Therm

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.

A therm factor is used by 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 it was 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.

The therm is equal to about 25 200 (large) calories or about 29.3 kilowatt hours of electrical energy. One therm can also be provided by about 96.7 cubic feet of natural gas. The therm has sometimes been confused with the thermie (see below). The names of both units come from the Greek word for heat, therme.

thermie (th) 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 1000 (large) calories , 4.1868 megajoules or 3968.3 Btu.

Definitions

• Therm (EC) ≡ 100,000 BTU_IT

= 105,506,000 joules

≈ 29.3072222 kWh

The therm (EC) is often used by engineers in the US.

• Therm (US) ≡ 100,000 BTU_59°F

= 105,480,400 joules

≈ 29.3001111 kWh.

• Therm (UK) = 105,505,585.257 348 joules

≈ 29.30710701583 kWh

Ten therms are a decatherm. Common abbreviations are as follow:

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 (30.528 MW). In the United States, however, natural gas is commonly billed in CCFs (100 cubic feet) or therms.

References