HVAC Glossary
The words on your quote, your thermostat and your furnace's nameplate, explained.
A
- AFUE
Annual fuel utilization efficiency: the share of the fuel a furnace or boiler burns that ends up as heat in the house over a season. An 80% furnace sends about a fifth of its fuel up the chimney. A 96–98% condensing furnace pulls extra heat out of the exhaust, cools it enough to make water, and vents through PVC pipe. Over a long Minnesota heating season, that gap adds up.
- Auxiliary heat
The backup heat a heat pump system calls on when the heat pump alone can't keep up. It's usually electric resistance strips in the air handler or, on a dual-fuel system, the furnace. The thermostat brings it on by itself during a cold snap, during defrost, or when you bump the setpoint up several degrees. Seeing AUX on the thermostat on a below-zero morning is normal.
B
- Balance point
The outdoor temperature where a heat pump's output just matches the heat the house is losing. Below it, the heat pump needs help from backup heat. On a dual-fuel system the installer also sets a switchover temperature where the furnace takes over, based on the heat pump's capacity, the house, and what you pay for gas or propane compared with electricity. Every house has its own balance point.
- Boiler
A heating appliance that heats water (or makes steam) and sends it through pipes to radiators, baseboard or in-floor tubing, instead of blowing air through ducts. Plenty of 1920s–1950s houses in Minneapolis and Saint Paul still heat with a boiler and cast-iron radiators. Modern condensing boilers are far more efficient than the old cast-iron ones, but they need a condensate drain and their own venting.
- BTU
British thermal unit: the heat needed to warm one pound of water by one degree Fahrenheit. Furnaces are rated in BTUs per hour of input and output, so a furnace with an 80,000 BTU input at 96% AFUE delivers roughly 76,800 BTUs per hour to the house. Air conditioners are rated in BTUs per hour too, usually expressed in tons.
C
- C-wire
The common wire from the 24-volt transformer in the furnace or air handler to the thermostat. It gives a smart thermostat steady power. Older homes often have only two to four wires at the thermostat and no C-wire, which is why a new Wi-Fi thermostat may go blank, reboot or eat batteries. The fix is pulling a new wire or adding an adapter at the furnace.
- Capacitor
A small can-shaped electrical part in the outdoor AC or heat pump unit that gives the compressor and fan motor the kick they need to start and keep running. It's one of the most common summer failures: the fan doesn't spin, the unit hums, and the house warms up. A swollen or leaking top is a giveaway. Capacitors hold a charge, so leave them to a technician.
- Chimney liner
A metal pipe run down the inside of a masonry chimney so exhaust from a furnace, boiler or water heater vents safely. When an 80% furnace is replaced with a 90%+ model that vents through PVC, the water heater may be left alone in a chimney sized for two appliances. Its exhaust can cool too much, condense and damage the chimney, so a liner is often needed.
- Circulator pump
The pump on a hot-water boiler system that moves heated water through the pipes to the radiators, baseboard or floor loops. It's usually a small motor bolted into the piping near the boiler. A failed circulator means the boiler gets hot but the rooms don't. Zoned boiler systems may have a circulator for each zone, or one pump with zone valves.
- CO alarm
An alarm that sounds when carbon monoxide builds up in the air. CO has no smell or color, and a cracked heat exchanger, blocked flue or backdrafting water heater can produce it. Minnesota law requires a working CO alarm within 10 feet of every room used for sleeping. Replace alarms when they reach the end-of-life date printed on the unit.
- Cold-climate heat pump
A heat pump built to keep producing useful heat well below zero, usually with a variable-speed inverter compressor. Mitsubishi rates its Hyper-Heat models to run down to -13°F, for example. Capacity still drops as the temperature falls, so most Minnesota installs keep a backup: a gas or propane furnace (dual fuel) or electric heat. NEEP keeps a public list of models that meet its cold-climate specification.
- Communicating system
A matched furnace or air handler, outdoor unit and thermostat that talk to each other over a digital data line instead of simple on-off signals. The thermostat can run variable-speed equipment at the exact stage it needs and show fault codes on its screen. Carrier Infinity, Trane ComfortLink II and Lennox iComfort are examples. The catch is that the controls generally have to stay within that brand's system.
- Compressor
The pump in the outdoor unit of an air conditioner or heat pump that squeezes refrigerant vapor and pushes it through the system. It's the most expensive part in the unit. A single-stage compressor is either on or off. Two-stage and variable-speed (inverter) compressors can run at lower output for longer, which evens out temperatures and pulls more moisture out of the air in July.
- Condensate
The water that forms when warm, moist air or flue gas cools. An AC coil pulls condensate out of the house air in summer, and a 90%+ furnace makes it from its own exhaust all winter. It drains through a trap and tubing to a floor drain or a condensate pump. A plugged trap or failed pump can shut the system down or leave water on the basement floor.
- COP
Coefficient of performance: the heat a heat pump delivers divided by the electricity it uses, measured at one specific outdoor temperature. A COP of 3 means three units of heat for every unit of electricity. Electric resistance heat has a COP of 1. A heat pump's COP falls as the outdoor temperature drops, which is why cold-climate models list their COP at 5°F.
D
- Damper
A plate inside a duct that opens or closes to control airflow. Manual dampers have a small lever on the outside of the duct, often set to shift airflow between floors for summer and winter. Motorized dampers are part of a zoning system and open or close on their own when a zone's thermostat calls for heating or cooling.
- Defrost cycle
In winter, frost builds on a heat pump's outdoor coil as it pulls heat out of cold air. Every so often the heat pump reverses for a few minutes to melt it off. The outdoor fan stops, steam rises off the unit, and you may hear a whoosh when it switches over. That's normal. A unit that stays iced solid is not, and needs a service call.
- Design temperature
The cold outdoor temperature a heating system is sized to handle, based on local weather records rather than the coldest night ever recorded. Minnesota design temperatures are well below zero, and they generally get colder the farther north you go. A Manual J load calculation uses the local design temperature to work out how much heat a house needs on those nights.
- Dew point
The temperature at which moisture in the air starts condensing into water. It explains why windows sweat in January: the glass is colder than the dew point of the room air. In summer, a high outdoor dew point is what makes a July day feel sticky, and why an air conditioner that removes moisture matters as much as one that drops the temperature.
- Draft
The pull that carries exhaust from a furnace, boiler or water heater up and out of the house. Older natural-draft appliances rely on hot flue gas rising up the chimney. Modern furnaces use an inducer motor to create draft mechanically. Poor draft, from a blocked chimney, a disconnected flue pipe or a house under negative pressure, can spill carbon monoxide into the basement.
- Dual fuel
A heat pump paired with a gas or propane furnace. The heat pump heats the house through fall, spring and milder winter days, and the furnace takes over below a switchover temperature set at install. In summer the heat pump works as the air conditioner. It's a common way to add a heat pump to a Minnesota home that already has a good gas furnace.
E
- ECM motor
Electronically commutated motor: a variable-speed DC blower motor used in most higher-efficiency furnaces and air handlers. It uses less electricity than the older PSC motors, especially when the fan runs all the time, and it can ramp up slowly for quieter starts. An ECM motor has an electronic control module on the end that can fail separately from the motor itself.
- EER2
Energy efficiency ratio 2: an air conditioner's or heat pump's cooling efficiency at a single hot condition, 95°F outside, measured under the DOE test procedure that took effect January 1, 2023. SEER2 averages across a season; EER2 tells you how the unit holds up on the hottest afternoons. Some utility rebate programs list a minimum EER2 as well as a minimum SEER2.
- Emergency heat
A thermostat setting on a heat pump system that shuts the heat pump off and heats with backup only, usually electric resistance strips. It's meant for when the outdoor unit is broken or iced up, not for ordinary cold days. Leaving it on by accident can make an electric bill jump. If you've had to switch to it, the heat pump needs a service call.
- ERV
Energy recovery ventilator: a box with two fans that brings fresh outdoor air into a tight house and exhausts stale air, passing both streams through a core that transfers heat and some moisture between them. In a Minnesota winter it keeps the house from drying out as much as an HRV would. In summer it keeps some of the outdoor humidity out.
- Evaporator coil
The indoor coil, sitting on top of the furnace or inside the air handler, where cold refrigerant pulls heat and moisture out of the house air. A leaking coil means the system slowly loses refrigerant and cooling. A dirty or iced coil chokes off airflow. On a heat pump the same coil works in reverse in winter and heats the air instead.
- Expansion tank
A small tank on a hot-water boiler system, usually hanging from the piping near the boiler, that gives heated water room to expand. When its air charge is gone, pressure climbs every time the boiler fires and the relief valve may drip or discharge. Older systems may have a larger steel tank up in the floor joists instead.
F
- Flame sensor
A thin metal rod that sits in the burner flame and proves to the control board that the gas actually lit. If it's coated with residue, the board can't read the flame and shuts the gas off a few seconds after ignition. The furnace retries and may lock out. It's one of the most common causes of a no-heat call, and often a cleaning fixes it.
H
- Head (mini-split)
The indoor unit of a ductless mini-split. Most are mounted high on a wall, and there are also floor-mounted consoles and ceiling cassettes. Each head has its own fan, coil and washable filter, and is run from its own remote or thermostat. The filter needs rinsing a few times a year, and the head needs a condensate line to carry water away in summer.
- Heat exchanger
The metal chambers or tubes inside a furnace that keep combustion gases separate from the air blown through your house. Heat passes through the metal, and the exhaust goes out the flue. A crack or hole can let carbon monoxide into the airstream, and a technician who finds one will usually shut the furnace off. A 90%+ furnace adds a secondary heat exchanger to pull out more heat.
- Heat pump
An air conditioner that can also run in reverse. In summer it moves heat out of the house; in winter it pulls heat from the outdoor air and moves it inside. Because it moves heat instead of making it by burning fuel, it can deliver more heat than the electricity it uses. Ducted, ductless, cold-climate and geothermal versions all work on the same principle.
- HRV
Heat recovery ventilator: brings fresh outdoor air into the house and exhausts stale air through a core that moves heat from the outgoing stream to the incoming one. Newer Minnesota homes are built tight and have mechanical ventilation designed in. An HRV removes more moisture than an ERV does, which can help in a house with sweating windows in winter.
- HSPF2
Heating seasonal performance factor 2: a heat pump's heating efficiency averaged across a season, under the DOE test procedure used since January 1, 2023. The federal minimum for heat pumps is 7.5 HSPF2. It's useful for comparing models, but the test assumes a milder winter than Minnesota's, so for a cold-climate install also look at capacity and efficiency at 5°F.
I
- Igniter
The part that lights the gas burner in a modern furnace. Most are hot-surface igniters, a ceramic element that glows orange; some furnaces use a spark igniter instead. Hot-surface igniters are brittle and wear out. When one cracks, the inducer runs, nothing glows, and after a few tries the furnace locks out. Touching the element with bare fingers shortens its life.
- Inducer motor
The small fan motor on a modern furnace that starts first and pulls combustion gases through the heat exchanger and out the flue. The control board waits for a pressure switch to prove the inducer is running before it lets the igniter light. A worn inducer often squeals or rattles before it quits, and a failed one means no heat.
- Inverter
A variable-speed drive for a compressor. Instead of running full blast or sitting off, an inverter-driven compressor speeds up and slows down to match what the house needs. Nearly all ductless mini-splits and cold-climate heat pumps use one. The payoff is steadier temperatures and better efficiency at part load, and it's what lets cold-climate models keep heating well below zero.
K
- Kettling
A rumbling or popping noise from a boiler, like a kettle about to boil. It usually means scale or sludge has built up on the heat exchanger, or water isn't flowing fast enough, so water trapped against the hot surface flashes to steam. It wastes fuel and stresses the boiler. Flushing or descaling, and fixing the flow or water problem behind it, usually stops it.
L
- Limit switch
A safety switch that shuts the burner off if the furnace gets too hot inside. The usual cause is poor airflow: a clogged filter, closed registers, a failing blower or a dirty AC coil sitting on top of the furnace. A furnace that keeps tripping its high limit will short-cycle while the switch does its job. Replacing the switch without fixing the airflow solves nothing.
- Line set
The pair of copper tubes, one of them insulated, that carry refrigerant between the outdoor unit and the indoor coil or mini-split head. On a ductless install it runs through a small hole in the wall, usually inside a cover on the outside of the house. Reusing an old line set with new equipment is sometimes possible and sometimes not, depending on its size, condition and refrigerant.
M
- Make-up air
Outdoor air brought into a building to replace the air that exhaust fans push outside. A big kitchen range hood, or the exhaust hood in a restaurant, can pull a tight building into negative pressure, which can backdraft a water heater or furnace flue. Large residential range hoods and commercial kitchen hoods usually need a make-up air system sized to match.
- Manual D
The ACCA method for designing duct systems: sizing each trunk and branch so every room gets the airflow its Manual J load calls for. Ducts in many older homes were never designed this way, which is behind a lot of cold back bedrooms and hot upstairs rooms. A new furnace won't fix a duct system that can't move enough air to the rooms that need it.
- Manual J
The ACCA method for calculating how much heating and cooling a house needs, room by room, from its insulation, windows, air leakage, orientation and local design temperatures. We size replacements with a Manual J instead of matching the old furnace's nameplate, because the old unit may not have been sized right to begin with. An oversized furnace or AC short-cycles, and an oversized AC leaves the house clammy.
- MERV
Minimum efficiency reporting value: a 1–16 scale for how well an air filter catches particles of different sizes. MERV 8 catches dust and pollen; MERV 11–13 also catches finer particles like smoke. Higher-MERV filters restrict more air, especially in thin 1-inch sizes, so a 4- or 5-inch media cabinet is the better way to get good filtration without choking the blower.
- Multi-zone
A ductless system with one outdoor unit feeding two or more indoor heads, each controlled separately. It keeps the outside of the house down to one condenser. The tradeoff is that one outdoor unit problem takes out every room it serves, and a small room calling alone on a big multi-zone unit can make it cycle. Separate single-zone systems are sometimes the better choice.
O
- Orphaned water heater
A gas water heater left alone in a chimney after the furnace it shared the flue with is replaced by a high-efficiency model that vents through PVC. The chimney was sized for both appliances, and the water heater's smaller, cooler exhaust can condense inside it and fail to draft properly. The usual fix is a properly sized chimney liner or a different venting setup for the water heater.
P
- Pressure switch
A safety switch on a modern furnace that confirms the inducer motor is pulling exhaust through the flue before the burner lights. A blocked vent, a cracked hose, a plugged condensate trap or a weak inducer keeps it from closing, and the furnace won't fire. In Minnesota, frost or drifted snow blocking a PVC vent outside is a common winter cause.
R
- R-22
The refrigerant, often called Freon, in most home air conditioners installed before about 2010. Production and import ended January 1, 2020, so an R-22 system can only be recharged with reclaimed or stockpiled refrigerant, which is expensive. A leaking R-22 unit is usually a replace decision instead of a recharge, since the leak will need refilling again next summer.
- R-32
A single-component refrigerant already common in ductless mini-splits sold around the world, and now used in some new US central air and heat pump systems, including Daikin and Goodman equipment. Under the EPA's AIM Act rules, new residential split systems have moved away from R-410A to lower-GWP refrigerants like this one. It's mildly flammable (class A2L), so installers follow updated handling rules.
- R-410A
The refrigerant, sometimes sold under the name Puron, in most central air conditioners and heat pumps installed from about 2010 until the recent switch to lower-GWP refrigerants. R-410A systems can still be repaired and recharged. New split systems now ship with R-454B or R-32 instead, and the refrigerants aren't interchangeable, so a new outdoor unit usually means a matching new indoor coil.
- R-454B
A lower-GWP refrigerant blend that Carrier, Trane, Lennox and other manufacturers adopted for new central air conditioners and heat pumps as the EPA's AIM Act rules moved the industry off R-410A. It's mildly flammable (class A2L), so many indoor coils and furnaces paired with it now carry a refrigerant leak sensor. It can't be added to an R-410A system.
- Radiator
The cast-iron units under the windows in many older Minneapolis and Saint Paul homes, fed by a hot-water or steam boiler. They heat quietly and evenly and hold warmth after the boiler shuts off. Hot-water radiators have a small bleed valve for letting trapped air out, and a cold radiator on the top floor often just needs bleeding.
- Red tag
A tag a technician or gas utility hangs on an appliance that isn't safe to run, usually after shutting off its gas. Common reasons include a cracked heat exchanger, carbon monoxide spilling into the house, or a venting problem. Don't restart a red-tagged appliance. It needs repair or replacement and, in some cases, an inspection before the gas is turned back on.
- Relative humidity
How much moisture the air holds compared with the most it could hold at that temperature. In a Minnesota winter, indoor humidity has to come down as it gets colder outside or windows sweat and frost: roughly 25–35% in cold weather, lower in deep cold. A whole-home humidifier on the furnace, with an outdoor-sensing control, handles this on its own.
- Rooftop unit
A packaged heating and cooling unit that sits on a building's roof, with the burner, compressor and fans in one cabinet and ducts dropping straight into the space below. They're standard on restaurants, strip-mall retail and small offices. Filters, belts, economizers and the gas heat section all need regular service, usually on a spring and fall schedule.
S
- SEER2
Seasonal energy efficiency ratio 2: an air conditioner's or heat pump's cooling efficiency averaged over a season, under the DOE test procedure used since January 1, 2023. SEER2 numbers read lower than old SEER numbers for the same unit, so don't compare a SEER2 quote against an old SEER label. The federal minimum for split central AC in Minnesota's region is 13.4 SEER2.
- Static pressure
The resistance the blower works against pushing air through filters, coils and ducts, measured in inches of water column with a manometer. High static, from undersized ducts, a restrictive filter or a dirty coil, makes the blower work harder, cuts airflow and can overheat a furnace. Many residential furnaces are rated for about 0.5 inches of total external static pressure.
T
- Ton
A measure of cooling capacity equal to 12,000 BTUs per hour, from the days of cooling with ice. Most home central air conditioners run from 1.5 to 5 tons. Bigger isn't better: an oversized AC cools the house fast, shuts off before it pulls the moisture out, and leaves the house cold and clammy. A load calculation sets the right size.
U
- UV light
An ultraviolet lamp mounted in the ductwork or near the AC coil to keep mold and bacteria from growing on the damp coil and drain pan. It works best on surfaces it shines on all the time, and less well against germs moving past in the airstream. The bulbs lose output over time and need replacing on the maker's schedule, often every year or two.
Z
- Zoning
Splitting one duct system into areas with their own thermostats, using motorized dampers and a zone control panel. It's a common fix for a two-story house where the upstairs is hot and the main floor is cold, or a walkout lower level that stays chilly. Zoning works best with two-stage or variable-speed equipment and ducts designed to handle it.
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