Plain English
HVAC Glossary
The words on your quote, your thermostat and your technician's invoice, explained in two or three sentences each.
$89 diagnostic, credited toward the repair when you approve the work. Answered 24/7.
Short answer
What do HVAC terms like AFUE, SEER2 and BTU mean?
AFUE is the share of a furnace's fuel that becomes heat over a season; SEER2 and HSPF2 rate a heat pump's or air conditioner's seasonal cooling and heating efficiency; a BTU is the unit of heat, and a ton of cooling is 12,000 BTU per hour. Every term below has a short definition and a link to the page that explains it in full.
A
- AFUE (Annual Fuel Utilization Efficiency)
The share of a furnace's or boiler's fuel that becomes heat in the house over a whole season. An 80% AFUE furnace sends about a fifth of its fuel's energy up the chimney; a condensing furnace rated 90% or higher recovers most of that.
- Air handler
The indoor cabinet holding the blower that moves air through the ductwork, usually paired with a heat pump or air conditioner. It plays the role a furnace's blower plays when there is no furnace.
- Auxiliary heat (Emergency heat)
The backup heat a heat pump system uses when the heat pump alone cannot keep up, typically electric resistance strips or a furnace. Emergency heat is the thermostat setting that runs only the backup, for when the heat pump itself has failed.
B
- Balance point
The outdoor temperature below which a heat pump can no longer supply all of a building's heat by itself. Below it, auxiliary heat or a furnace makes up the difference.
- BTU (British thermal unit)
The amount of heat needed to raise one pound of water by one degree Fahrenheit. Furnace, boiler and air conditioner capacity is stated in BTU per hour.
- Building automation system (BAS)
The central control system that schedules and monitors a commercial building's heating, cooling and ventilation equipment. It turns equipment on and off by occupancy, holds setpoints and raises alarms when something fails.
C
- Carbon monoxide (CO)
A colourless, odourless and poisonous gas produced when fuel burns incompletely. Any home with a gas, propane or oil appliance needs working CO alarms, and a combustion appliance should be tested rather than assumed safe.
- Cold-climate heat pump
A heat pump built to keep producing useful heat far below freezing, using a variable-speed compressor and refrigerant circuits designed for low outdoor temperatures. In the Twin Ports most are installed with a furnace as backup for the coldest nights.
- Combustion analysis
A test that samples a gas or oil appliance's flue gas with a calibrated analyser, reading oxygen, carbon monoxide, flue temperature and efficiency. It is the only way to confirm an appliance is burning cleanly and safely.
- Compressor
The pump at the heart of an air conditioner or heat pump that pressurises refrigerant and drives it around the system. It is usually the most expensive single part to replace.
- Condensate
Water produced by an air conditioner's cooling coil, or by a high-efficiency furnace or boiler when it cools its own exhaust. It has to drain away, and a blocked or frozen condensate line is a common cause of leaks and shutdowns.
- Condensing boiler
A boiler that cools its exhaust enough to condense water vapour out of it, recovering heat a conventional boiler loses up the chimney. It reaches its best efficiency with low return-water temperatures, which is why it pairs well with outdoor reset control.
- Condensing furnace
A furnace with a second heat exchanger that recovers heat from the exhaust by condensing its water vapour, reaching 90% AFUE or higher. It vents through plastic pipe rather than a chimney and produces condensate that needs a drain.
- COP (Coefficient of performance)
The ratio of heat a heat pump delivers to the electrical energy it uses at a given moment. Electric resistance heat has a COP of 1; a heat pump's COP is higher and falls as the outdoor temperature drops.
D
- Defrost mode
The cycle in which a heat pump briefly reverses to melt frost off its outdoor coil in winter. The cloud of steam from the outdoor unit during defrost is normal.
- Draft
The upward flow that carries combustion gases out through a chimney or vent. A water heater left alone in an oversized masonry chimney after a furnace upgrade often drafts poorly and can spill exhaust into the house.
- Dual fuel
A heating system that pairs an electric heat pump with a gas or propane furnace and switches between them by outdoor temperature or fuel cost. It is the most common way heat pumps are installed in the Twin Ports.
- Duct sealing
Closing the leaks in ductwork joints and seams with mastic or approved tape so the air a system heats or cools reaches the rooms instead of the basement, attic or wall cavities.
- Ductless mini-split
A heat pump or air conditioner with an outdoor unit connected by refrigerant lines to one or more wall, ceiling or floor units indoors, with no ductwork. It is the usual way to add cooling to a house heated by radiators.
E
- Economizer
A damper system on a commercial rooftop unit that uses cool outdoor air for free cooling when conditions allow, instead of running the compressor. A stuck economizer wastes energy or lets in too much cold air.
- ERV (Energy recovery ventilator)
A ventilator that exchanges stale indoor air for fresh outdoor air while transferring both heat and some moisture between the two streams. It limits how much a tight house dries out in winter.
- Evaporator coil
The indoor coil of an air conditioner or heat pump, where refrigerant absorbs heat from the house air. Low airflow or low refrigerant can make it freeze into a block of ice.
- Expansion tank
A small tank on a hot-water heating system that absorbs the water's expansion as it heats. A failed expansion tank is a common reason a boiler's pressure climbs until the relief valve drips.
F
- Flame sensor
A metal rod in a furnace's burner flame that confirms the flame is lit; if it cannot detect flame, the furnace shuts the gas off within seconds. A dirty flame sensor is one of the most common causes of a furnace that lights and then quits.
H
- Heat exchanger
The sealed metal chamber in a furnace or boiler that passes heat from the burning fuel to the air or water without letting combustion gases mix with it. A cracked furnace heat exchanger is a safety problem and usually means replacement.
- Heat pump
An electric system that moves heat rather than making it: out of the house in summer, like an air conditioner, and into the house from outdoor air in winter. Because it moves heat, it can deliver more heat energy than the electricity it uses.
- Hot surface ignitor
The part that lights a modern furnace's burners by glowing hot, replacing the standing pilot light. It is fragile and wears out, and a failed ignitor leaves the furnace blowing air with no heat.
- HRV (Heat recovery ventilator)
A ventilator that exchanges stale indoor air for fresh outdoor air while passing heat from the outgoing air to the incoming air. It brings in fresh air without paying the full cost of heating it.
- HSPF2 (Heating Seasonal Performance Factor 2)
The U.S. rating of a heat pump's heating efficiency over a season, under the test procedure in force since 2023. Higher is better, but capacity at low outdoor temperatures matters more in the Twin Ports than the seasonal number.
- Hydronic heating
Heating with hot water circulated from a boiler through radiators, baseboard or tubing in floors. It is the heating in a large share of older Twin Ports houses.
L
- Load calculation (Manual J)
An engineering calculation of how much heating and cooling a specific building needs, room by room, from its insulation, windows, air leakage and local design temperatures. Manual J is the industry-standard method, and it is how equipment should be sized, rather than by square footage or the old unit's nameplate.
M
- Make-up air
Outdoor air brought into a building to replace air that exhaust fans remove, such as a kitchen hood. Without it the building goes into negative pressure: doors are hard to open, hoods stop pulling and appliances can backdraft.
- MERV (Minimum Efficiency Reporting Value)
A 1-to-16 rating of how well an air filter catches particles; higher numbers catch smaller particles. A filter more restrictive than the system was designed for can starve it of airflow.
- Modulating furnace
A furnace whose gas valve and blower can run anywhere across a range of output instead of simply on or off (single-stage) or at two fixed levels (two-stage). It runs longer at low output, giving steadier temperatures and quieter operation.
N
- Negative pressure
The condition where more air is exhausted from a building than is supplied, so outdoor air is drawn in through every gap. In commercial kitchens it is the usual cause of hoods that will not pull and doors that are hard to open.
O
- Outdoor reset
A boiler control that lowers the water temperature on mild days and raises it as the weather gets colder. It saves fuel, keeps rooms more even and lets a condensing boiler condense more of the time.
P
- Pressure relief valve
The safety valve on a boiler or water heater that opens to release water if pressure or temperature gets too high. A relief valve that drips is warning you about something else, often a failed expansion tank; it should never be capped.
- Propane conversion (LP conversion)
Refitting a gas appliance to burn propane instead of natural gas, or the reverse, with the manufacturer's kit: different burner orifices, a different gas-valve setting and a combustion check afterwards. Propane runs at a much higher manifold pressure than natural gas.
R
- Radiant floor heating
Heating from warm-water tubing or electric mats under the floor, which warm the floor and the people in the room directly rather than blowing air. It is even and quiet but slow to respond to setpoint changes.
- Refrigerant
The fluid an air conditioner or heat pump circulates to carry heat, absorbing it as it evaporates and releasing it as it condenses. A system does not use refrigerant up, so if it is low there is a leak to find.
- Relative humidity
How much water vapour the air holds compared with the most it could hold at that temperature. In a Twin Ports winter, indoor humidity has to come down as the outdoor temperature drops, or it condenses and freezes on windows.
- Rooftop unit (RTU)
A packaged heating and cooling unit installed on a commercial roof, containing the burner or heat pump, compressor, blower and controls in one cabinet. Most small commercial buildings are heated and cooled by one or more of them.
S
- SEER2 (Seasonal Energy Efficiency Ratio 2)
The U.S. rating of an air conditioner's or heat pump's cooling efficiency over a season, under the test procedure in force since 2023. With a short cooling season, a higher SEER2 pays back more slowly in the Twin Ports than in the south.
- Short cycling
When a furnace, boiler or air conditioner turns on and off in short bursts instead of running full cycles. It wears equipment out and is usually caused by oversizing, a dirty filter, a faulty sensor or overheating.
- Static pressure
The resistance the ductwork and filter put against the blower, measured in inches of water column. High static pressure from undersized ducts or a restrictive filter reduces airflow, comfort and equipment life.
- Steam heating
Heating with steam from a boiler that rises to radiators and returns as condensed water, in one-pipe or two-pipe systems. Banging pipes usually mean condensate is trapped where steam is trying to pass.
T
- Tankless water heater
A water heater that heats water only as it flows, instead of keeping a tank hot. In a cold climate the incoming water is colder, so the flow it can deliver at a comfortable temperature is lower than the brochure suggests.
- Thermostat
The control that tells a heating or cooling system when to run. Smart and communicating thermostats add scheduling, remote access and, with some equipment, finer control of variable-speed systems.
- Ton
The unit of air conditioning and heat pump capacity: one ton moves 12,000 BTU of heat per hour. It comes from the cooling effect of melting a ton of ice over a day.
U
- Unit heater
A self-contained heater with a fan, usually hung from the ceiling of a garage, shop or warehouse, that heats the space without ductwork. It can be gas-fired, fed with hot water from a boiler, or electric.
W
- Walk-in cooler
A refrigerated room for commercial food storage, cooled by an evaporator inside and a condensing unit outside or on the roof. Door gaskets, defrost and airflow are where most problems start.
No term matches that yet. Try a shorter word, browse by letter, or ask us — we answer 24/7.
Keep reading
No heat? Call us now.
We answer 24 hours a day, every day of the year — Duluth, Superior and every town within 45 miles, in both states. $89 diagnostic, credited toward the repair when you approve the work.