Therm

From Wikipedia, the free encyclopedia
Jump to: navigation, search

The therm (symbol, thm) is a non-SI unit of heat energy equal to 100000 British thermal units (Btu[1]). It is approximately the energy equivalent of burning 100 cubic feet (2.83 cubic metres) – 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 expressed in units of therms per 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, 25200 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.

Definitions[edit]

  • Therm (EC) ≡ 100000 BTUISO[2]
    = 105506000 joules
    29.3072 kWh
    The therm (EC) is often used by engineers in the US.
  • Therm (US) ≡ 100000 BTU59 °F[3]
    = 105480400 joules
    29.3001111111111 kWh.
  • Therm (UK) ≡ 105505585.257348 joules[4]
    29.3071070159300 kWh

10 therms are known as a decatherm (sometimes, dekatherm;[5] commonly abbreviated Dth), which is 1000000 Btu (of whichever type). Further common abbreviations are MDth for a 1000 decatherms, and MMDth for 1000000 decatherms.[5][not in citation given]

Usage[edit]

United Kingdom regulations were amended to replace therms with joules with effect from 1999, with natural gas usually retailed in the derived unit, kilowatt-hours. Despite this, the wholesale UK gas market trades in therms. In the United States, natural gas is commonly billed in CCFs (hundreds of cubic feet) or therms.

Carbon footprint[edit]

According to the EPA burning one therm of natural gas produces on average 5.3 kg (11.7 lb) of carbon dioxide.[6]

See also[edit]

References[edit]