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Absolute Humidity

Actual mass of water vapour in the air per unit volume.

Air Conditioner (AC)

An air conditioner is a type of heat pump in which heat is removed from the indoor air and rejected to the outdoor air, thereby conditioning the air in the home by reducing its temperature and humidity.  Dehumidification is achieved because as heat is removed from warm humid air, some of the moisture in the air condenses on the heat exchanger coil and collects in a drain pan below.  Air conditioners differ from other heat pumps in that they can only be used to cool a home.

Air Handler

Part of the heating and cooling system that conditions and circulates air, often in a large metal box containing a blower, heating/cooling elements, filters, sound attenuators, and dampers. They are usually connected to a ductwork ventilation system that distributes the air through the building and then back to the air handler.

Air Source Heat Pump (ASHP)

A heat pump system that pulls heat from or rejects heat to outdoor air, usually through the use of a fan and heat exchanger unit installed outside. Air source heat pumps function much like ground or water source heat pumps, but have the added complexity of dealing with more drastic temperature ranges, as well as the formation of frost or ice on the coil during some winter conditions. Frost or ice buildup is commonly dealt with by periodically running the heat pump in a “defrost cycle”, in which the heat pump is temporarily reversed and the outdoor coil is heated.


A boiler is a closed vessel that is used to heat a fluid, typically water, and circulate it through a closed loop for various heating applications. Boilers generally use natural gas or propane as fuel.

British Thermal Unit (BTU)

A British thermal unit is a measure of energy and is equal to approximately 1055 joules. In the HVAC industry, BTUs are used to describe heat energy. A BTU is defined as the amount of heat energy needed to raise the temperature of 1 pound of liquid water by 1°F at one atmosphere.

British Thermal Unit Per Hour (BTU/h or BTUH)

A British thermal unit per hour is a power measurement. In the HVAC industry, it is typically used to describe the output capacity of a heating system or the amount of heating power required by a building under design conditions.

Buffer Tank

A storage tank used to increase the overall thermal mass and storage of a system. This helps to deal with the drastic variations of heating and cooling loads of a building throughout the year and greatly reduces short cycling of the system. In heat pump applications, the heat pump operates to maintain the buffer tank at a set temperature and the buffer tank is used to meet the loads of the system.


A pump used to circulate a fluid in a closed circuit. These pumps only need to overcome the friction losses of the piping in the system. Circulators are commonly used in hydronic heating and cooling applications.

Coefficient of Performance (COP)

The ratio of the heat energy delivered by a heat pump to the amount of electrical energy consumed by its components (compressor, pumps, fans, etc.). The COP is a measure of heat pump efficiency, and varies with operating conditions.

Combo System

A combo system provides hot water for both space heating and domestic usage. The same water source is used for both applications. For example, a building may have a hot water tank used to provide domestic water as well as supply a hydronic air handler for space heating.


A mechanical device that increases the pressure of a gas by decreasing its volume. Compressors are one of the major components of a heat pump, and the quality and type used can greatly impact overall heat pump performance.


The liquid formed by condensation, which is the change of state from a gas to a liquid. Water condensate frequently appears on the indoor evaporator coil of an air conditioner, where it is collected in a drain pan and piped or pumped to a floor drain.


A heat exchanger in which heat is removed from a fluid as it changes phase from gas to liquid. Such a phase change is known as “condensation”. In an air-conditioner, the condenser delivers heat to the outdoor air. In a reversing heat pump, the condenser and evaporator change roles with the seasons.

Defrost Cycle

In order to remove excessive ice or frost buildup on an air source heat pump’s outdoor coil, a heat pump will periodically run a “defrost cycle”, in which it is temporarily reversed and the outdoor coil is heated. Some air source heat pumpsrun defrost cycles based only on time (e.g., defrost for 10 minutes every hour), while others are more sophisticated and control defrost based on actual measured refrigerant and outdoor air conditions.


The process of removing moisture from the air in order to decrease humidity and increase comfort. This process is typically achieved by cooling the air below its dew point, which causes some of the moisture in the air to condense.

Design Day

A representative winter or summer day, based on historical high and low temperature trends, and used to calculate the heating or cooling load for a given home. For example, Toronto has a winter design day of 0°F (-18°C), while Ottawa has a winter design day of -13°F (-25°C).

Desuperheater/ “Hot Water Booster”

An auxiliary heat exchanger that is present on some heat pumps and is used to heat domestic water.  A desuperheater removes some heat from the “superheated vapour” leaving the compressor and transfers this heat to domestic water.  This means that during the cooling season, some of the excess heat that would normally be rejected to the outside ground or air is instead reclaimed, providing a “free” supply of hot water.

Digital Scroll Compressor

Digital scroll compressors achieve variable capacity operation by engaging and disengaging the scroll mechanism in cycles at a few times each minute, and so maintaining intermediate refrigerant flow rates and system pressures. Because the compressor motor continues to run, the system experiences very little energy penalty. Digital scroll compressors allow heat pumps to run anywhere from 10% to 100%, providing the best available range of capacity control.

Direct Exchange (DX) Heat Pump

A direct exchange (DX) heat pump is a ground source heat pump that uses copper refrigerant tubing installed in the ground to transfer heat between the refrigerant and the soil. Using refrigerant tubing in the ground instead of plastic water piping eliminates the need for an additional water-to-refrigerant heat exchanger and circulating pump, thereby increasing system performance.

Diverting Valve

A valve used to split the flow of a fluid into two directions. In a heat pump, diverting valves can be used to redirect refrigerant from one load to another, allowing a single heat pump to be used to heat a home’s air as well as provide hot domestic water.

Domestic Hot Water (DHW)

The water used in residential and office buildings for everyday purposes, such as cooking, washing dishes, cleaning, or bathing.

Domestic On-demand

A feature on some heat pumps, describing their ability to deliver domestic hot water in addition to their primary heating/cooling. Heat pumps with domestic on-demand have a diverting valve and an additional refrigerant to water heat exchanger. Despite the term “on-demand”, most heat pump systems require a storage tank for domestic hot water.

EC motor vs. PSC motor

An EC (electronically commutated) motor is a high efficiency programmable brushless DC motor that consists of a permanent magnet rotor and a built-in inverter. A PSC (permanent split capacitor) motor is an AC motor with a run capacitor permanently connected in series with the start winding. PSC motors are   not as efficient as EC motors.

Electronic Expansion Valve (EXV)

An electronic device that controls the refrigerant flow rate in a heat pump through a stepper motor valve, continuously adjusted through an electronic controller based on measured refrigerant conditions. The goal of this device is to ensure that the refrigerant is fully evaporated as it leaves the evaporator, in order to maximize heat transfer capacity and prevent any liquid from entering the compressor. The device does this by very accurately maintaining a set level of “superheat”. EX valves have a much faster response than thermal expansion valves, which means better refrigerant control and better heat pump performance. Heat pumps that use Digital Scroll compressor technology require this fast response, since they change conditions drastically during normal operation.

Energy Efficiency Ratio (EER)

A measure of energy efficiency for an air-conditioner or a heat pump in cooling mode. It is equal to the ratio of total cooling capacity (in btu/h) to electrical energy input (in watts).


A measure of the total internal energy of a system. As enthalpy cannot be measured directly, quantities are typically presented as a change in enthalpy over a given process. Enthalpy change takes into account heat transfer, moisture addition, and change in volume or pressure.

Expansion or Throttling device

A device or valve in a heat pump that is used to control refrigerant flow and maintain a difference between the high- and low-pressure sides of the system. In simple heat pumps, this device can be simply a fixed orifice (a small piece of metal with a pinhole opening). In more sophisticated heat pumps, the opening of the expansion valve is actively controlled in order to maintain ideal refrigerant conditions for various capacities, improving system performance and reliability.

Forced Air Heating/Cooling

In general, all forced air heating/cooling systems consist of an air filter, blower, heat exchanger(s), and controls. In heating, the blower draws in air from various locations in the building via the return plenum and reheats the air at the heat exchanger. The air is then distributed throughout the building using the supply branches and grilles attached to the supply plenum. The process is the same in cooling, except a heat exchanger is used to remove energy (sensible and latent) from the return air in order to cool and dehumidify it before it is distributed back throughout the building.

Freeze protection (Glycol /Antifreeze)

Used in place of water as the working fluid in hydronic systems that may be subjected to temperatures below the freezing point of water (0°C). Typically, the fluid is a water-antifreeze mixture in order to lower the freezing point of the fluid to protect the system from freezing. The percentage and type of antifreeze used will determine the freezing point (freeze protection) and heat transfer properties of the fluid.

Global Warming Potential (GWP)

A relative measurement of how much effect a given refrigerant will have on Global Warming compared to carbon dioxide, which has a GWP of 1. The lower the GWP, the better the refrigerant is for the environment. The value is based on the heat absorbing ability of the gas relative to carbon dioxide.

Ground Source Heat Pump (GSHP)

A ground source heat pump, or geothermal heat pump, pulls heat from or dumps heat to water taken from the ground. The moderate temperature of the ground lets it act as a heat source during the cooler months, and a heat sink during the warmer seasons. Ground source heat pump systems can be “closed loop”, in which a mass of water or antifreeze solution is continually circulated through the heat pump and heat exchanger pipes installed in the ground, or “open loop”, in which water is pulled from a nearby lake or well and later rejected back to the surface or into a different well.

Heat Exchanger

A heat exchanger is a component used to transfer heat/energy from one fluid (such as water) to another. They come in various shapes and sizes depending on the application. Typical heat exchangers in the HVAC industry deal primarily with any combination of air, water, and refrigerant.

Heat Loss/ Heat Gain (HL/HG)

The amount of heat lost from a building on a cold day or gained by a building on a hot day. When sizing heating and cooling equipment, heat loss/heat gain calculations are typically performed based on the temperature conditions of a representative winter and summer day (known as a “design day”). Design day heat loss is used to size heating equipment, while design day heat gain is used to size cooling equipment. Heat loss/heat gain are typically measured in BTUH or BTU/h (British Thermal Units per Hour) and are affected by the difference between indoor and outdoor temperature, surface area of the building, level of insulation, quality and airtightness of windows and doors, heat and moisture given off by people and appliances, etc.

Heat Pump (HPP)

A heat pump uses mechanical work to move heat from a mass (source) to a warmer mass (load), going against the natural tendency for heat to move from hot to cold. Heat pumps work by taking advantage of the heat transfer that takes place when a fluid changes phase from liquid to gas and vice-versa, in a process known as the “refrigeration cycle”. Most heat pumps consist of a compressor to pressurize a refrigerant, two heat exchangers to transfer heat between the refrigerant and air or water, and some type of expansion valve. Heat pumps can be found in varieties that can only be used for heating, though “reversing” models are commonly available which can be used to both heat and cool a home. This switching ability is achieved by reversing the direction of refrigerant flow via a “reversing valve”.

Heat Recovery Ventilator (HRV) & Energy Recovery Ventilator (ERV)

Heat or Energy Recovery Ventilators are used to reduce the energy cost of bringing in fresh air for ventilation of a building. The incoming fresh (outdoor) air is passed through a heat exchanger core, allowing it to recover heat and/or moisture from the outgoing exhaust air. This procedure preconditions the fresh air, reducing the amount of energy required to bring it to comfortable indoor conditions. ERVs transfer both heat and moisture, while HRVs transfer only heat. Recovery Ventilators pre-cool and dehumidify fresh air during warmer seasons, and pre-heat and humidify fresh air during colder seasons.


HVAC stands for heating, ventilation, and air conditioning. It represents the industry and technology of indoor environmental comfort.

Hydronic Coil (or Heat Exchanger)

A water-to-air heat exchanger commonly found in forced-air heating and cooling systems. Hydronic coils use hot or chilled water to heat or cool air as it passes through. Gas-burning appliances are traditionally used to supply hot water to hydronic coils, though it is becoming more common to use heat pumps to supply both hot and chilled water.

Hydronic Pump Module

A family of Ecologix products that consists of assemblies of pumps, valves, pipes, and gauges designed to meet a variety of common plumbing needs for the heating and cooling industry. Specialized hydronic pump modules exist for applications such as zoning of in-floor heating, temperature control, pool heating, snow melting, etc.

In-floor heating

In-floor heating is a form of radiant heating and there are two types: electric and hydronic. In general, heat is applied within or underneath the floor, which warms the floor and allows it to radiate heat up into the conditioned space. Electric in-floor heat converts electricity into heat energy using resistance wires. These resistance wires are installed within or underneath the floor in a grid-like pattern. The circuits are generally controlled using a thermostat to turn the unit on. Hydronic in-floor heating systems circulate hot water through closed loop PEX pipe circuits installed within or underneath the floor. The return water circulates back to the heat source to be reheated. The system is also generally controlled using a thermostat to turn on the circulating pump(s) and the heating source. The output of the system is determined by the spacing of the pipes, the supply water temperature and flow rate, and the floor covering.

Inverter drive or Variable Frequency Drive (VFD)

Inverter drives, or variable frequency drives (VFDs), control the speed of an alternating current motor by controlling the frequency of electrical power supplied to the motor. This reduces the amount of energy required and makes operation smoother, increasing the life of the motor. Heat pumps with inverter drive compressor motors can operate at a range of capacities, typically from about 50% to 110% of full-load conditions.

Kilowatt (kW)

A standard unit of measurement used for electrical power (energy rate). 1 kilowatt is equal to 1000 watts and 1 watt is equal to 1 joule per second.

Kilowatt-hour (kWh)

A unit of energy most commonly used for billing energy delivered by electric utility companies. For example, a device rated at 1 kW that operates for one hour would consume 1 kWh of electrical energy. 1 kWh = 3.6 MJ

Liquid Injection

Liquid injection is a method that can be used in the refrigeration cycle to allow the heat pump to operate at lower source temperatures without overheating the compressor. A small amount of liquid refrigerant leaving the condenser bypasses the expansion valve and outdoor coil (evapourator) and is injected into the low pressure side of the compressor.

Outdoor Condenser

The outdoor portion of an air conditioning system, which consists of a heat exchanger, fan, and typically a compressor. The fan pushes outdoor air past the heat exchanger, removing heat from the refrigerant as it cools and condenses. For reversible air source heat pumps, the outdoor condenser is more appropriately referred to as an outdoor module or outdoor unit, since in the heating season it actually acts as an evaporator.

Ozone Depletion Potential (ODP) (wrt regfrigerants)

The potential of a given refrigerant to destroy the ozone layer. The refrigerant R11 is used as the datum reference point and has an ODP of 1.0. The smaller the ODP value, the better the refrigerant is for the environment.

Packaged Terminal Air Conditioner (PTAC)

A Packaged Terminal Air Conditioner (PTAC) is a self-contained air-conditioning unit installed through an exterior wall. A PTAC may be connected to ductwork or include electrical resistance heating. It often uses only electricity to heat or cool.


A plenum is a contained pathway for directing air either to or from a heating or air conditioning system. The supply and return air pathways typically branch off the main plenums attached to the heating/cooling system.

Pressure drop

The decrease in pressure of a fluid from one point in a flow path to another. It is the result of resistance to the fluid flow due to the combination of the fluid viscosity and the velocity of the flow. Higher velocities and viscosities will result in larger pressure drops.

Pulse Width Modulation (PWM)

Pulse width modulation is a technique commonly used for controlling the power to inertial electrical devices, such as motors. PWM is used as a speed control technique by cycling the power off and on to the device. The higher the percentage of time the power is on, the higher the output of the device being controlled.


A mechanical device used for moving a fluid by means of a piston, plunger, or a set of rotating vanes. A pump consumes energy, typically electricity, and converts it to mechanical work in order to move the fluid.

“Puron” or R-410A

R-410A is a refrigerant used in many residential and commercial heat pump and air conditioning applications. It is a blend of difluoromethane (R-32) and pentafluoroethane (R-125) refrigerants and, because it contains no chlorine, will not contribute to depletion of the ozone layer. Therefore it is more environmentally friendly than older refrigerants such as R-22.

Radiant Heating

Radiant heating transfers heat from a higher temperature thermal mass to a lower temperature thermal mass through radiation heat transfer.

Reciprocating Compressor (or Piston compressor)

A type of compressor that uses pistons to drive a gas into a cylinder, increasing its pressure until it is finally discharged. The pistons are driven in a reciprocating (back-and-forth) motion by a crankshaft, with workings similar to that of a typical car engine. Such compressors can be capable of achieving very high pressures, but are less efficient and more susceptible to damage due to entering liquid.


A refrigerant is a substance that is used as the working fluid in a heat pump/ refrigeration cycle. The refrigerant undergoes a change of phase from liquid to gas in the evaporator and from gas to liquid in the condenser, allowing it to absorb and release a large amount of heat energy in relation to its volume.

Relative Humidity

The amount of water vapour in the air relative to a saturated mixture at the same temperature and pressure.

Reversing Valve

A valve that changes the direction of refrigerant flow in a heat pump, allowing it to switch between heating and cooling modes. A heat pump that has a reversing valve is known as a reversible or reversing heat pump. The use of a reversing valve allows a building to be heated and cooled by a single piece of equipment.

Scroll Compressor

A type of compressor that increases the pressure of a gas by forcing it between two interleaved scrolls, or spirals, while one moves in a circular motion inside the other. This motion forces the gas into smaller and smaller spaces as it moves through. Scroll compressors operate more quietly and efficiently than reciprocating compressors, and are used almost exclusively in modern residential and small commercial heat pumps. Scroll compressors also tend to be more robust, as the scroll mechanism can handle small amounts of liquid without causing failure.

Seasonal Energy Efficiency Ratio (SEER)

A measure of equipment energy efficiency over a typical cooling season. This ratio represents the total cooling output of an air conditioner or a heat pump during the normal cooling season (in BTU) divided by the total electricity (in watt-hours) consumed during the same period.

Sensible heat-recovery efficiency (SRE) & Total Energy-Recovery Efficiency (TRE)

SRE or TRE A measure of how effectively a Heat or Energy Recovery Ventilator (HRV or ERV) recovers the energy from the exhausted air. Measurements of effectiveness take into account fan energy, leakage (exhaust air transfer), mass and flow imbalance, frost control, and other external and internal energy gains and losses. SRE applies to HRVs and is based on heat transfer only. TRE applies to ERVs and is based on both heat and humidity transfer.

Solar Heat Gain

Heat gain due to the effects of sun shining in through a window and being absorbed by items within the building. A large window facing the sun can be a significant contributor to heat gain, and can lead to a substantial amount of energy required in order to maintain comfortable indoor temperatures.

Subcooled Liquid & Subcool

A liquid that is cooler than the temperature at which it would begin to evaporate at a given pressure. That is, it is colder than its saturation point. The temperature difference between a subcooled liquid and its saturation point is known as “subcool”, and is a measure commonly used by heat pump technicians. In order to get the most heat transfer capacity out of a heat pump, it is important to ensure that refrigerant is leaving the condenser in a subcooled condition.

Superheated Vapor & Superheat

A gas that is hotter than the temperature at which it would begin to condense into a liquid at a given pressure. That is, it is hotter than its saturation point. The temperature difference between a superheated vapor and its saturation point is known as “superheat”, and is a measure commonly used by heat pump technicians. In order to get the most heat transfer capacity out of a heat pump, it is important to ensure that refrigerant is leaving the evaporator in a superheated condition.

Tankless Water Heater (e.g. Rinnai)

Also known as an instantaneous, continuous flow, inline, flash, or on-demand water heater. The device heats water as the water flows through it, giving the user an endless supply of hot water without requiring a storage tank. This eliminates standby losses. When there is a demand for hot water, the tankless water heater senses the flow through the unit and fires to heat the water up to the required temperature as it circulates through the copper heat exchanger. Tankless water heaters can be used for both space heating and domestic water.

Thermal Expansion Valve (TXV or TEV)

A mechanical device that controls the refrigerant flow rate in a heat pump, based on indirectly measured refrigerant temperature and pressure. TX valves use a gas-filled temperature sensing bulb and a diaphragm that opens a valve as the temperature increases. The goal of this device is to ensure that the refrigerant is fully evaporated as it leaves the evaporator, in order to maximize heat transfer capacity and prevent any liquid from entering the compressor. It does this by maintaining a set level of “superheat”. Systems with a TX valve are usually more efficient than those with a fixed orifice expansion device.

Two-stage compressors

Two-stage compressors can operate at full-load (100%) and part-load (typically about 70%) capacities, allowing the heat pump to better match the current heating or cooling requirement. In scroll compressors, part-load capacities are achieved by allowing the gas to discharge from the scroll mechanism before fully reaching the end.

Vapour injection

Vapour injection is a method that can be used in the refrigeration cycle to allow the heat pump to operate at lower source temperatures without overheating the compressor. A small amount of liquid refrigerant leaving the condenser is diverted from the main cycle through an additional thermal expansion valve and then a small heat exchanger. In this heat exchanger, the refrigerant crosses paths with the liquid refrigerant out of the condenser causing it to evapourate before being injected into the compressor on the low pressure side. Vapour injection proves to be more efficient than liquid injection by providing a slight boost in capacity at the lower temperatures.

Variable Capacity

The performance of a heating or cooling device can be improved through the use of variable capacity technology, which allows the device’s capacity to be reduced to better match the current demand. Performance improvements are due in part to a reduction of “short-cycling”, in which an oversized heating or cooling device quickly satisfies a call for heating or cooling and so cycles on and off over only a few minutes. In the context of heat pumps, a variety of variable capacity compressor technologies can be used.

Water source heat pump (WSHP)

A heat pump that pulls heat from or rejects heat to outside water, usually by using a pump to circulate it through a heat exchanger. Outside water can be taken from a nearby lake or river, or can be pumped from a ground well. Water source heat pumps are also commonly referred to as ground-source heat pumps.

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