HVAC Glossary in Plain English

A class of lower-global-warming refrigerants that are mildly flammable (the “2L” rating). R-454B and R-32 are the A2L refrigerants now used in new residential air conditioners and heat pumps; equipment includes leak sensors or other safeguards.

Which refrigerant does my unit use? →

Annual Fuel Utilization Efficiency: the share of a furnace’s fuel that becomes heat in the home over a season. A 96% AFUE furnace turns about 96 cents of every gas dollar into heat; the rest goes up the vent.

In Colorado: Since July 31, 2026 the ENERGY STAR spec for gas furnaces in northern states is about 97% AFUE, which also matters for Colorado’s furnace NOx rule.

Colorado gas furnace NOx rules →

The indoor cabinet with a blower and coil used with a heat pump or AC when there is no furnace. It often includes electric backup heat strips.

Reducing a gas furnace’s input at high elevation, where thinner air holds less oxygen. Manufacturers specify orifice and gas-pressure changes; a common rule is about 4% per 1,000 feet, though methods vary by model.

In Colorado: At 5,280 ft that is roughly a fifth of the nameplate input. A furnace left at sea-level settings can overfire.

Furnace altitude calculator →

Backup heat — electric strips or a gas furnace — that helps a heat pump when it can’t keep up alone, usually in very cold weather or during defrost.

Heat pump blowing cold air →

The outdoor temperature at which a heat pump’s output exactly matches the home’s heat loss. Below it, backup heat is needed. In a dual-fuel system, the switch-over temperature is set near the economic or thermal balance point.

Dual-fuel systems in Colorado →

The fan that moves air through the furnace or air handler and the ducts. PSC motors run at fixed speeds; ECM (electronically commutated) motors adjust speed and use less electricity, and many can hold airflow steady as filters load.

British thermal unit, the energy needed to warm one pound of water by 1°F. Heating and cooling capacity is given in Btu per hour; 12,000 Btu/h equals one ton of cooling.

An electrical part that helps the compressor and outdoor fan motors start and run. A failed capacitor is one of the most common reasons an outdoor unit hums but doesn’t start.

In Colorado: Big day-to-night temperature swings and summer heat are hard on capacitors.

AC won’t turn on →

A colorless, odorless gas produced by incomplete combustion. Fuel-burning appliances must vent it outside; CO alarms on every level and near bedrooms are the early warning.

CO and furnace safety →

A heat pump designed and tested to keep delivering a large share of its rated heating capacity at low outdoor temperatures, typically using a variable-speed (inverter) compressor.

In Colorado: Colorado incentives focus on cold-climate models.

Do heat pumps work in Denver winters? →

Water produced when a high-efficiency furnace cools its exhaust or when an AC coil pulls moisture from the air. It drains through a trap, line or condensate pump.

In Colorado: Frozen condensate lines are a common winter lockout cause.

Water leaking from the AC or furnace →

A furnace (generally 90% AFUE and up) with a second heat exchanger that captures heat from the water vapor in the exhaust. It vents through PVC pipe and needs a condensate drain.

Coefficient of performance: heat delivered divided by electricity used, at a given moment. A COP of 3 means three units of heat for one unit of electricity. COP falls as the outdoor temperature drops.

Heat pump vs. furnace →

In heating mode a heat pump’s outdoor coil can frost over; the unit briefly reverses to melt it. Steam and a pause in heat are normal.

Heat pump covered in ice →

The outdoor temperature a heating or cooling system is sized for, usually the 99% (heating) or 1% (cooling) value from ASHRAE data.

In Colorado: ENERGY STAR’s reference lists about 0–5°F for Front Range counties; below that a correctly sized furnace runs continuously.

Furnace can’t keep up in extreme cold →

A heat pump paired with a gas furnace. The heat pump heats in mild weather; the furnace takes over in deep cold or when gas is cheaper.

Dual-fuel guide →

A heat pump with an outdoor unit and one or more indoor heads connected by refrigerant lines, no ducts required. Each head has its own control.

Mini-splits for older homes →

The indoor coil, usually on top of the furnace, where refrigerant absorbs heat from the house air in cooling mode. It must match the outdoor unit and refrigerant.

Frozen AC coil →

A safety switch near a furnace’s burners that opens if flames roll out of the burner area. A tripped rollout switch is a reason to shut the furnace down and call, not to keep resetting it.

Furnace error codes →

A metal rod that proves a flame is present. When it gets coated, the furnace lights and then shuts off within seconds.

Furnace short cycling →

The sealed metal chamber that transfers burner heat to the house air while keeping combustion gases separate. Cracks are a safety concern because they can let combustion gases into the air stream.

A system that moves heat instead of making it: outdoors to indoors in winter and indoors to outdoors in summer, using a reversing valve.

Heat pump vs. furnace in Colorado →

A temperature switch that shuts the burners off if the furnace overheats, most often because of poor airflow from a dirty filter or closed registers.

A ceramic element that glows hot to light the burners on most modern furnaces. It is fragile and wears out over time.

Furnace won’t turn on →

Heating Seasonal Performance Factor (2023 test procedure): a heat pump’s seasonal heating efficiency in Btu per watt-hour. Higher is better; the federal minimum for split heat pumps is 7.5 HSPF2.

A small fan that pulls combustion gases through the heat exchanger and out the vent before the burners light. The pressure switch proves it is working.

Furnace noises →

The pair of insulated copper refrigerant lines between the outdoor unit and the indoor coil.

ACCA’s standard methods for a heating and cooling load calculation (J), duct design (D) and equipment selection (S). Sizing by square footage alone is not a substitute.

In Colorado: Altitude and local design temperatures are part of a correct Manual J.

Heat pump sizing at altitude →

Minimum Efficiency Reporting Value: a filter rating from 1 to 16. Higher numbers capture finer particles but can restrict airflow on systems not designed for them.

In Colorado: Wildfire smoke seasons make better filtration worth discussing with your technician.

Furnaces that run at more than one heat level. Two-stage models have low and high fire; modulating models adjust in small steps. Both run longer, quieter cycles than single-stage.

Nitrogen oxides, an air pollutant formed in combustion. Colorado limits NOx from new gas furnaces and water heaters.

In Colorado: Since January 1, 2026, new gas furnaces sold in Colorado must meet 14 ng/J NOx or the current ENERGY STAR specification.

Colorado gas furnace NOx rules →

Proves the inducer has created enough draft before the furnace lights. Blocked vents, frozen condensate or a cracked hose can trip it.

In Colorado: Snow-blocked PVC vents are a frequent winter cause.

Furnace blowing cold air →

The refrigerant used in most home ACs before 2010. New equipment charged with it stopped in 2010, and U.S. production and import ended in 2020; repairs now rely on recycled supply.

The refrigerant in most systems built from 2010 to 2024. It is being replaced by R-454B and R-32 in new equipment; existing systems remain serviceable.

The two A2L refrigerants replacing R-410A in new residential systems. Carrier, Trane and Lennox ducted systems use R-454B; Goodman, Amana and Daikin use R-32.

Goodman vs. Amana →

The ducts and grilles that bring house air back to the furnace or air handler. Too little return air causes noise, high static pressure and rooms that won’t heat.

One room colder than the rest →

Seasonal Energy Efficiency Ratio (2023 test procedure) for cooling. Higher is better. In the DOE North region, which includes Colorado, split-system ACs must be at least 13.4 SEER2 and split heat pumps 14.3 SEER2.

A furnace, AC or heat pump turning on and off much more often than normal. Causes range from a dirty flame sensor or filter to an oversized system.

Furnace short cycling →

The resistance the blower works against, measured in inches of water column. High static from small ducts, closed registers or restrictive filters cuts airflow and comfort.

How much warmer the air leaving a furnace is than the air returning to it. Every rating plate lists an acceptable range; a reading outside it points to airflow or combustion problems.

How to check it safely →

Low-voltage wires connect the thermostat to the equipment. The C (common) wire gives smart thermostats steady power; R is power, W heat, Y cooling, G fan.

Thermostat not working →

A unit of cooling capacity equal to 12,000 Btu/h. Most homes use 1.5- to 5-ton systems.

A compressor that changes speed to match the load instead of switching fully on and off. It improves comfort, humidity control and cold-weather heat pump output.

Motorized dampers and separate thermostats that let different floors or areas be heated or cooled independently from one system.

Fixing uneven temperatures →