HVAC glossary

HVAC glossary: the terms, in plain English

The cross-system terms every homeowner runs into — capacity, efficiency ratings, and the core parts — defined once and clearly. System-specific jargon lives in each system's own glossary, linked below.

AFUEAnnual Fuel Utilization Efficiency

The share of fuel a furnace turns into heat over a year, as a percentage. Standard furnaces are 80%; high-efficiency condensing furnaces run 90–98.5%. Full detail on the AFUE page.

Air handler

The indoor unit that circulates conditioned air — blower, coil, and filter — pairing with a heat pump or AC. It's the all-electric counterpart to a furnace; see the air handler page.

BTUBritish Thermal Unit

The basic unit of heat — the energy to raise one pound of water by 1°F. HVAC capacity is measured in BTUs per hour; 12,000 BTU/hr equals one ton of cooling.

Condenser

The outdoor unit that holds the compressor and outdoor coil, releasing heat outside (in cooling) or gathering it (in a heat pump). It pairs with the indoor air handler or furnace coil.

Evaporator coil

The indoor coil where refrigerant absorbs heat from your home's air. It sits in the air handler or on top of a furnace, and it's where condensation (and sometimes ice) forms.

HSPF2Heating Seasonal Performance Factor 2

The efficiency rating for a heat pump's heating, the winter counterpart to SEER2. Because a heat pump moves heat instead of burning fuel, it delivers more heat energy than the electricity it draws. Full detail on the HSPF2 page.

Load calculation

The engineering estimate (Manual J) of how much heating and cooling a home actually needs, based on size, insulation, windows, and climate. It's how a system is sized correctly rather than by rule of thumb.

MERVMinimum Efficiency Reporting Value

The rating of how much an air filter captures, from 1 to 16 for home filters. Higher catches finer particles but restricts airflow more; most homes do well with MERV 8–13. See air filters.

Refrigerant

The fluid that carries heat through the sealed system, absorbing it in one coil and releasing it in the other. Home systems use R-410A or the newer low-GWP R-454B; handling it requires EPA certification.

SEER2Seasonal Energy Efficiency Ratio 2

The current rating for cooling efficiency — cooling output over a season divided by the electricity used. Higher is more efficient; central systems run 14–20 and premium mini-splits reach 30+.

Split system

The common home setup with an indoor unit and an outdoor unit joined by refrigerant lines — as opposed to a packaged unit that houses everything in one outdoor cabinet.

Static pressure

The resistance to airflow inside the duct system, like blood pressure for your HVAC. Too high — from undersized ducts or a clogged filter — strains the blower and cuts efficiency.

Ton

A unit of cooling capacity equal to 12,000 BTU per hour. A typical home AC is 2–5 tons; sizing it to the home's actual load matters more than buying the biggest unit.

Zoning

Dividing a home into areas with independent temperature control, using dampers in the ducts or separate units. It lets you condition only the spaces in use.