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In the United States, U-value is expressed as British thermal units (Btu) per hour-square feet-degrees Fahrenheit: Btu/(h⋅ft 2 ⋅°F) Within this article, U-values are expressed in SI unless otherwise noted. To convert from SI to US customary values, divide by 5.678. [1]
The SC, or shading coefficient, is used widely in the evaluation of heat gain through glass and windows. [ 1 ] [ 5 ] Finally, the SCL, or solar cooling load factor, accounts for the variables associated with solar heat load.
Equivalence of direct radiation (EDR) is a standardized comparison method for estimating the output ability of space-heating radiators and convectors.. Measured in square feet, the reference standard for EDR is the mattress radiator [1] invented by Stephen J. Gold in the mid 19th century.
[1] [2] It therefore has units of BTU/watt-hr. The higher the HSPF rating of a unit, the more energy efficient it is. An electrical resistance heater, which is not considered efficient, has an HSPF of 3.41. [3] Depending on the system, an HSPF ≥ 9 can be considered high efficiency and worthy of a US Energy Tax Credit. [4]
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.
The therm (symbol, thm) is a non-SI unit of heat energy equal to 100,000 British thermal units (BTU), [1] and approximately 105 megajoules, 29.3 kilowatt-hours, 25,200 kilocalories and 25.2 thermies. One therm is the energy content of approximately 100 cubic feet (2.83 cubic metres) of natural gas at standard temperature and pressure .
Cooling capacity is the measure of a cooling system's ability to remove heat. [1] It is equivalent to the heat supplied to the evaporator/boiler part of the refrigeration cycle and may be called the "rate of refrigeration" or "refrigeration capacity".
In those contexts, the unit of heat capacity is 1 BTU/°R ≈ 1900 J/K. [5] The BTU was in fact defined so that the average heat capacity of one pound of water would be 1 BTU/°F. In this regard, with respect to mass, note conversion of 1 Btu/lb⋅°R ≈ 4,187 J/kg⋅K [ 6 ] and the calorie (below).