVAC and appliance ratings. For example, a typical residential gas furnace is rated at 100,000 BTU/h, meaning it supplies 100,000 BTU of heat each hour at full capacity, guiding installation and performance assessments.

Applications

Energy Measurement

The British thermal unit (BTU) serves as a fundamental measure for quantifying total in the United States, particularly for sources like fossil fuels, where it allows aggregation across diverse fuels on a common thermal basis. In 2023, U.S. consumption totaled 93.6 quadrillion BTU, or quads (where 1 quad equals 10^{15} BTU), according to the U.S. Energy Information Administration (EIA). This scale underscores the vast magnitude of national use, with quads providing a standardized unit for policy analysis and reporting. Sectoral breakdowns highlight BTU's utility in tracking direct fuel consumption. The transportation sector accounted for 28.0 quads in 2023, predominantly from petroleum products like , enabling equivalents such as gasoline's energy content to be expressed in BTU for comparative purposes. Meanwhile, the residential and commercial sectors together consumed about 20.6 quads, reflecting direct uses of and other fuels, while the industrial sector used 26.1 quads for processes like . For non-electric sources, BTU measurements dominate EIA reports on fossil fuels; for instance, one barrel of crude equates to approximately 5.8 million BTU, facilitating conversions in and trade assessments. Globally, the BTU is less prevalent outside the U.S. and , where the (SI) predominates, but it persists in international oil and gas trade for its compatibility with thermal equivalents. For example, one (toe), a common global benchmark, corresponds to roughly 40 million BTU, bridging imperial and metric systems in commodity markets. Recent trends illustrate shifts in BTU-denominated consumption. Coal's share of U.S. fell to 9% in 2023, continuing a post-2020 decline driven by the rise of renewables in , which reduced coal use by over 17% from 2022 levels alone. EIA projections as of November 2025 anticipate total consumption rising slightly to 95.71 quads in 2025, but with accelerating —particularly in transportation and buildings—diminishing reliance on direct BTU-based in favor of grid-supplied power. In contemporary applications, BTU continues to feature in updated U.S. Department of (DOE) standards for appliances, such as revised HVAC efficiency metrics incorporating BTU/h ratings as of 2025.

Heating, Ventilation, and Air Conditioning

In (HVAC) systems, the British thermal unit per hour (BTU/h) serves as a fundamental measure for determining the heating or required to maintain comfortable indoor conditions in residential and commercial buildings. Sizing HVAC equipment accurately using BTU/h prevents issues like short cycling, uneven temperatures, or excessive energy use, with a basic for cooling loads estimating 20–30 BTU/h per of floor area, adjusted for factors such as ceiling height, sun exposure, and . For heating, similar area-based multipliers apply but vary more widely by regional , often ranging from 30 BTU/h per in warmer zones to 50 BTU/h or higher in colder areas. Efficiency in HVAC heating systems is quantified using the Annual Fuel Utilization Efficiency (AFUE) rating, which expresses the percentage of fuel energy converted into usable output in BTU, accounting for losses through venting and standby operation over a full heating season. For instance, a furnace with a 95% AFUE rating delivers 95 BTU of for every 100 BTU of input , minimizing waste compared to older models at 80% or below. In cooling applications, capacity is often specified in tons of refrigeration, where 1 ton equals 12,000 BTU/h of removal, a standard derived from the energy needed to melt one ton of ice in 24 hours. Design considerations significantly influence BTU/h requirements, including building insulation levels—which can reduce loads by up to 50% in well-insulated structures—and U.S. Department of Energy (DOE) climate zones that adjust needs by 20–50% across regions. In DOE Zone 1 (hot-humid areas like southern Florida), heating demands are minimal at around 20 BTU/h per square foot, while Zone 5 (cold, like northern Minnesota) may require 45–60 BTU/h per square foot to offset extreme winters. Modern HVAC design relies on tools like the ACCA Manual J procedure, a longstanding standard (originally developed in the late 20th century, with ongoing updates) often implemented via software that performs detailed room-by-room load calculations in BTU/h, incorporating variables like window types and duct efficiency for precise sizing. Cooling efficiency is further evaluated via Seasonal Energy Efficiency Ratio (SEER) ratings, which measure BTU/h of cooling output per watt-hour of electricity over a season; these can be converted to Energy Efficiency Ratio (EER) for peak conditions, where higher SEER values (e.g., 14–20) indicate better performance in variable climates. As an example, an average U.S. single-family of 2,000 square feet typically requires 24,000–60,000 BTU/h for heating, depending on location and insulation, ensuring the system matches the calculated design load without oversizing that leads to inefficiency.

Associated Units

Therm

The is a unit of heat energy defined as exactly 100,000 British thermal units (BTU) on the International Table (IT) basis. This corresponds precisely to 105.505585262 megajoules (MJ). The unit provides a convenient multiple of the BTU for measuring larger quantities of , particularly in contexts involving fuels. The therm gained prominence in the for billing in the 1960s amid the transition from manufactured town gas to imported and domestic supplies following discoveries. In the , regulations were amended to replace therms with joules (retail in kilowatt-hours) effective January 2000. It was subsequently adopted in the United States for commercial gas metering and billing, with widespread implementation by the mid-20th century to standardize energy accounting beyond volumetric measures. In practice, the therm is primarily used for invoicing consumption, where one therm equates to approximately 96.7 standard cubic feet (scf) of gas assuming a heating value of 1,036 BTU per scf, the U.S. annual average for delivered . This avoids the cumbersome use of large numbers of individual BTUs; for instance, a consuming 1,000 cubic feet of gas might be billed for about 10.36 therms rather than 1,036,000 BTU. To convert total energy usage to therms, the is applied as follows: Therms=Total BTU100,000\text{Therms} = \frac{\text{Total BTU}}{100,000} This direct division ensures straightforward calculation from meter readings adjusted for local gas composition. The therm offers advantages over billing in cubic feet by directly reflecting the actual heat content of the gas, which varies by source and can range from 950 to 1,150 BTU per depending on purity and impurities. This heat-based approach simplifies structures and ensures equitable pricing, as consumers pay for delivered rather than alone; it has become a standard for billing in most U.S. states. In recent years, including since 2020, demand has declined in regions with increasing adoption and , shifting heating loads to measured in kilowatt-hours (kWh).

Ton of Refrigeration

The ton of refrigeration, abbreviated as RT, is a unit of power that quantifies the of and systems by measuring the rate at which is removed, encompassing both (associated with phase changes like ) and (associated with temperature changes). It is defined as 12,000 British thermal units per hour (BTU/h) of cooling effect. This unit traces its origins to early 20th-century practices tied to production, where one RT represents the cooling power needed to completely melt one (2,000 lb or 907 kg) of pure at 32°F (0°C) over 24 hours, drawing on the of fusion. The standardization emerged from efforts by industry leaders, including a committee formed under the in 1893 to establish a uniform measure, with further refinement by the American Society of Refrigerating Engineers (ASRE), founded in 1904 as a predecessor to . The cooling capacity in BTU/h can be calculated using the relation: Cooling capacity (BTU/h)=Tons (RT)×12,000\text{Cooling capacity (BTU/h)} = \text{Tons (RT)} \times 12{,}000 For instance, a system rated at 1 RT removes 12,000 BTU/h, while larger installations scale accordingly. In contemporary usage, the RT remains prevalent in U.S. commercial HVAC applications, particularly for specifying and cooling equipment capacities under , despite the global shift toward SI metrics. A typical large-scale example is a 500 RT , which delivers 6,000,000 BTU/h of cooling for industrial or building-scale needs. For conversion to the International System of Units, 1 RT equals approximately 3.517 kW. The unit is standardized on the short ton in the U.S., though a rare variant based on the metric tonne (1,000 kg) also approximates 3.517 kW in practice, with the short ton basis prevailing in American standards.

References

  1. https://www.eia.gov/energyexplained/units-and-calculators/british-thermal-units.php
  2. https://www.nrc.gov/reading-rm/basic-ref/glossary/british-thermal-unit-btu
  3. https://projects.sare.org/media/pdf/9/1/3/913715british-thermal-unit.pdf
  4. https://energyeducation.ca/encyclopedia/British_thermal_unit
  5. https://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors/nist-guide-si-appendix-b8
  6. https://www.nist.gov/glossary-term/19511
  7. https://www.akenergyauthority.org/LinkClick.aspx?fileticket=aRAYvPdxxKk%3D&portalid=0
  8. https://www.engineeringtoolbox.com/heat-work-energy-d_292.html
  9. https://www.greenbuildingadvisor.com/article/understanding-the-british-thermal-unit
  10. https://physics.nist.gov/cuu/pdf/sp811.pdf
  11. https://royalsocietypublishing.org/doi/10.1098/rsta.2014.0348
  12. https://heatinghelp.com/news-and-media/dead-men-tales/the-origin-of-the-btu/
  13. https://www.nature.com/articles/149613c0
  14. https://www.iso.org/standard/3632.html
  15. https://usma.org/wp-content/uploads/2015/07/US_Metric_Board_Summary_Report_1982-July.pdf
  16. https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication811e1995.pdf
  17. https://www.simetric.co.uk/sibtu.htm
  18. https://www.nist.gov/pml/special-publication-811/nist-guide-si-footnotes
  19. https://www.eia.gov/totalenergy/data/monthly/pdf/sec12_19.pdf
  20. https://www.eia.gov/totalenergy/data/monthly/pdf/sec12.pdf
  21. https://www.eia.gov/tools/faqs/faq.php?id=45&t=8
  22. https://www.epa.gov/sites/default/files/2020-09/documents/1.4_natural_gas_combustion.pdf
  23. https://www.engineeringtoolbox.com/fuels-higher-calorific-values-d_169.html
  24. https://www.eia.gov/totalenergy/data/monthly/pdf/mer_a_doc.pdf
  25. https://www.naesb.org/pdf2/wgq_bps100605w2.pdf
  26. https://www.eia.gov/naturalgas/monthly/pdf/table_25.pdf
  27. https://epa.illinois.gov/topics/air-quality/planning-reporting/annual-emission-reports/reference-tables/conversion-factors.html
  28. https://classes.lanecc.edu/mod/resource/view.php?id=3624113
  29. https://www.greenbuildingadvisor.com/article/understanding-energy-units
  30. https://www.bpa.gov/-/media/Aep/energy-efficiency/momentum-savings/2016-2020-hvac-sales-data-summary.pdf
  31. https://www.jstor.org/stable/1626874
  32. https://files.eric.ed.gov/fulltext/ED025119.pdf
  33. https://www.energy.gov/sites/prod/files/2013/11/f5/53827-5.pdf
  34. https://www.eia.gov/energyexplained/us-energy-facts/images/consumption-by-source-and-sector.pdf
  35. https://www.eia.gov/tools/faqs/faq.php?id=86&t=1
  36. https://www.eia.gov/energyexplained/units-and-calculators/
  37. https://www.bp.com/content/dam/bp/business-sites/en/global/corporate/pdfs/energy-economics/statistical-review/bp-stats-review-2022-approximate-conversion-factors.pdf
  38. https://www.eia.gov/energyexplained/us-energy-facts/
  39. https://www.eia.gov/outlooks/steo/pdf/steo_full.pdf
  40. https://www.energy.gov/eere/buildings/hvac
  41. https://www.energy.gov/energysaver/room-air-conditioners
  42. https://thefurnaceoutlet.com/blogs/hvac-tips/the-sizing-cheat-sheet-btu-per-square-foot-by-region
  43. https://www.trane.com/residential/en/resources/blog/what-btu-and-afue-rating-for-new-furnace/
  44. https://www.acca.org/standards/technical-manuals/manual-j
  45. https://www.carrier.com/residential/en/us/products/heat-pumps/heat-pump-efficiency/
  46. https://www.eia.gov/tools/faqs/faq.php?id=45&t=3
  47. https://www.inchcalculator.com/convert/therm-to-megajoule/
  48. https://www.oxfordenergy.org/wpcms/wp-content/uploads/2019/07/Gas-Production-from-the-UK-Continental-Shelf-NG-148.pdf
  49. https://www.govinfo.gov/content/pkg/SERIALSET-10152_00_00-010-0088-0000/pdf/SERIALSET-10152_00_00-010-0088-0000.pdf
  50. https://www.eia.gov/energyexplained/units-and-calculators/energy-conversion-calculators.php
  51. https://www.inchcalculator.com/convert/british-thermal-unit-to-therm/
  52. https://www.eia.gov/outlooks/aeo/
  53. https://terminology.ashrae.org/?entry=ton%20of%20refrigeration
  54. https://www.ashrae.org/about/mission-and-vision/ashrae-industry-history/air-conditioning-and-refrigeration-timeline
  55. https://taylorandfrancis.com/knowledge/Engineering_and_technology/Engineering_support_and_special_topics/Ton_of_refrigeration/
Add your contribution
Related Hubs
User Avatar
No comments yet.