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Flashcards in Mechanical Systems Deck (27)
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British thermal unit (Btu)

the amount of heat required to raise the temperature of 1 lbm water by 1 degree F


U (coefficient of heat transmission)

the overall rate of heat flow through any combination of materials, including air spaces and air layers on the interior and exterior of a building assembly;; it is the reciprocal of the sum of all the resistances in the building assembly


C (conductance)

the number of BTUs per hour that pass through 1 ft^2 of homogeneous material of a given thickness when the temperature differential is 1 degree F


k (conductivity)

the number of BTUs per hour that pass through 1 ft^2 of homogeneous material 1 inch thick when the temperature differential is 1 degree F


dew point

the temperature at which water vapor in the air becomes saturated and begins to condense into drops of water


dry-bulb temperature

the temperature of the air-water mixture as measured with a standard dry-bulb thermometer



the total heat in a substance, including latent heat and sensible heat


latent heat

heat that causes a change of state of a substance, such as the heat required to change water into steam;; the amount of heat required to change the state of a substance is much greater than the heat required to raise the temperature of the substance (sensible heat). The average value of latent heat per pound of moisture is 1061 Btu.



the number of hours needed for 1 Btu to pass through 1 ft^2 of material or assembly of a given thickness when the temperature differential is 1 degree F;; it is the reciprocal of conductance.


sensible heat

heat that causes a change in temperature of a substance but not a change of state;; the sensible heat needed to raise the temperature of 1 lbm of water from 50 deg F to 100 deg F is 50 Btu. In contrast, the latent heat needed to change liquid water at 212 deg F to steam at 212 deg F is 1061 Btu.


specific heat

the number of BTUs required to raise the temperature of a specific material by 1 deg F;; specific heat is a measure of a material's capacity to store heat as compared with the storage capacity of water.


wet-bulb temperature

the temperature of the air as measured with a sling psychrometer;; the wet-bulb temperature is a more critical measure of heat in high humidity because it is an indicator of physical stress caused when the human body is near the upper limits of temperature regulation by perspiration.


mean radiant temperature (MRT)

the uniform temperature of an imaginary surrounding enclosure in which radiant transfer from the human body would equal the radiant heat transfer in the actual nonuniform enclosure;; MRT is a calculated variable and cannot be directly measured


cross-ventilation vs. stack ventilation

Natural ventilation. Cross-ventilation is driven by windows and relies on narrow plans with large openings on either side; it is naturally compatible with daylighting. Stack ventilation depends on very low openings to admit outside air and high openings to exhaust it; it's generally weaker than cross-ventilation except when there is no wind.



a device in a building control system that receives commands from a controller and activates a piece of equipment


annual fuel utilization efficiency (AFUE)

the ratio of annual fuel output energy to annual input energy;; this includes nonseasonal pilot light input losses


coefficient of performance (COP)

a unites number that is a rating of the efficiency of heating or cooling equipment;; it is derived by dividing the steady-state rate of energy output (or the rate of heat removal, in the case of cooling equipment) of the equipment by the steady-state rate of energy input to the equipment. The output and input values must be in equivalent units, such as watts out to watts in.



a device that measures, analyzes, and initiates actions in a building control system



in a building control system, the range of temperatures within which neither heating nor cooling is needed


energy efficiency ratio (EER)

the ratio of net cooling capacity in Btu/hr to the total rate of electrical input in watts under designated operating conditions


energy management system (EMS)

a computer- based system used to monitor and control facility energy use;; an EMS is typically part of a building automation system


ground-coupled cooling

a method of cooling a building by direct contact with the earth or by circulating air through underground tunnels to cool it


heating seasonal performance factor (HSPF)

a measure of the performance of a heat pump operating in the heating cycle


home energy rating system (HERS)

a standardized system for rating the energy efficiency of residential buildings using the HERS Council Guidelines and the Mortgage Industry HERS Accreditation Procedures;; a HERS score is a numeric value between 0 and 100 indicating the relative energy efficiency of a given home as compared with the HERS Energy-Efficient Reference Home.


integrated part load value (IPLV)

the single-number figure of merit based on part-load EER or COP expressing part-load efficiency for air conditioning and heat pump equipment on the basis of weighted operation at various load capacities for the equipment, as determined using the applicable test method in the Appliance Efficiency Regulations


relative solar heat gain (RSHG)

the ratio of solar heat gain through a window, corrected for external shading, to the incident solar radiation;; this heat gain includes directly transmitted solar heat and absorbed solar radiation, which are conducted or convected into the space.


seasonal energy efficiency ratio (SEER)

the total cooling output of a central air conditioning system or heat pump in the cooling mode, measured in Btus per hour, during its normal usage period for cooling divided by the total electrical input in watt-hours, as determined by specific test procedures;; the higher the SEER, the more efficient the equipment performance. The minimum SEER permitted for residential air conditioning equipment in the US is 13.