Therm
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The therm (symbol thm) is a non-SI unit of heat energy equal to 100,000 British thermal units (BTU). It is approximately the energy equivalent of burning 100 cubic feet (often referred to as 1 CCF) of natural gas.
Since natural gas meters measure volume and not energy content, a therm factor is used by (Natural) gas companies to convert the volume of gas used to its heat equivalent, and thus calculate the actual energy use. The therm factor is usually in the units therms/CCF. It will vary with the mix of hydrocarbons in the natural gas. Natural gas with a higher than average concentration of ethane, propane or butane will have a higher therm factor. Impurities, such as carbon dioxide or nitrogen, lower the therm factor.
The volume of the gas is calculated as if measured at standard temperature and pressure (STP). The heat content of natural gas is solely dependent on the composition of the gas, and is independent of temperature and pressure.
One therm is equal to about 105.5 megajoules, 25,200 kilocalories or 29.3 kilowatt-hours. One therm can also be provided by about 96.7 cubic feet (2.74 m3) of natural gas. The therm sometimes has been confused with the thermie (see below). The names of both units come from the Greek word for heat.
A thermie (th) is a metric unit of heat energy, part of the meter-tonne-second system sometimes used by European engineers. The thermie is equal to the amount of energy required to raise the temperature of 1 tonne of water by 1 °C. The thermie is equivalent to 1,000 kilocalories, 4.1868 megajoules or 3968.3 BTU.
Definitions
-
- = 105,480,400 joules
- ≈ 29.3001111111111 kWh.
- Therm (UK) = 105,505,585.257 348 joules[3]
-
- ≈ 29.30710701583 kWh
Ten therms are a decatherm. Common abbreviations are as follows:[4]
| Dth | decatherm |
| MDth | thousand decatherms |
| MMDth | million decatherms |
Usage
United Kingdom regulations were amended to replace therms with joules with effect from 1999. Despite this, natural gas is now usually retailed in kilowatt-hours, although the wholesale UK gas market trades in therms, with a typical forward transaction being for 25,000 therms/day (31 MW). In the United States, however, natural gas is commonly billed in CCFs (100 cubic feet) or therms.
Carbon footprint
According to the EPA burning one therm of natural gas produces on average 11.7 lbs or 5.3 kilogram of carbon dioxide, see http://www.epa.gov/cleanenergy/energy-resources/calculator.html#results.
