ASHRAE Terminology

equivalent evaporation of 34.5 lb of water per hour from and at 212°F (100°C). This is equal to heat output of 970.3 Btu/lb/h × 34.5 lb = 33,475 Btu/h, approximately 9809.5 W.

design maximum rating of a steam or water boiler expressed as the total heat transferred by the heating surfaces in Btu/h (kW). Sometimes expressed in horsepower or pounds (torque, kilograms) of steam per hour. Compare to boiler capacity.

the quantity of heat required to raise the temperature of one pound of water by one degree Fahrenheit at a specified temperature. This standard uses the International Table (IT) definition, where the quantity of heat is defined to be 1055.05585262 J (1 BtuIT).

metering system capable of measuring the energy added to or extracted from an electric or fluid stream. Also called thermal energy meter, heat meter, or thermal meter. Compare to watt meter.

(thermal conductance), time rate of steady-state heat flow through unit area of a material or construction, induced by a unit temperature difference between the body surfaces. Units of C are Btu/h·ft² or °FW/m²·K. Note that the C-factor does not include soil or air films.

the cooling effect produced by the change in enthalpy between the refrigerant liquid entering the expansion valve and the vapor leaving the evaporator, generally measured in Btu per hour (kW or tons of refrigeration).

available capacity of an air conditioner for removing sensible and latent heat from the space to be conditioned (Btu/h).

the rate, expressed in watts (Btu/h), at which the equipment removes latent heat from the air passing through it under specified conditions of operation.

the rate, expressed in watts (Btu/h), at which the equipment removes sensible heat from the air passing through it under specified conditions of operation.

the rate, expressed in watts (Btu/h), at which the equipment removes heat from the air passing through it under specified conditions of operation.

insulation composed principally of natural or synthetic elastomers or both, processed to form a flexible, semirigid or rigid foam, having a predominately closed-cell structure. Insulation is usually expressed in k value (Btu/h·ft·°F [w/m·°K]).

a unit used to express the thermal insulation provided by garments and clothing ensembles, where 1 clo = 0.88 ft2·h·°F/Btu (0.155 m2·°C/W).

a ratio calculated by using the formula: CER = (C+ FE)/E where: C = cooling capacity, Btu/h (W), FE = fan electrical input, W × 3.413 Btu/W (W), E = total electrical input (W).

total heat rejection capacity of a cooling tower, traditionally, 15,000 Btu/h. Note: this value is based on 25% compressor heat added to a ton of refrigeration. Current energy-efficient equipment may have lower values than traditional values.

(1) the time rate of energy flow. In the United States, demand usually refers to electric power and is measured in kW (equals kWh/h) but can also refer to natural gas, usually as therms or ccf per day. In many other countries, demand is commonly used with other energy sources, especially district heat. (2) the highest amount of power (average Btu/h over an interval) recorded for a building or facility in a selected time frame.

(1) ratio of net cooling capacity in Btu/h to total rate of electric input in watts under designated operating conditions. (2) ratio of the net total cooling capacity to the effective power input at any given set of rating conditions, in watts per watt.

an expression of the annual energy used or calculated to be used by a building or building space per unit of gross floor area. Expressed in MBtu/ft2·yr (watts-hours/m2·yr).

that lower portion of the building envelope, including opaque area and fenestration, that has conditioned or semiheated space above and is horizontal or tilted at an angle of less than 60°F (15°C) from horizontal but excluding slab-on-grade floors. For the purposes of determining building envelope requirements, the classifications are defined as follows: (a) mass floor: a floor with a heat capacity that exceeds (1) 7 Btu/ft²·°F (39.7 watt h/m² °C) or (2) 5 Btu/ft²·°F (28.4 watt h/m²·°C) provided that the floor has a material unit mass not greater than 120 lb/ft³ (1924.6 kg/ m³), (b)  steel joist floor: a floor that (1) is not a mass floor and (2) that has steel joist members supported by structural members, (c) wood framed and other floors: all other floor types, including wood joist floors.

thermal radiation expressed in terms of square foot of steam radiator surface emitting 240 Btu/hr. The metric equivalent is 757 watts/m². Note: this term is generally not used in SI.

refrigerating effect in watts, Btu/h, or tons (12,000 Btu/h) produced by the evaporation of refrigerant passed by the valve under specified conditions.

the perimeter heat loss factor for slab-on-grade floors, expressed in Btu/h·ft2·°F (W/m2·°K).

rate at which fuel is supplied to an appliance. The rate may be expressed in Btus per hour, watts, or thousands of Btus per hour (MBH), in cubic feet per hour, liters per second, or thousands of cubic feet per hour, in therms (th) or dekatherms (dth) per hour, or in gallons per hour (GPH).

(1) (also known as hemispherical irradiance), quantity of solar energy incident on a unit surface area in unit time through a unit hemisphere above the surface, expressed in Btu/h·ft2 (W/m2). (2) the global (or hemispheric) solar radiant energy is the quantity of solar energy incident upon a unit surface area in unit time through a unit hemisphere above the surface, expressed in W/m2 (Btu/[h·ft2]).

the heat released when a substance is combusted, determined as the difference in the enthalpy between the reactants (refrigerant[s] and air) and their products after combustion as defined in Section 6.1.3.5. The heat or enthalpy of combustion is often expressed as energy per mass (e.g., kJ/kg or Btu/lb).

(1) heat energy required to cause a change of state from solid to liquid at constant temperature. For ice to water, 143.5 Btu/lb (333.8 kJ/kg). (2) latent heat involved in changing between the solid and the liquid states.

(also called heat transmission, heat transport, and thermal transmission), transfer of energy, in Btu/h (W), induced by a temperature difference. May occur by conduction, convection, radiation, mass transfer, or any combination of these.

the rate of heat that the equipment adds to the conditioned space or heat transfer fluid in a defined interval of time, expressed in Btu/h (W).

for the space-heating season, the ratio of the total space heating delivered to the total electrical energy input if the combined appliance operated exclusively in a space heating only mode. The quantity is expressed in units of Btu/Wh. For SI use, compare coefficient of performance.

the amount of heat produced per unit of fuel when complete combustion takes place at constant pressure, the products of combustion are cooled to the initial temperature of the fuel and air, and the vapor formed during combustion is condensed. HHV is expressed in Btu/lb (J/kg) or Btu/ft3 (W/m3) for gaseous fuel and in Btu/lb (J/kg) or Btu/gal (J/l) for liquid fuel.

units using inches, pounds, and other designations. Examples are: foot, Btu, horsepower, gallon. As compared to SI unit system.

latent heat absorbed by ice at 32°F (0°C) in liquefying into water phase at 32°F (0°C). For precise data, 143.5 Btu/lb. (334.4 kJ/kg), for definition of ton, 144 Btu/lb (2.326 kJ/kg).

fuel burning capacity of an appliance in British thermal units per hour (Btu/h) [kilowatts (kW)] as specified by the manufacturer. Appliance input ratings are based on sea-level operation and need not be changed for operation up to 2000 ft (600 m) altitude.

the rate at which solar radiation is incident on a unit surface area in unit time, measured in Btu/(h·ft2) [W/m2].

time rate of steady-state heat flow through a unit area of a homogeneous material, induced by a unit temperature gradient in a direction perpendicular to that unit area. Units are Btu in./h·ft2·°F or Btu/h·ft·°F [W/(m·K)].

the rate, expressed in Btu/h (W), at which the equipment removes latent heat (reduces the moisture content) of the air passing through it under specified conditions of operation.

quantity of heat released on change of unit mass of saturated vapor to saturated liquid with no change in temperature, measured in Btu/lbm (J/kg).

quantity of heat required to change a unit mass of ice to water at 32°F (0°C) temperature, measured in Btu/lbm (J/kg).

quantity of heat required to cause a change of state of a substance from a saturated liquid to a saturated vapor with no change in temperature, measured in Btu/lbm (J/kg).

a floor with a heat capacity that exceeds (a) 7 Btu/ft2·°F (143 kJ/m2·K) or (b) 5 Btu/ft2·°F (102 kJ/m2·K), provided that the floor has a material unit mass not greater than 120 lb/ft3 (1920 kg/m3).

a wall with a heat capacity exceeding (1) 7 Btu/ft2·°F (143 kJ/m2·K) or (2) 5 Btu/ft2·°F (102 kJ/m2·K), provided that the wall has a material unit weight not greater than 120 lb/ft3 (1920 kg/m3).

one thousand Btu per hour. An I-P unit of power. In SI, use watts or kilowatts.

capacity for doing work, usually expressed in work units (foot-pounds or newton-meters), sometimes in heat units (Btu or joule). Energy may be inherent in the speed of a body (kinetic energy) or in its position relative to another body (potential energy).

heat produced by oxidation of food elements (i.e., metabolism) in humans or animals. The met represents the average heat produced by a sedentary man, approximately 90 kcal/h or 100 W (340 Btu/h).

(1) valve having a very small hole in the seat, fitted with a needle-shaped plunger. (2) valve in which the obturator is a stem ending in a conical point which mates with a corresponding valve seat.

a factor or parameter that allows for the normalization of energy use by the factor or parameter (e.g., Btu/student·yr or kWh/widget·yr) chosen to allow for expression of energy performance.

the capacity of the permanent bleed provision, expressed either as a percentage of the nominal capacity or in kW (Btu/h or tons) of refrigerating effect produced by the evaporation of that amount of refrigerant flow.

the quotient of the flux incident on an element of a surface containing the point, by the area of that element, measured in watts per square meter, W/m2 (Btu/ft2).

the thermal resistance of the insulation alone as specified by the manufacturer in units of h·ft2·°F/Btu (m2·K/W) at a mean temperature of 75°F (24°C). Rated R-value refers to the thermal resistance of the added insulation in framing cavities or insulated sheathing only and does not include the thermal resistance of other building materials or air films.

energy per unit moisture removed expressed in kJ/kg (Btu/lb).

for the cooling season, the ratio of the total heat removed from the conditioned space to the total electrical energy input required to remove that heat, evaluated over all appliance operating modes. The quantity is expressed in units of Btu/Wh.

for the cooling season, the ratio of the total heat removed from the conditioned space to the total electrical energy input if the combined appliance operated exclusively in a space-cooling-only (COOL) mode. The quantity is expressed in units of Btu/Wh.

an enclosed space within a building that is heated by a heating system whose output capacity is greater than or equal to 3.4 Btu/h·ft2 (10 W/m2) of floor area but is not a conditioned space.

the rate, expressed in Btu/h (W), at which the fan coil under test reduces or increases the dry-bulb temperature of the air passing through it.

the rate, expressed in W (Btu/h), at which the equipment lowers the dry-bulb temperature (removes sensible heat) of the air passing through it under specified conditions of operation.

the rate, expressed in Btu/h (W), at which the equipment raises the dry-bulb temperature (adds sensible heat) of the air passing through it under specified conditions of operation.

solar energy flowing into a space or structure. Units are expressed in kilowatts (Btu/h).

ratio of the quantity of heat required to raise the temperature of a given mass of any substance one degree to the quantity required to raise the temperature of an equal mass of a standard substance one degree (usually water at 59°F [15°C]). The units are expressed in Btu/lb·°F [J/(kg·K)].

the reciprocal of the heat transfer film coefficient, Ri or Ro, in h·ft2·°F/Btu ([m2·K]/W). (Subscripts i and o often denote inside and outside surface film resistances and conductances, respectively.) For convection to occur, the surrounding space must be filled with air or another fluid. If the space is evacuated, heat flow occurs by radiation only.

quantity of heat equivalent to 100,000 Btu.

time rate of steady-state heat flow through unit area of a material or construction, induced by a unit temperature difference between the body surfaces. Units are Btu/h·ft2·°F (W/[m2·K]). Note that the C-factor does not include soil or air films.

(k-factor), time rate of steady-state heat flow through unit thickness of unit area of a homogeneous material, induced by a unit temperature gradient in a direction perpendicular to the isothermal planes of that unit. Units of k are in Btu·in/(h·ft2·°F), Btu·ft/(h·ft2·°F), or W/(m·K). Thermal conductivity must be evaluated for a specific mean temperature, thickness, age, and moisture content. See also thermal conductance.

see Btu meter or watt meter.

(R-value), the reciprocal of the time rate of heat flow through a unit area induced by a unit temperature difference between two defined surfaces of material or construction under steady-state conditions. Units of thermal resistance are h·ft2·°F/Btu (m2·°K/W). Thermal resistance is the reciprocal of the thermal conductance.

(also known as U-factor), heat transmission in unit time through unit area of a material or construction and the boundary air films, induced by unit temperature difference between the environments on each side. Note: this heat transmission rate is also called the overall coefficient of heat transfer. U, in Btu/h·ft2·°F (W/[m2·K]). Thermal transmittance is sometimes called the overall coefficient of heat transfer or U-factor. Thermal transmittance includes surface film conductance.

mechanism that transfers energy from one system to another without accompanying transfer of entropy. Units of thermodynamic work are Btu (W·h).

heat removed by a ton of refrigeration operating for a day, 288,000 Btu (approximately 84.3 kWh).

time rate of cooling equal to 12,000 Btu/h (approximately 3517 W). It is a quantity approximately equal to the latent heat of fusion or melting of 1 ton (2000 lb) of ice, from and at 32°F (0°C).

unit thermal conductance or overall heat transfer coefficient, W/(m²·K) (Btu/(h·ft²·°F).

that part of a valve that closes off or stops fluid flow.

hot-water boiler having a volume not exceeding 120 gal (450 L), or a heat input not exceeding 200,000 Btu/h (60 kW), or an operating temperature not exceeding 250°F (120°C), to provide hot water to a system.

metering system capable of measuring the energy added to or extracted from an electric or fluid stream. Also called thermal energy meter, heat meter, or thermal meter. Compare to Btu meter.