Learning Center
Glossary · speak HVAC like a pro
HVAC terms, translated
Every term you'll meet in a quote, an inspection report, or a chatbot answer, defined in plain English, with the part that actually matters: what it means for your home and your money.
Efficiency ratings
SEER2 Seasonal Energy Efficiency Ratio 2, the current U.S. rating for how efficiently an air conditioner or heat pump cools across a season. It replaced SEER in 2023 with a tougher test that simulates real ductwork, so the same unit scores slightly lower on SEER2 than old SEER. HSPF2 Heating Seasonal Performance Factor 2, the seasonal heating-efficiency rating for heat pumps under the updated 2023 test procedure. Higher HSPF2 means more heat delivered per unit of electricity across the heating season. AFUE Annual Fuel Utilization Efficiency, a furnace's heating efficiency as a percentage of fuel actually converted to heat for your home. A 95% AFUE furnace turns 95 cents of every gas dollar into heat; an 80% unit sends about 20 cents up the flue.
Equipment
Heat exchanger The sealed metal component inside a gas furnace where combustion heat transfers to the air blown through your ducts, keeping exhaust gases separate from the air you breathe. R-454B refrigerant The lower-global-warming refrigerant replacing R-410A in new air conditioners and heat pumps under the federal transition that began in 2025. It's a mildly flammable (A2L) refrigerant requiring updated equipment and handling. Variable-speed Compressor or blower technology that adjusts output continuously instead of running full-blast or off. A variable-speed system might idle along at 40% capacity for hours rather than blasting for ten minutes and stopping. Dual fuel A system pairing an electric heat pump with a gas furnace: the heat pump handles heating and cooling most of the year, and the furnace takes over during the coldest stretches when gas heat is more effective or economical. Heat pump An all-electric system that moves heat rather than creating it, pulling warmth from outdoor air into your home in winter and reversing to cool in summer. One system replaces both an AC and most or all of a furnace's job. Ductless mini-split A heat pump that skips ductwork entirely: an outdoor unit connects by small refrigerant lines to one or more wall- or ceiling-mounted indoor 'heads,' each conditioning its own zone with its own remote or thermostat. Compressor The heart of an air conditioner or heat pump, the pump that pressurizes refrigerant and drives it through the system. It lives in the outdoor unit and is typically the most expensive single component. Evaporator coil The indoor coil (usually above your furnace) where refrigerant absorbs heat and moisture from your home's air. Its partner, the condenser coil outside, releases that heat outdoors. Condenser The outdoor half of an AC or heat pump, the cabinet with the fan and coil that releases the heat collected from inside your home (or, for a heat pump in winter, collects heat from outdoor air). ECM blower An Electronically Commutated Motor, a variable-speed, electronically controlled blower motor that adjusts airflow precisely and uses far less electricity than the old fixed-speed PSC motors. C-wire (common wire) The 'common' wire in thermostat wiring that provides continuous 24-volt power. Smart thermostats need constant power for their screens and Wi-Fi, which older thermostat wiring often didn't include. Two-stage Equipment with two output levels, a low stage (roughly 60 to 70% capacity) for most conditions and a high stage for extreme days. The middle tier between single-stage and fully variable-speed equipment. Air handler The indoor unit that moves air through your ducts, blower, coil, and controls in one cabinet. In all-electric homes it takes the furnace's place, pairing with a heat pump or AC and often housing electric backup heat strips.
Sizing & airflow
Manual J The industry-standard load calculation that determines how much heating and cooling your specific home actually needs, using insulation, windows, orientation, ceiling heights, and air leakage, not just square footage. Static pressure The resistance your ductwork puts up against airflow, HVAC's version of blood pressure. Measured in inches of water column, it tells a technician whether air can actually move through the system as designed. Balance point The outdoor temperature at which a heat pump's output exactly matches your home's heat loss, above it, the heat pump covers everything; below it, backup heat starts helping. BTU British Thermal Unit, the standard measure of heating or cooling energy. One BTU is the energy needed to raise a pound of water one degree Fahrenheit; HVAC equipment is rated in BTUs per hour of output. Tonnage An air conditioner or heat pump's cooling capacity. One 'ton' equals 12,000 BTU per hour, a reference to the melting power of a literal ton of ice per day. Residential systems typically run 1.5 to 5 tons. Short cycling When a system starts, runs only a few minutes, and shuts off, over and over. Causes range from a clogged filter or a failing sensor to the big one: equipment that's oversized for the home.
Comfort & air
MERV Minimum Efficiency Reporting Value, the 1 to 20 rating for how well an air filter captures particles. Higher MERV catches smaller particles: MERV 8 handles dust and pollen; MERV 13 also captures smoke, bacteria-sized particles, and most allergens. Relative humidity (RH) The amount of moisture in air compared to the maximum it could hold at that temperature, expressed as a percentage. Comfortable indoor range is roughly 30 to 50%.
Vocabulary is step one
Step two is applying it to your home. The Learning Center explains how systems work, the comparison guides weigh your options, and the decision guides give you honest verdicts.