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GLOSSARY
GLOSSARY
Learn how to talk just like your favorite HVAC contractor using these key terms.
AFUE (Annual Fuel Utilization Efficiency):
The percent (efficiency) of fossil fuel furnaces. Includes cycling and
flue losses and other factors.
BTU (British Thermal Unit):
The amount of heat needed to raise the temperature of one pound of water one de-
gree Fahrenheit.
BTUH:
The number of BTUs produced in one hour; used to signify the heating and cooling capacity of a system and
the heat losses and gains of buildings and homes.
CFM (Cubic Feet per Minute):
Volume of air movement. Used in duct design calculations.
Compressor:
The central part of a heat pump system. The compressor increases the pressure and temperature of
the refrigerant and simultaneously reduces its volume while causing the refrigerant to move through the system.
Condenser:
A heat exchanger in which hot, pressurized (gaseous) refrigerant is condensed by transferring heat to
cooler surrounding air, water or earth.
COP (Coefficient of Performance):
Heating efficiency rating for a geothermal system. BTU output divided by BTU
input.
Degree Days (Heating & Cooling):
Calculated by adding the high and low temperatures of the day, dividing by 2,
then subtracting from 65. (Example: High 42, Low 20 = 34 heating degree days.)
Delta T (or ΔT):
The change in temperature (degrees). Usually associated with water or air temperatures.
Desuperheater:
A device that recovers superheat from the compressor discharge gas for heating water.
EER (Energy Efficiency Ratio):
Cooling efficiency rating for geothermal systems. BTU output divided by Watt
input.
Fossil fuel:
Any of several types of combustible fuels formed from the decomposition of organic matter. Examples
are natural gas, propane, fuel oil, oil and coal.
GPM (Gallons per Minute):
A reference to the flow rate of water through the geothermal unit.
Heat Exchanger:
A device designed to transfer heat between two physically separated fluids or mediums (air, wa-
ter, or refrigerant) of different temperatures.
Heat Loss:
The amount of energy (BTUs/hr) lost from the home during cold weather that needs to be overcome/
supplied by the heating system. The amount varies based on outdoor air temperatures.
Heat Gain:
The amount of heat (BTUs/hr) that is absorbed by the home during hot weather, requiring removal by
the air conditioner in order to cool the home. Varies based on outdoor air temperatures.
Heat Sink:
The medium (air, water or earth) which receives heat rejected from a heat pump.
Heat Source:
The medium (air, water or earth) from which heat is extracted by a heat pump.
HSPF (Heating Season Performance Factor):
Heating efficiency rating for heat pumps over the course of the
entire heating season.
Infiltration / Exfiltration:
Air movement into the home through “leaks” (infiltration); or movement of air from inside
the home to the outside (exfiltration).
Relative Humidity:
The ratio of the amount of water vapor actually present in the air to the greatest amount pos-
sible at the same temperature. During heating, RH levels of 40-50% are generally recommended.
R-Value:
The resistance of heat movement through an insulating material. Varies with thickness and type.
SEER (Seasonal Energy Efficiency Ratio):
Cooling efficiency rating for heat pumps and air conditioners. Calcu-
lated over the entire cooling season.
Solar Gains:
Heat gain generated through the heating of walls and windows by the sun.
Ton:
In HVAC terms, 12,000 BTUs per hour.
Wind Chill:
A still-air temperature that would have the same cooling effect on exposed skin as a given combination
of temperature and wind speed. Does not affect heating requirements for your home.
