HVAC Glossary: Furnace, AC & Water Heater Terms

The words on a quote, a fault label or a home-inspection report, each defined in one sentence, then explained for an Ontario house: what the part does, how it fails and which repair or installation deals with it.

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14 terms

Furnace parts

Blower motor: A blower motor is the electric motor that spins the furnace's squirrel-cage fan to push heated or cooled air through the ducts, and the same motor serves the furnace, the central air conditioner and continuous-fan mode.
Condensate pump: A condensate pump is a small float-activated pump that collects the water produced by a high-efficiency furnace, air conditioner coil or dehumidifier and pushes it through a thin tube to a drain when no floor drain is close enough for gravity.
Condensate trap: A condensate trap is the water-filled trap on a high-efficiency furnace's drain that lets acidic condensate flow out while blocking flue gases from escaping and holding the draft pressure the pressure switch needs.
ECM motor: An ECM (electronically commutated motor) is a brushless DC blower motor with a built-in electronic control that varies its speed to match the airflow needed, using far less electricity than the older fixed-speed PSC (permanent split capacitor) motor.
Flame sensor: A flame sensor is a thin metal rod sitting in the burner flame that proves the burners are lit by passing a tiny DC current, measured in microamps, back to the control board, which closes the gas valve within seconds if that signal is missing.
Furnace control board: A furnace control board, also called an integrated furnace control, is the circuit board that runs every heating cycle in sequence, from inducer and igniter to gas valve, flame proving and blower, and stores the fault codes that flash on its LED.
Furnace gas valve: A furnace gas valve is an electrically operated safety valve that opens on a 24-volt signal from the control board to feed gas to the burners at a regulated manifold pressure, and closes as soon as flame is no longer proven.
Furnace pressure switch: A furnace pressure switch is a safety switch with a rubber diaphragm that closes only when the inducer motor creates enough draft, measured in inches of water column, proving the vent path is clear before the gas valve is allowed to open.
Heat exchanger: A heat exchanger is the sealed metal chamber inside a furnace that the burner flames heat from within, so that household air blown across its outer surface picks up the heat without ever mixing with the combustion gases.
Hot surface igniter: A hot surface igniter is a silicon carbide or silicon nitride element in a gas furnace that glows orange-hot, above 1,000 °C, to light the burners on each call for heat, replacing the standing pilot light used on older furnaces.
Inducer motor: An inducer motor, also called a draft inducer, is a small fan in a gas furnace that starts before the burners light, pulling combustion air through the heat exchanger and pushing the flue gases out through the vent.
Limit switch: A limit switch, also called a high-limit switch, is a temperature-activated safety switch in a furnace's heat exchanger compartment that shuts the burners off when the air leaving the furnace gets too hot, while the blower keeps running to cool it down.
Rollout switch: A rollout switch, or flame rollout switch, is a manual-reset thermal safety switch mounted beside a furnace's burner box that cuts power to the gas valve if flames spill out of the combustion chamber instead of being drawn into the heat exchanger.
Secondary heat exchanger: A secondary heat exchanger is the second coil in a condensing gas furnace, usually stainless steel, that pulls extra heat out of the flue gases until their water vapour condenses, which is why these furnaces vent through plastic pipe and need a drain.

10 terms

AC & heat pump

AC capacitor: An AC capacitor is a cylinder-shaped electrical part in the outdoor unit that stores and releases an electrical charge to start the compressor and fan motors and keep them running, with its strength rated in microfarads (µF).
AC compressor: An AC compressor is the motor-driven pump in the outdoor unit that squeezes refrigerant vapour to a high pressure and temperature so the system can reject the house's heat outdoors or, in a heat pump, deliver heat indoors.
AC contactor: An AC contactor is the relay in the outdoor unit whose 24-volt coil pulls a set of contacts closed when the thermostat calls for cooling, sending 240-volt power to the compressor and condenser fan.
Air handler: An air handler is the indoor cabinet of an all-electric or heat pump system that contains the blower, the indoor refrigerant coil and usually electric backup heat strips, moving and conditioning the air as a furnace does but burning no fuel.
Auxiliary heat: Auxiliary heat is the backup heat, either electric resistance strips in the air handler or the gas furnace in a hybrid system, that a heat pump's controls switch on automatically when the heat pump alone cannot hold the setpoint or during a defrost cycle.
Condenser coil: A condenser coil is the finned coil that wraps the outdoor AC or heat pump cabinet, where hot compressed refrigerant releases the heat it picked up indoors into the outside air drawn across it by the condenser fan.
Evaporator coil: An evaporator coil is the indoor coil, usually an A-shaped coil sitting on top of the furnace or inside the air handler, where cold refrigerant absorbs heat from the house air blown across it while moisture condenses on its fins.
Heat pump balance point: A heat pump balance point is the outdoor temperature at which the heat pump's heating output exactly equals the house's heat loss, so that below it the system needs help from backup heat to hold the setpoint.
Inverter compressor: An inverter compressor is a variable-speed compressor whose drive electronics change the frequency of the power fed to its motor, so it can run anywhere from a fraction of its capacity up to full output instead of only fully on or fully off.
Reversing valve: A reversing valve is the four-way valve in a heat pump that swaps the direction of refrigerant flow, turning the outdoor coil from a condenser (cooling mode) into an evaporator (heating mode) and back again for defrost.

7 terms

Water heaters

Anode rod: An anode rod is a magnesium or aluminum rod suspended inside a tank water heater that corrodes sacrificially, drawing the water's corrosive attack to itself so that the glass-lined steel tank does not rust through.
Atmospheric vent water heater: An atmospheric vent water heater is a gas water heater that vents by natural draft: hot exhaust rises through a draft hood on top of the tank and up a metal vent into a chimney, with no fan and no electrical connection.
Dip tube: A dip tube is the plastic tube inside a tank water heater that carries incoming cold water from the inlet at the top down to the bottom of the tank, so it is heated there instead of mixing with the hot water leaving at the top.
Recirculation pump: A recirculation pump is a small pump that keeps hot water moving through a home's supply piping and back to the water heater, through a dedicated return line or a crossover valve under the farthest sink, so distant taps get hot water in seconds.
T&P valve: A T&P valve (temperature and pressure relief valve) is the safety valve on a water heater that opens automatically if the tank reaches 150 psi or the water reaches 99 °C (210 °F), releasing water through a discharge pipe so the tank cannot rupture.
Thermocouple: A thermocouple is a heat-sensing probe that sits in a gas appliance's pilot flame and generates a small voltage, about 25 to 30 millivolts, that holds the gas valve open; if the pilot goes out, the voltage collapses and the valve shuts off the gas.
Water heater element: A water heater element is the electric resistance rod that heats the water in an electric tank water heater; most tanks have two, an upper and a lower, typically 3,000 to 4,500 watts each on a 240-volt circuit, with only one running at a time.

3 terms

Boilers & hydronic

Boiler expansion tank: A boiler expansion tank is a small steel tank with an air-charged rubber diaphragm that absorbs the extra volume of heating water as it warms and expands, keeping the pressure in a closed hydronic system steady.
Circulator pump: A circulator pump is the electric pump on a hot-water (hydronic) heating system that moves heated water from the boiler out through the radiators, baseboards or in-floor loops and back to the boiler to be reheated.
Combi boiler: A combi boiler, short for combination boiler, is a single gas appliance, usually wall-hung, that heats the water for a home's radiators or in-floor loops and also produces domestic hot water on demand, with no separate storage tank.

4 terms

Airflow & ducts

CSA F280 heat-loss calculation: A CSA F280 heat-loss calculation is the Canadian standard method for working out a house's room-by-room heat loss and heat gain from its insulation, windows, air leakage and local design temperature, so that heating and cooling equipment can be sized correctly.
HVAC zoning: HVAC zoning is a system of motorized duct dampers, a zone control panel and two or more thermostats that lets one furnace, air conditioner or heat pump heat and cool different areas of a house independently.
Return air: Return air is the house air drawn back to the furnace or air handler through return grilles and ducts, often called cold air returns, to be filtered, heated or cooled and sent out again through the supply registers.
Static pressure: Static pressure is the pressure air exerts against the inside of a duct system, measured in inches of water column (in. w.c.), and it indicates how much resistance the blower must overcome to move air through the filter, coil and ducts.

1 terms

Ventilation & air quality

Make-up air: Make-up air is outdoor air deliberately brought into a building to replace the air removed by exhaust fans, range hoods, clothes dryers and fuel-burning appliances, so that the building is not pulled into negative pressure.

2 terms

Controls

C-wire: A C-wire, or common wire, is the thermostat conductor that completes the 24-volt circuit back to the furnace transformer, giving a smart thermostat continuous power instead of relying on batteries or on power stolen through the heating and cooling wires.
Dual-fuel changeover temperature: The dual-fuel changeover temperature, also called the switchover or lockout temperature, is the outdoor temperature setting at which a hybrid heating system stops heating with the heat pump and hands the job to the gas furnace.

9 terms

Ratings & refrigerants

AC tonnage: AC tonnage is the cooling capacity of an air conditioner, where one ton equals 12,000 BTU per hour, the rate of heat removal needed to melt one ton of ice in 24 hours, and has nothing to do with the unit's weight.
BTU: A BTU (British thermal unit) is the amount of heat needed to raise the temperature of one pound of water by one degree Fahrenheit, and furnaces, boilers, air conditioners and water heaters are rated in BTU per hour of heating or cooling capacity.
COP: COP (coefficient of performance) is the ratio of the heat a heat pump delivers to the electricity it consumes at one specific outdoor temperature, so a COP of 3 means 3 kWh of heat for every 1 kWh of electricity.
EER2: EER2 (Energy Efficiency Ratio 2) is an air conditioner's or heat pump's cooling output in BTU per hour divided by its power draw in watts at one fixed test condition, 35°C (95°F) outdoors, under the M1 test procedure introduced in 2023.
HSPF2: HSPF2 (Heating Seasonal Performance Factor 2) is a heat pump's seasonal heating efficiency rating: the total heat it delivers over a standard heating season in BTU divided by the watt-hours of electricity it uses, measured under the M1 test procedure adopted in 2023.
NEEP cold-climate heat pump list: The NEEP cold-climate heat pump list is a free public database, run by Northeast Energy Efficiency Partnerships, of air-source heat pumps that meet its cold-climate specification, showing each model's reported heating capacity and efficiency at low outdoor temperatures, including -15°C.
R-22: R-22 is an ozone-depleting HCFC refrigerant, sold under the trade name Freon, that was the standard charge in home air conditioners and heat pumps built before about 2010 and has since been phased out under the Montreal Protocol.
R-410A: R-410A is the HFC refrigerant, sold under brand names such as Puron, used in most air conditioners and heat pumps installed in Ontario from about 2010 to 2025, which does not deplete ozone but has a high global warming potential and is being phased down.
UEF: UEF (Uniform Energy Factor) is the efficiency rating for residential water heaters: the share of the energy a unit consumes that ends up as delivered hot water in a standardized 24-hour test, where a higher number means less energy used for the same hot water.

1 terms

Rules & paperwork

TSSA: The TSSA (Technical Standards and Safety Authority) is the not-for-profit regulator that enforces Ontario's Technical Standards and Safety Act, 2000, certifying gas technicians and registering the fuel contractors allowed to install and service gas appliances in the province.

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