HVAC Glossary
Heating & Cooling Terms, Explained
Plain-language definitions of the ratings, equipment, and service terms you'll come across when choosing or maintaining an HVAC system.
Efficiency Ratings
- SEER2
- Seasonal Energy Efficiency Ratio (2023 standard) — measures how efficiently an air conditioner or heat pump cools over a season. A higher SEER2 means lower cooling energy use.
- AFUE
- Annual Fuel Utilization Efficiency — the percentage of fuel a furnace or boiler converts to usable heat. A 95% AFUE furnace turns 95% of its fuel into heat for your home.
- HSPF2
- Heating Seasonal Performance Factor (2023 standard) — rates a heat pump's heating efficiency over a season. Higher HSPF2 means more heat delivered per unit of electricity.
- EER2
- Energy Efficiency Ratio (2023 standard) — a system's cooling efficiency at a single peak-load condition, useful for comparing performance on the hottest days.
- BTU
- British Thermal Unit — a measure of heating or cooling energy. Equipment capacity is often expressed in BTUs per hour; 12,000 BTU/hr equals one ton of cooling.
- Tonnage
- The cooling capacity of an AC or heat pump, measured in tons. Correct tonnage comes from a load calculation, not just square footage.
Equipment
- Heat Pump
- A system that both heats and cools by moving heat rather than burning fuel — pulling heat from outdoor air (or the ground) into your home in winter and reversing in summer.
- Furnace
- A heating appliance that burns natural gas or propane (or uses electric elements) to warm air, which is then distributed through ductwork by a blower.
- Boiler
- A system that heats water and circulates it (or steam) through radiators, baseboards, or in-floor tubing to warm a space.
- Air Handler
- The indoor unit that circulates conditioned air. Paired with a heat pump or AC, it houses the blower and evaporator coil.
- Condenser
- The outdoor unit of a split system. It releases heat removed from your home (in cooling mode) and contains the compressor and condenser coil.
- Evaporator Coil
- The indoor coil that absorbs heat and humidity from your home's air as refrigerant evaporates inside it.
- Compressor
- The heart of an AC or heat pump — it pressurizes and circulates refrigerant so the system can move heat. Compressor type (single-, two-stage, or variable-speed) affects comfort and efficiency.
- Ductless Mini-Split
- A system with an outdoor unit connected to one or more wall/ceiling indoor heads, delivering heating and cooling without ductwork — ideal for additions, garages, and zoning.
- Geothermal Heat Pump
- A ground-source system that exchanges heat with the stable temperature of the earth through buried loops, offering very high year-round efficiency.
Comfort & Air Quality
- MERV
- Minimum Efficiency Reporting Value — rates how well an air filter captures particles. Higher MERV traps smaller particles, but the rating should match what your system can handle.
- IAQ
- Indoor Air Quality — the condition of the air inside your home, affected by filtration, humidity, ventilation, and contaminants like dust, pollen, and VOCs.
- HRV / ERV
- Heat- and Energy-Recovery Ventilators — bring in fresh outdoor air while recovering heat (and, for ERVs, moisture) from the air being exhausted.
- Humidifier / Dehumidifier
- Whole-home accessories that add moisture in dry winter months or remove it in humid seasons to keep indoor humidity in a comfortable, healthy range.
- Zoning
- Using dampers or multiple indoor units and thermostats to control temperatures independently in different areas of a home.
- Variable-Speed
- Equipment that adjusts output in fine increments instead of simply on/off, delivering steadier temperatures, quieter operation, and better humidity control.
Service & Operation
- Load Calculation (Manual J)
- An engineering calculation of a home's exact heating and cooling needs, used to size equipment correctly instead of guessing.
- Refrigerant
- The fluid that carries heat through an AC or heat pump. Modern systems use low-GWP refrigerants; older R-22 (Freon) systems are being phased out.
- Short Cycling
- When a system turns on and off too frequently. It wastes energy, wears out parts, and often signals an oversized unit, low refrigerant, or a control issue.
- Thermostat
- The control that tells your system when to heat or cool. Smart and programmable thermostats add scheduling, remote control, and energy insights.
- Ductwork
- The network of channels that distributes conditioned air. Leaky or poorly designed ducts can waste a large share of heating and cooling energy.
- Preventive Maintenance
- Scheduled tune-ups and inspections that keep equipment running efficiently, catch small problems early, and help protect manufacturer warranties.
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