Therm

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The therm (symbol thm) is a non-SI unit of heat energy equal to 100,000 British thermal units. It is approximately the energy equivalent of burning 100 cubic feet (often referred to as 1 CCF) of natural gas at standard temperature and pressure (STP).

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. It is usually in the units therms/CCF. Since a therm is close to the heat output of 1 CCF of gas under normal conditions, the factor usually is close to 1.

Definitions

Therm (EC) = 105 506 000 joules ([Official Journal L 073, P. 0114] 27 March 1972). This is equivalent to 100,000 BTU_IT. The therm (EC) is often used by engineers within the US.
Therm (US) = 105 480 400 joules ([15 USC Chapter 6]). This is equivalent to 100,000 BTU_59°F.
Therm (UK) = 105 505 585.257 348 joules ([The Units of Measurement Regulations 1995]).

A decatherm is 10 therms. Common abbreviations are as follows (Reference: [washingtonpost.com]):

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.

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