Glossary of Commonly Used Terms

As with most technical industries there’s a fair bit of technical jargon in HVAC, a lot of specific terms and acronyms that might come across a little overwhelming to the uninitiated. For an explanation to HVAC please read our guide. 

With four decades experience in the design, supply and manufacture of portable air conditioners, large ventilation fans, Industrial electric fan heaters, large cooling fans, portable boilers and commercial dehumidifiers we’ve come to grips with HVAC tech speak. With that in mind we have put together this list of HVAC terms and their explanations. If you can’t find the term you’re looking for or know of one we have missed out let us know at

Glossary of Commonly Used HVAC Terms

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

ABSOLUTE PRESSURE – Pressure measured with the base of zero.

ABSOLUTE TEMPERATURE – A temperature scale expressed in degrees oF or oC using absolute zero as a base.

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

AIR HANDLER – The name for the blower fan, the heating coil, and the materials that house them. They are often connected to heating or cooling equipment to help with distribution.

AIRFLOW– The distribution or movement of air.

AIR, STANDARD CONDITIONS – Standardised conditions at which capacity ratings for air conditioning equipment is rated.

BLOWER FAN – A fan used to move air over heating/cooling mechanisms in a given unit.

BOILER – A fuel-burning apparatus or container for heating water.

BTUs – An abbreviation for British Thermal Units, the measurement for heat output.

BTUH – British thermal units per hour.

CAPILLARY TUBE – A refrigerant control consisting of a small diameter tube which controls flow by restriction. Each tube is carefully sized by inside diameter and length for every unique application.

CARBON MONOXIDE – An odourless, colourless, tasteless, poisonous and flammable gas that is produced when carbon burns with insufficient air. Symptoms of exposure are similar to the flu and high levels of exposure can be lethal.

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

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

COMPRESSOR – The compressor applies pressure to the coolant so it can move through the system.

CONDENSER – The condenser puts pressure on the coolant, changing it from a gas to a liquid.

CONDENSER COIL – The outdoor portion of an air con unit or heat pump that either releases or collects heat, depending upon its application at that point in time.

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

CONDENSING UNIT – The area in a refrigeration system where the compression and condensation of refrigerant is carried out.

CONDUCTION – The transfer of heat from molecule to molecule within a substance.

CONVECTION – The transfer of heat by a moving fluid.

COEFFICIENT OF PERFORMANCE (COP) – Ratio of work performed or accomplished as compared to the energy used.

CUBIC FEET PER MINUTE (CFM) – Stands for Cubic Feet per Minute. This measurement indicates how many cubic feet of air pass by a stationary point in one minute. Often used in relation to ductwork. The higher the number, the more air is being moved through the ductwork by the system. Often abbreviated to CFM.

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

DAMPERS – These control the direction of the airflow in your system.

DIRECT FIRED HEATER – a heater with an airflow that is warmed directly over a flame.

DISCHARGE LINE – A tube used to move compressed refrigerant vapour from the compressor to the condenser inlet.

DUCTS – Ducts are the tubes or ‘tunnels’ that carry conditioned air (hot or cold) througha building. Supply ducts carry conditioned air through a building while return ducts bring air back to the unit for conditioning.

DRY BULB TEMPERATURE – Temperature read with an ordinary thermometer.

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

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).

EVAPORATIVE COOLING – The cooling effect of vapourisation of a liquid in a moving air stream. More information on evaporative coolers can be found here.

EVAPORATOR COIL – The part of the portable air conditioning unit or heat pump that is located inside the air handler or attached to the furnace. Its primary function is to absorb the heat from the air in your space.

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

EXHAUST TUBE AIR CONDITIONER – A large portable air conditioner that ventilates processed warm air via an external exhaust tube.

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

FAN – an apparatus with rotating blades that creates a current of air for cooling or ventilation.

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

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

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

FREON – A coolant that was once highly popular, but has been phased out due to environmental concerns. More information.

FUEL EFFICIENCY – A comparison of the amount of heat your unit produces vs. the amount of energy/fuel it burns.

HVAC – An abbreviation of the words heating, ventilation and air conditioning (just in case you didn’t know!)

HEAT EXCHANGER – A heat exchanger is a piece of equipment that facilitates the transfer of heat from one medium to another. The medium can either be separated by a wall or plate to avoid cross-contamination or can come into direct contact.

HEAT OF COMPRESSION – Heat added to a vapour by the work carried out during compression.

HEAT PUMP – A heat pump is a HVAC unit that heats or cools by moving heat. During the winter, a heat pump draws heat from outdoor air and circulates it through your home’s air ducts. In the summer, it reverses the process and removes heat from your house and releases it outdoors.

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

INDIRECT FIRED HEATER – a heater with a heat exchanger that warms air that passes over it.

LATENT HEAT – Heat that produces a change of state without a change in temperature; i.e., ice to water at 32 oF or water to steam at 212oF.

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 vapour 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 vapour at the same temperature.

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

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

MBH (Thousands of BTUs per hour) – 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.

MUFFLER – A device installed in a 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.

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

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

PITOT TUBE – A device comprising a small diameter orifice projecting directly into an air stream. It measures total pressure and is 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.

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 vapour as it passes through a tube, duct or lift.

PSYCHROMETER – A device 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 vapour content, volume, etc. You can view a chart here.

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

QUICK CONNECT – Name given to the end connections on precharged 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.

R-22 REFRIGERANT – The old standard for residential air conditioners, R-22 refrigerant has now been phased out globally.

R-410A REFRIGERANT – A chlorine-free refrigerant that became a commonly used alternative to R22 post phase out. However, this is now also being phased out in favour of less environmentally harmful refrigerants such as R290, R32 etc.

RADIATION – The transfer of heat without an intervening medium. It is absorbed on contact with a solid surface. This method is used on some types of space heaters.

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

REFRIGERANT – A chemical that produces a cooling effect while expanding or vaporizing.

REFRIGERANT CONTROL – A device used to meter the amount of refrigerant supplied 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 EFFECT – The amount of heat a given quantity of refrigerant will absorb in changing from a liquid to a vapour at a given evaporating pressure.

RELAY – A device used to open and close an electrical circuit.

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.

SEER – An abbreviation for Seasonal Energy Efficiency Ratio. SEER measures the efficiency of your air conditioner and heat pump.

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

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

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

SPACE HEATER – a self-contained appliance for heating an enclosed space within a building.

SPECIFIC HEAT – The amount of heat necessary to change the temperature of one pound of a substance 10 F.

SPECIFIC VOLUME – The volume of a substance per unit of mass.

SPLIT SYSTEM – An HVAC system in which some components are located inside the structure of the house and some are located outside. Split systems should be matched for optimal efficiency.

STANDARD AIR DENSITY – 0.075 pounds per cubic foot. Equivalent to dry air at 70 oF and at sea level pressure.

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 vapour from the evaporator outlet to the suction inlet of a 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 vaporised.

THERMIDISTAT – Monitors temperature and humidity and adjusts a heating or cooling system to maintain desired levels.

THERMOSTAT – Usually found on an inside wall, this device operates as a control to regulate your heating and cooling equipment, allowing you to adjust your home comfort at the touch of a switch.

TON – Unit of measurement for determining cooling capacity. One ton equals 12,000 Btuh.

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.

TWO-STAGE OPERATION – Provides two levels of heating or cooling output for greater temperature control, energy efficiency and improved indoor air quality.

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

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 – A system that exchanges stale, recirculated indoor air with fresh, filtered outside air.

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

ZONE – The area that is heated/cooled by a given thermostat.

For full details on our range of portable air conditioner units, industrial electric heaters, warehouse fans, building site dehumidifiers, evaporative coolers and portable boilers please call on 01527 830610, where a member of our expert team will be available to help. Alternatively you can also find details and specifications on our website.