Learning Center · GlossaryEst. 1950
HVAC terms, made simple
32 heating and cooling terms defined, so you can talk to any contractor with confidence.
- AFUE (Annual Fuel Utilization Efficiency)
- A percentage measuring how much of a furnace's fuel becomes usable heat over a year. A 95% AFUE furnace converts 95% of its gas to heat; an older unit may be 70–80%.
- Air Handler
- The indoor unit that moves conditioned air through the ducts, typically paired with a heat pump or AC. Contains the blower and evaporator coil.
- Boiler
- A heating system that warms water or steam and circulates it through radiators or in-floor loops, rather than blowing hot air.
- BTU (British Thermal Unit)
- A unit of heat energy. Furnace output and AC capacity are rated in BTUs (or BTUs per hour) to size equipment to a home.
- Carbon Monoxide (CO)
- A colorless, odorless gas produced by incomplete combustion. A working CO detector and annual furnace inspection are the key protections for any gas-heated home.
- Compressor
- The pump at the heart of an AC or heat pump that circulates and pressurizes refrigerant. The most expensive component to replace.
- Condensate Drain
- The line that carries away water produced by a high-efficiency furnace or an AC's evaporator coil. A clog can shut the system down or cause leaks.
- Condenser
- The outdoor unit of an AC or heat pump that releases heat (in cooling mode). Houses the compressor and condenser coil.
- Dehumidifier
- Removes excess moisture, improving summer comfort and reducing mold risk. Can be standalone or whole-home.
- Dual-Fuel System
- A heat pump paired with a gas furnace. The heat pump heats efficiently most of the year; the furnace takes over automatically on the coldest days. Common in Minnesota.
- Ductless Mini-Split
- A heat pump that delivers heating and cooling to specific rooms without ductwork, using an outdoor unit and one or more wall/ceiling indoor heads. Ideal for additions and zoning.
- EPA Section 608
- Federal certification required for technicians who handle refrigerant. A sign your installer is qualified to work on AC and heat-pump systems legally and safely.
- Evaporator Coil
- The indoor coil where refrigerant absorbs heat from your home's air. When airflow or refrigerant is low, it can freeze.
- Flame Sensor
- A safety component that confirms the burner has lit. A dirty flame sensor is a common cause of a furnace that lights then shuts off seconds later.
- Furnace
- A forced-air heater that burns gas (or uses electricity) to warm air, then distributes it through ductwork with a blower.
- Heat Exchanger
- The metal chamber in a furnace that transfers combustion heat to your home's air while keeping exhaust gases separate. A crack is a safety concern (CO risk).
- Heat Pump
- A system that heats and cools by moving heat rather than burning fuel. In winter it extracts heat from outdoor air; in summer it removes heat from your home.
- HEPA
- A very high-efficiency particulate filter standard. True HEPA usually requires a dedicated air-cleaning unit, not a slot filter, to avoid choking system airflow.
- HRV / ERV
- Heat- or Energy-Recovery Ventilator. Brings measured fresh air into a tightly sealed home while recovering heat (and, for an ERV, moisture) from the outgoing air.
- HSPF2
- The heating-efficiency rating for heat pumps. Higher HSPF2 means more heat delivered per unit of electricity over a heating season.
- Humidifier
- A whole-home device that adds moisture to dry winter air, reducing static, dry skin, and gaps in wood floors. Set to match the outdoor temperature to avoid window condensation.
- Load Calculation (Manual J)
- The industry method for sizing equipment to a home's actual heat loss/gain — using insulation, windows, and layout, not just square footage. The right way to size a replacement.
- Mechanical Permit
- The local authorization required to install or replace heating/cooling equipment, pulled by a licensed contractor and followed by an inspection that confirms code compliance.
- MERV
- Minimum Efficiency Reporting Value — how well an air filter captures particles. Higher catches more, but too-high a rating can restrict airflow if the system can't handle it.
- NATE Certification
- North American Technician Excellence — a respected, independent HVAC technician certification covering installation and service competency.
- Refrigerant
- The fluid that carries heat through an AC or heat pump. It isn't consumed — low refrigerant means a leak, not normal use. Modern systems use R-410A or newer low-GWP blends.
- SEER2
- The current rating for air-conditioner and heat-pump cooling efficiency. Higher SEER2 means more cooling per unit of electricity. It replaced the older SEER standard.
- Short-Cycling
- When a furnace or AC starts and stops in rapid bursts without completing a cycle. Wastes energy and wears components; often caused by a dirty filter or oversizing.
- Static Pressure
- The resistance to airflow in your duct system. High static pressure (often from restrictive filters or undersized ducts) strains the blower and hurts comfort.
- Ton (of cooling)
- A measure of air-conditioner capacity equal to 12,000 BTU/hour. A typical home AC is 2–5 tons, set by a load calculation — not square footage alone.
- Variable-Speed / Two-Stage
- Equipment that can run at partial capacity instead of only full-on. It delivers steadier temperatures, quieter operation, and better efficiency than single-stage units.
- Zoning
- Dividing a home into independently controlled comfort areas using dampers or multiple units, so you heat and cool only the spaces you're using.
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