Comfort Glossary – Heating and Cooling Terms

Your service technician should always talk in terms that you can relate to. If you’ve had another company’s technician at your home and heard a term that you did not understand, we hope our comfort glossary will help.

ABSOLUTE HUMIDITY – The weight of water vapor in a given amount of air.

ABSOLUTE PRESSURE – Pressure measured with the base of zero.

ABSOLUTE TEMPERATURE – A temperature scale expressed in degrees using absolute zero as a base. Referred to as the Rankin or Kelvin scale.

ABSOLUTE ZERO – The temperature at which molecular activity theoretically ceases.

ABSORBENT – Substance with the ability to take up or absorb another substance.

ABSORPTION REFRIGERATOR – Refrigerator that creates low temperature by using the cooling effect formed when a refrigerant is absorbed by chemical substance.

ACCA – A leading HVAC/R Association.

ACCUMULATOR – Storage tank, which receives liquid refrigerant from evaporator and prevents it from flowing into suction line before vaporizing.

ACH, AIR CHANGES PER HOUR – The number of times that air in a house is completely replaced with outdoor air in one hour.

ACTUATOR – That portion of a regulating valve, which converts mechanical fluid, thermal energy or electrical energy into mechanical motion to open or close the valve seats.

ADD ON HEAT PUMP – Installing a heat pump in conjunction with an existing fossil fuel furnace.

ADIABATIC COMPRESSION – Compressing refrigerant gas without removing or adding heat.

ADSORBENT – Substance with the property to hold molecules of fluids without causing a chemical or physical damage.

AERATION – Act of combining substance with air.

AFUE – Annual Fuel Utilization Efficiency -ratio of annual output of useful energy or heat to the annual energy input to the furnace

AGITATOR – Device used to cause motion in confined fluid.

AHU (AIR HANDLER UNIT) – The inside part of the A/C system that contains the blower, cooling (evaporator) coil, and heater.

AIR CHANGE – The amount of air required to completely replace the air in a room or building; not to be confused with re-circulated air

AIR CLEANER – Device used for removal of airborne impurities.

AIR COIL – Coil on some types of heat pumps used either as an evaporator or condenser.

AIR CONDITIONER – Device used to control temperature, humidity, cleanliness and movement of air in a confined space.

AIR CONDITIONING – The process of controlling the temperature, humidity, cleanliness and distribution of the air.

AIR COOLER – Mechanism designed to lower temperature of air passing through it.

AIR DIFFUSER – Air distribution outlet or grille designed to direct airflow into desired patterns.

AIR DIFFUSION – Distribution of the air in a space, called the treated space, by means of devices, called air terminal devices, in a manner so as to meet certain specified conditions, such as air change rate, pressure, cleanliness, temperature, humidity, air velocity and noise level.

AIR DISTRIBUTION – The transportation of a specified air flow to or from the treated space or spaces, generally by means of ductwork.

AIR GAP – The space between magnetic poles or between rotating and stationary assemblies in a motor or generator.

AIR HANDLER – Fan-blower, filter and housing parts of a system.

AIR INFILTRATION – Leakage of air into rooms through cracks, windows doors and other openings.

AIR SOURCE EQUIPMENT – Heat pumps or air conditioners that uses the outdoor air to transfer heat to and from the refrigerant in the unit.

AIR TERMINAL DEVICE – A device located in an opening provided at the boundaries of the treated space to ensure a predetermined motion of air in this space.

AIR-COOLED CONDENSER – Heat of compression, plus the heat of absorption, is transferred from refrigerant within coil to surrounding air, either by convection or fan or blower.

AIRFLOW – The distribution or movement of air

AK VALUE (OF AN AIR TERMINAL DEVICE) – Quotient obtained by dividing a measured airflow rate by a measured air velocity according to a specific process and a specific instrument.

ARI (AIR-CONDITIONING AND REFRIGERATION INSTITUTE) – Air-Conditioning and Refrigeration Institute is a nonprofit, voluntary organization comprised of heating, air conditioning and refrigeration manufacturers. ARI publishes standards for testing and rating heat pumps and air conditioners to provide you with a standardized measure of comparison. So, ARI ensures a level of performance within the industry.

ASHRAE – A leading HVAC/R Association – American Society of Heating, Refrigerating and Air Conditioning Engineers –

ASTM – American Society for Testing and Materials.

AMBIENT – Refers to the temperature surrounding a body or unit under test.

ATMOSPHERIC PRESSURE – The weight of a 1-unit column of the earth’s atmosphere.

BACKDRAFTING – Reverse flow of combustion gases down the chimney of a vented combustion appliance, which is often caused by depressurization of the room where the appliance is located.

BALANCE POINT – The lowest outdoor temperature at which the refrigeration cycle of a heat pump will supply the heating requirements without the aid of a supplementary heat source.

BALANCING – Process of adjusting the flow of air in duct systems, or water flow in hot-water heating systems.

BIMETAL – Two metals with different rates of expansion fastened together. When heated or cooled they will warp and can be made to open or close a switch or valve.

BLOWER (FAN) – An air-handling device for moving air in a distribution system.

BTU (BRITISH THERMAL UNIT) – Quantity of heat required to raise the temperature of one pound of water one degree Fahrenheit.

BOILING POINT – The temperature at which the addition of any heat will begin a change of state from a liquid to a vapor.

BRITISH THERMAL UNIT (BTU) – The amount of heat necessary to change the temperature of 1 pound of pure water 1 degree Fahrenheit.

CAE (COMBINED ANNUAL EFFICIENCY) – A measure of the amount of heat produced for every dollar of fuel consumed for both home heating and water heating.

CAPACITY – The ability of a heating or cooling system to heat or cool a given amount of space. For heating, this is usually expressed in BTUs. For cooling, it is usually given in tons.

CAPILLARY TUBE – A refrigerant control consisting of a small diameter tube which controls flow by restriction. They are carefully sized by inside diameter and length for each particular application.

CARBON MONOXIDE – A colorless, odorless, highly poisonous gas produced when carbon burns without sufficient air nearby.

CARBOXYHEMOGOBLIN SATURATION – Carbon monoxide poisoning.

CFC (CHLOROFLUOROCARBON) – A class of refrigerants. Generally refers to the Chlorofluorocarbon family of refrigerants. Sometimes called Freon

CFM (CUBIC FEET PER MINUTE) – A standard measurement of airflow that indicates how many cubic feet of air pass by a stationary point in one minute. The higher the number, the more air is being forced through the system. A typical system produces 400 CFM per ton of air conditioning.

CHARGE – Amount of refrigerant placed in a refrigerating unit.

CHECK VALVE – A valve designed to permit flow in one direction only.

COMFORT ZONE – The range of temperatures, humidities and air velocities at which the greatest percentage of people feel comfortable.

COMPRESSION – The reduction of volume of a vapor or gas by mechanical means.

COMPRESSOR – Pump of a refrigerating mechanism, which draws a low pressure on cooling side of refrigerant cycle and squeezes or compresses the gas into the high pressure or condensing side of the cycle.

COMPRESSION RATIO – The ratio determined by dividing the discharge pressure, in PSI (Pa), by the suction pressure in PSI (Pa).

COMPRESSOR – A mechanical device used to compress gases. Three main types – reciprocating, centrifugal and rotary.

CONDENSER COIL – Part of the outdoor portion of a split-system air conditioner or heat pump. By converting refrigerant that is in a gas form back to a liquid, the coil sends heat carried by the refrigerant to the outside.

CONDENSATION POINT – The temperature at which the removal of any heat will begin a change of state from a vapor to a liquid.

CONDENSING MEDIUM – The substance, usually air or water, to which the heat in a condenser is transferred.

CONDENSING UNIT – Part of a refrigerating mechanism which pumps vaporized refrigerant from the evaporator, compresses it, liquefies it in the condenser and returns it to the refrigerant control

CONDUCTION – The transfer of heat through a solid material.

CONTACTOR – An electromagnetic actuated relay. Usually used to refer to the relay which closes the circuit to a compressor.

CONVECTION – The movement of heat by air flow.

COOLING ANTICIPATOR – A resistance heater (usually not adjustable) in parallel with the cooling circuit. It is ‘on’ when the current is ‘off”, adding heat to shorten the off cycle.

COP (COEFFICIENT OF PERFORMANCE) – COP compares the heating capacity of a heat pump to the amount of electricity required to operate the heat pump in the heating mode.

CUBIC FEET PER MINUTE – A common means of assigning quantitative values to volumes of air in transit, usually abbreviated CFM.

CYCLE – The complete course of operation of a refrigerant back to a selected starting point in a system.

DAMPER – A device that is located in ductwork to adjust air flow.

DB – Dry Bulb Temperature

DB (DECIBEL) – A decibel describes the relative loudness of a sound.

DEFROST CYCLE – The process of removing ice or frost buildup from the outdoor coil during the heating season.

DEHUMIDIFICATION – The reduction of water vapor in air by cooling the air below the dew point; removal of water vapor from air by chemical means, refrigeration, etc.

DENSITY – Mass or weight per unit of volume.

DESIGN CONDITIONS – Cooling loads vary with inside and outside conditions. A set of conditions specific to the local climate are necessary to calculate the expected cooling load for a home. Inside conditions of 75 degrees Fahrenheit and 50% relative humidity are usually recommended as a guideline. Outside conditions are selected for the 2.5% design point.

DIRECT GAS-FIRED HEATER – The burner fires directly in the air stream being heated, rather than through a heat exchanger. 100% of available BTUs are delivered to the heated space because no flue or heat exchanger is required. This results in no wasted energy.

DISCHARGE LINE – A tube used to convey the compressed refrigerant vapor from the compressor to the condenser inlet.

DISCHARGE PRESSURE – The pressure read at the compressor outlet. Also called head pressure or high side pressure.

DOE (DEPARTMENT OF ENERGY) – The Department of Energy is a federal agency in charge of setting industry efficiency standards and monitoring the consumption of energy sources.

DOWNFLOW – A type of furnace that takes cool air from the top and blows warm air to the bottom.

DRIER – Sometimes called filter/drier, it removes moisture and keeps the refrigerant clean.

DRY AIR – Air which contains no moisture vapor.

DRY BULB TEMPERATURE – Temperature read with an ordinary thermometer.

DUCT – A pipe or closed conduit made of sheet metal, fiberglass board, or other suitable material used for conducting air to and from an air handling unit.

DUCTWORK – The delivery system through which warm air from the furnace is brought to where it’s needed.

EER (ENERGY EFFICIENCY RATIO) – A ratio calculated by dividing the cooling capacity in Btu’s per hour (Btuh) by the power input in watts at any given set of rating conditions, expressed in Btuh per watt (Btuh/watt). EER & SEER can not be compared equally. Air source equipment is rated by SEER and geothermal equipment is rated by EER. EER changes with the inside and outside conditions, falling as the temperature difference between inside and outside gets larger.

EFFICIENCY – A rating on comfort equipment is similar to the miles per gallon rating on your car.

EFFECTIVE TEMPERATURE – An arbitrary concept which combines into a single value the effect of temperature, humidity, and air movement as sensed by the human body.

ENERGY RECOVERY VENTILATOR (ERV) – This device preheats incoming outside air during the winter and pre-cools incoming air during the summer to reduce the impact of heating and or cooling the indoor air.

ENTHALPY – Total amount of heat in one pound (kg) of a substance calculated from accepted temperature base, expressed in BTU’s per pound mass (J/kg).

ENERGY STAR – Products/Homes/Buildings that earn the ENERGY STAR prevent greenhouse gas emissions by meeting strict energy efficiency guidelines set by the U.S. Environmental Protection Agency and the U.S. Department of Energy.

EPA (ENVIRONMENTAL PROTECTION AGENCY) – Environmental Protection Agency

EQUIVALENT LENGTH – That length of straight tubing, which has the same pressure drop as the fitting, valve or
accessory (of the same nominal size) being considered.

EVAPORATIVE COOLING – The cooling effect of vaporization of a liquid in a moving air stream.

EVAPORATOR – A device in which a liquid refrigerant is vaporized. Some superheating usually takes place.

EVAPORATOR COIL – Part of a split-system air conditioner or heat pump located indoors. The evaporator coil cools and dehumidifies the air by converting liquid refrigerant into a gas, which absorbs the heat from the air. The warmed refrigerant is then carried through a tube to the outdoor unit (condenser coil).

EVAPORATOR SUPERHEAT – The actual temperature of the refrigerant vapor at the evaporator exit as compared to the saturated vapor temperature indicated by the suction pressure.

EXHAUST – The airflow leaving the treated space.

EXTERNAL STATIC PRESSURE – The sum of the static and velocity pressures of a moving air system at the point of measurement.

FAHRENHEIT – A temperature scale with the freezing point of water 320 F and the boiling point 2120 F at sea level.

FEET PER MINUTE – A term assigned to a velocity of a moving air stream, usually express FPM.

FILTER – A device for removing dust particles from air or unwanted elements from liquids.

FILTER-DRIER – A device that removes moisture, acid and foreign matter from the refrigerant.

FLASH GAS – Instantaneous evaporation of some liquid refrigerant at the metering device due to pressure drop, which cools the remaining liquid refrigerant to, desired evaporation temperature.

FLOW HOOD – A diagnostic tool used to measure air flow through ducts, supply registers, and return grilles.

FORCED AIR – This describes a type of heating system that uses a blower motor to move air through the furnace and into the ductwork.

FURNACE – That part of an environmental system which converts gas, oil, electricity or other fuel into heat for distribution within a structure.

FREEZING POINT – The temperature at which the removal of any heat will begin a change of state from a liquid to a solid.

FREON – A refrigerant used in cooling units for air conditioning.

GAUGE PRESSURE – Pressure measured with atmospheric pressure as a base.

GEOTHERMAL EQUIPMENT – Heat pumps that uses the ground to transfer heat to and from the refrigerant in the unit. The unit circulates water through a heat exchanger in the to a closed loop buried in the ground or by pumping water from a well through the unit.

GPH (Gallons Per Hour) – You might see this rating if you are looking at an oil furnace. In addition to input and output, an oil furnace also has a rating of gallons per hour, the volume of oil a furnace is capable of burning in 60 minutes.

HCFC (Hydro chlorofluorocarbon) – A class of refrigerants. Generally refers to Halogenated Chlorofluorocarbon family of refrigerants.

HEAT – A form of energy causing the agitation of molecules within a substance.

HEAT EXCHANGER – A device for the transfer of heat energy from the source to the conveying medium.

HEAT GAIN – The amount of heat gained, measured in BTU’s, from a space to be conditioned, at the local summer outdoor design temperature and a specified indoor design condition.

HEAT FLOW – Heat flows from a warmer to a cooler substance. The rate depends upon the temperature difference, the area exposed and the type of material.

HEAT LOSS – The amount of heat lost, measured in BTU’s from a space to be conditioned, at the local winter outdoor design temperature and a specified indoor design condition.

HEAT OF COMPRESSION – The heat added to a vapor by the work done on it during compression.

HEAT OF THE LIQUID – The increase in total heat (Enthalpy) per pound of a saturated liquid as its temperature is increased above a chosen base temperature. (Usually – 400F for refrigerants).

HEAT PUMP – Compression cycle system used to supply heat to a temperature controlled space. Same system can also remove heat from the same space.

HEAT RECOVERY VENTILATOR (HRV) – This device bring fresh, outside air into a home while simultaneously exhausting stale indoor air outside. In the process of doing this, an HRV removes heat from the exhaust air and transfers it to the incoming air, pre-heating it.

HFC (HYDRO FLUOROCARBON) – A class of refrigerants. Generally refers to Hydro fluorocarbon family of refrigerants.

HUMIDIFIER – A device that adds moisture to warm air being circulated or directed into a space.

HUMIDISTAT – A device designed to regulate humidity input by reacting to changes in the moisture content of the air.

HUMIDITY – The amount of moisture in the air. Air conditioners remove moisture for added comfort.

HSPF – This is a measurement of a heat pump’s heating efficiency. There is no governmental minimum rating. The higher the HSPF, the more efficient the heat pump’s heating performance. HSPF stands for Heating Seasonal Performance Factor.

HEAT TRANSFER – The three methods of heat transfer are conduction, convection and radiation.

HVAC – Heating, Ventilating and Air Conditioning

HVAC/R – Heating, Ventilating, Air Conditioning, & Refrigeration

IAQ – Indoor Air Quality

INCHES OF MERCURY – Atmospheric pressure is equal to 29.92 inches of mercury.

INDOOR/EVAPORATOR COIL – If your furnace is the air handler section of your split system, then you’ll need an indoor coil added to your furnace to complete the system. The coil transfers heat to give you cool air and also aids in dehumidification.

INDUCTION – Process by which the primary air sets into motion an air volume, called secondary air, in the room.

INDUCTION RATIO (I) – Ratio of the total air flow rate to the primary air flow rate.

INFILTRATION – Air flow inward into a space through walls, leaks around doors and windows or through the building materials used in the structure.

ISO 9000 – A family of international standards for quality management and assurance by the ISO (International Standards Organization).

KW – kilowatt, equals 1,000 watts.

KWH – kilowatt hour is the amount of kilowatts of electricity used in one hour of operation of any equipment.

LATENT COOLING LOAD – The net amount of moisture added to the inside air by plants, people, cooking, infiltration, and any other moisture source. The amount of moisture in the air can be calculated from a combination of dry-bulb and wet-bulb temperature measurements.

LATENT HEAT – Heat that produces a change of state without a change in temperature.

LATENT HEAT OF CONDENSATION – The amount of heat energy in BTU’s that must be removed to change the state of one pound of a vapor to one pound of liquid at the same temperature.

LATENT HEAT OF FUSION – The amount of heat energy, in BTU’s required to change the state of one pound of a liquid to one pound of solid at the same temperature.

LATENT HEAT OF MELTING – The amount of heat energy, in BTU’S, that must be removed to change the state of one pound of solid to one pound of liquid at the same temperature.

LATENT HEAT OF VAPORIZATION – The amount of heat energy in BTU’s required to change the state of one pound of a liquid to one pound of vapor at the same temperature.

LIFT – To elevate a fluid from one level to a higher level.

LIQUID LINE – A tube used to convey the liquid refrigerant from the condenser outlet to the refrigerant control device of the evaporator.

LOAD ESTIMATE – A series of studies performed to determine the heating or cooling requirements of your home. An energy load analysis uses information such as the square footage of your home, window and door areas, insulation quality and local climate to determine the heating and cooling capacity needed by your furnace, heat pump or air conditioner.

MANOMETER – A tube filled with a liquid used to measure pressures.

MBH – One MBH is equivalent to 1,000 BTU’s per hour.

MEAN TEMPERATURE DIFFERENCES – The mean of difference between the temperature of a fluid receiving and a fluid yielding heat.

MELTING POINT – The temperature at which the addition of any heat will begin a change of state from a solid to a liquid.

MERCURY MANOMETER – Used to measure vacuum in inches of mercury.

MICRON – A unit used to measure high vacuums. One micron equals 1/25,400 of one-inch mercury.

MOLLIER CHART – A psychometric chart. How-to convert between Mollier and Psychometric charts?

MUFFLER – Device installed in hot gas line to silence discharge surges.

OIL SEPARATOR – A device for separating out oil entrained in the discharge gas from the compressor and returning it to the crankcase.

PACKAGED SYSTEM – Packaged units provide both heating and cooling from one unit that is placed outside the home – on the ground, on the roof, or sometimes mounted through the walls of the building. Packaged units come in several combinations of fuel sources – gas heat/electric cooling; heat pump; electric heat/electric cooling; oil heat/electric cooling.

PARTIAL PRESSURE – The pressure exerted by any individual gas in a mixture.

PASCALS (PA) – A small unit of air pressure.

PITCH – The slope of a pipeline for the purpose of improving drainage.

PITOT TUBE – A device comprising a small diameter orifice projecting directly into an air stream measuring total pressure and surrounded by an annular section with small diameter entrances normal to the flow, measuring static pressure; both sections are usually connected to a manometer to indicate velocity pressure.

PLENUM – Airflow passage made of duct board, metal, drywall, or wood. Joins supply and return ducts with HVAC equipment.

PRECHARGED LINES – Refrigerant line’s which are filled with refrigerant and are sealed at both ends. The seals are broken when the lines are installed and the line charge becomes part of the total system charge.

PRESSURE DROP – The decrease in pressure due to friction of a fluid or vapor as it passes through a tube or duct or/and lift.

PRESSURE – TEMPERATURE RELATIONSHIP – The change affected in temperature when pressure is changed or vice versa. Only used at saturated conditions. An increase in pressure results in a temperature increase. A decrease in temperature results in a pressure decrease.

PUMPDOWN – Process of pumping refrigerant out of the evaporator and suction line at the end of the on- cycle by closing a solenoid valve in the liquid line and letting the compressor shut-off by the low pressure control.

PSYCHROMETER – A devices having both a dry and wet bulb thermometer. It is used to determine the relative humidity in a conditioned space. Most have an indexed scale to allow direct conversion from the temperature readings to the percentage of relative humidity.

PSYCHROMETRIC CHART – A chart on which can be found the properties of air under varying conditions of temperature, water vapor content, volume, etc.

QUICK CONNECT – Name given to the end connections on pre-charged lines which screw on to mated fittings of the outdoor and indoor sections. Tightening the quick connections ruptures the seals on the fittings and the line charge becomes part of the total system charge.

RADIATION – The transfer of heat without an intervening medium. It is absorbed on contact with a solid surface.

RECEIVER – A vessel for holding refrigerant liquefied by the condenser.

RECIPROCATING COMPRESSOR – A type of compressor used in air conditioners that compresses refrigerant by using a type of “piston” action.

REFRIGERANT – A substance, which produces a refrigerating effect while expanding or vaporizing.

REFRIGERANT CONTROL – A device used to meter the amount of refrigerant to an evaporator. It also serves as a dividing point between the high and low-pressure sides of the system.

REFRIGERANT DISTRIBUTOR – A device which meters equal quantities of refrigerant to independent circuits in the evaporator coil.

REFRIGERANT MIGRATION – The movement of refrigerant through the system to the compressor crankcase during the off-cycle, caused by its attraction to oil.

REFRIGERANT OPERATING CHARGE – The total amount of refrigerant required by a system for correct operation.

REFRIGERANT VELOCITY – The rate at which refrigerant is moving at a given point in a system, usually given in feet per minute (FPM).

REFRIGERATION – The transfer of heat from a place where it is not wanted to a place where its presence is not desirable.

REFRIGERATION EFFECT – The amount of heat a given quantity of refrigerant will absorb in changing from a liquid to a vapor at a given evaporating pressure.

REGISTER – Combination grille and damper assembly covering an air opening or end of an air duct.

RELATIVE HUMIDITY – The percentage of water vapor present in a given quantity air compared to the amount it can hold at its temperature.

RELAY – A device used to open and close an electrical circuit. The relay may may be actuated by a bimetal electrically heated strip, a rod wrapped with a fine resistance wire causing expansion when energized, a bellows actuated by expansion of a fluid or gas or an electromagnetic coil.

REVERSING VALVE – A device in a heat pump that is electrically controlled to reverse the flow of refrigerant as the system is switched from cooling to heating; also called a four-way valve.

RETURN AIR – Air drawn into a heating unit after having been circulated from the heater’s output supply to a room.

RH – Relative Humidity

RISER – A vertical tube or pipe, which carries refrigerant in any form from a lower to a higher level.

RSES – A leading HVAC/R Association – Refrigeration Service Engineers Society –

SATURATED VAPOR – Vapor in contact with a liquid.

SATURATION – A condition of stable equilibrium of a vapor and a liquid.

SATURATION TEMPERATURE – Also referred to as the boiling point or the condensing temperature. This is the temperature at which a refrigerant will change state from a liquid to a vapor or visa versa.

SEER (SEASONAL ENERGY EFFICIENCY RATIO) – The total cooling of a central unitary air conditioner or unitary heat pump in Btu’s during its normal annual usage period for cooling divided by the total electric energy input in watt-hours during the same period.

SENSIBLE COOLING LOAD – The heat gain of the home due to conduction, solar radiation, infiltration, appliances, people, and pets. Burning a light bulb, for example, adds only sensible load to the house. This sensible load raises the dry-bulb temperature.

SENSIBLE HEAT – Heat that can be measured or felt. Sensible heat always causes a temperature rise.

SETPOINT – The temperature to which a thermostat is set to result in a desired heated space temperature.

SIGHT GLASS – A glass installed in the liquid line permitting visual inspection of the liquid refrigerant for the purpose of detecting vapor in the liquid. They also generally have a device included to monitor moisture content of the refrigerant.

SIZING – refers to the procedure a heating contractor goes through to determine how large a furnace (measured in BTUH) is needed to heat a house efficiently.

SLUGGING – A condition in which a quantity of liquid enters the compressor causing hammering and possible compressor damage.

SOUND ATTENUATORS – Components, which are inserted into the air distribution system and designed to reduce airborne noise, which is propagated along the ducts.

SPECIFIC HEAT – The amount of heat necessary to change the temperature of one pound of a substance 100 F.

SPECIFIC VOLUME – The volume of a substance per unit of mass; i.e., standard air 13.33 cubic feet per pound. The reciprocal of density.

SPLIT SYSTEM – Refrigeration or air conditioning installation, which places condensing unit outside or away from evaporator. These unit are connected together by a supply and return refrigerant lines.

SPREAD (LS) – Maximum distance between two vertical planes tangent to a specified envelope and perpendicular to a plane through the core center. The spread are generally referred to the envelope corresponding to 0.25 m/s for zero supply temperature differential (i.e., under isothermal conditions).

SUBCOOLED LIQUID – Liquid refrigerant which is cooled below its saturation temperature.

SUPERHEATED VAPOR – Refrigerant vapor which is heated above its saturation temperature. If a refrigerant is superheated, there is no liquid present.

SUPPLY – The ductwork that carries air from the air handler to the rooms in the house.

STANDARD AIR DENSITY – .075 pounds per cubic foot. Equivalent to dry air at 700 F and at sea level pressure.

STATE CONDITION – Substances can exist in three states – solid, liquid or vapor.

STATIC PRESSURE – The normal force per unit area at a small hole in the wall of a duct.

STATIC TAP – A means by which static pressures of a duct system may be read directly, usually consisting of a small diameter hole in the side of the duct connected to a manometer.

SUB COOLING – Cooling of a liquid, at a constant pressure, below the point at which it was condensed.

SUBLIMATION – A condition where a substance changes from a solid to a gas without becoming a liquid.

SUCTION LINE – A tube used to convey the refrigerant vapor from the evaporator outlet to the suction inlet of compressor.

SUCTION LINE ACCUMULATOR – A device located in the suction line that intercepts quantities of a liquid refrigerant and thereby prevents damage to the compressor.

SUPERHEAT – Heat added to a vapor after all liquid has been vaporized.

SWITCHOVER VALVE – A device in a heat pump that reverses the flow of refrigerant as the system is switched from cooling to heating. Also called a reversing valve or four-way valve.

TEMPERATURE – A measurement of heat intensity.

THERM – Another measurement of heat. One therm equals 100,000 BTUH.

THERMISTOR – Basically a semi-conductor, which has electrical resistance, that varies inversely with temperature.

THERMOSTAT – A bimetal actuated switch to close and open a circuit to indicate or terminate operation of a heating or air conditioning system.

THERMOSTATIC EXPANSION VALVE – Refrigerant control which monitors the flow rate according to the superheat at the evaporator outlet.

THROW (LT) – The maximum distance between the center of the core and a plane which is tangent to a specified envelope and perpendicular to the intended direction of flow The throw is generally referred to as the envelope corresponding to 0.25 m/s for zero supply temperature differential (i.e., under isothermal conditions).

TIME DELAY – Usually refers to a device that will not allow the condenser to restart for an average of 5 minutes.

TON – You’ll often see this as a measurement of the capacity of an air conditioning system. Don’t panic, it doesn’t measure weight! Just like gas and oil furnaces, air conditioners and heat pumps are rated in Btus. One ton of air conditioning is 12,000 Btus per hour. This means that a “one ton” air conditioning system has the capability of removing 12,000 Btus of heat per hour from your home.

TON OF REFRIGERATION – The amount of heat necessary to completely melt one ton of 32oF ice in 24 hours.

TOTAL AIR FLOW RATE (QL) – Sum of the primary and secondary air flow rates which are moved in the treated space.

TOTAL HEAT (ENTHALPY) – Total heat energy in a substance. The sum of sensible and latent heat.

TOTAL PRESSURE – The sum of all partial pressures in a mixture of gases.

TRAP – A depression or dip in refrigerant piping in which oil will collect. A trap may be placed at the base of a suction or hot gas riser to improve oil return up the riser.

UPFLOW – A type of furnace that draws cool air from the bottom and blows the warmed air out the top into the ductwork. This type of furnace is usually installed in a basement or an out-of-the-way closet.

VACUUM – Any pressure below atmospheric pressure.

VAPOR BARRIER – The term applied to an impervious layer of material superimposed upon a layer of insulation. Vapor barriers are always applied on the warm side of the insulation layer.

VAPOR PRESSURE – The pressure exerted by vapor.

VAV – Variable Air Volume.

VELOCITY PRESSURE – In a moving fluid, the pressure capable of causing an equivalent velocity as applied to move the same fluid through an orifice such that all pressure energy expanded is converted into kinetic energy.

VENTILATOR – Captures heating or cooling energy from stale indoor air and transfers it to fresh incoming air.

VFD (VARIABLE FREQUENCY DRIVE) – Electronic speed control for motors.

WATER MANOMETER – Used to measure pressure in inches of water.

WATT (W) – a unit of electricity.

WATER COLUMN (WC) – Common measure of air pressure used in HVAC systems.

WET BULB (WB) TEMPERATURE – Temperature read with a thermometer whose bulb is encased in a wetted wick.

ZONE – 1) Conditioned space in a house under the control of a thermostat. 2) A space within a house with a distinct pressure compared to other pressure zones.

ZONING – A system in which living areas or groups of rooms are divided into separate spaces and each space’s heating/air conditioning is controlled independently.