HVAC Glossary
Every technical term used across HVACPool, explained once, properly.
HVAC has an unusual amount of jargon for an industry ordinary people have to buy from. A contractor says “you want a two-stage 96 AFUE with a variable-speed blower” and the homeowner nods. This page is here so you do not have to nod. Terms are grouped by what they actually do rather than alphabetically, because that is how they make sense.
Efficiency Ratings
AFUE (Annual Fuel Utilisation Efficiency) — The percentage of fuel a furnace or boiler turns into usable heat across a full heating season. A 96% AFUE furnace sends 96 cents of every fuel dollar into the house and 4 cents up the flue. Anything at 90% or above is a condensing appliance and needs a condensate drain and plastic venting.
SEER2 (Seasonal Energy Efficiency Ratio 2) — Seasonal cooling efficiency under the test procedure that replaced SEER in 2023. SEER2 uses a higher external static pressure, so it is a slightly harder test; a SEER2 number is roughly 4.5–7% lower than the old SEER number for the same equipment. Higher is better. Ratings are certified for a specific indoor and outdoor combination, not for the outdoor unit alone.
EER2 (Energy Efficiency Ratio 2) — Efficiency at one fixed high-temperature condition rather than across a season. It matters more than SEER2 in hot-dry climates where the system spends its life near design conditions.
HSPF2 (Heating Seasonal Performance Factor 2) — The heating-mode equivalent of SEER2 for heat pumps. Higher is better.
COP (Coefficient of Performance) — Heat delivered divided by energy consumed, at a single operating point. A heat pump with a COP of 3 delivers three units of heat per unit of electricity. COP falls as outdoor temperature drops.
IEER (Integrated Energy Efficiency Ratio) — Part-load cooling efficiency, used mainly for commercial and packaged equipment that rarely runs at full capacity.
ENERGY STAR — A US government-backed certification. Criteria differ by product category and are revised periodically, so always confirm current status for the specific model on energystar.gov rather than trusting a label in a brochure.
Capacity and Sizing
BTU (British Thermal Unit) — The heat needed to raise one pound of water by one degree Fahrenheit. Equipment is rated in BTU per hour (BTU/h), which is a rate, not a quantity.
Ton — 12,000 BTU/h of cooling capacity. The name comes from the cooling a ton of melting ice provides in a day. A three-ton air conditioner has nothing to do with its weight.
Load Calculation — The engineering process that determines how much heating and cooling a specific building actually needs, accounting for area, insulation, window area and orientation, air leakage, occupancy and local design temperatures. In North America this is an ACCA Manual J calculation. It is the only defensible basis for selecting equipment.
Manual J, S and D — ACCA procedures for calculating the load (J), selecting equipment to match it (S), and designing the duct system to deliver it (D). A contractor who skips these and sizes from floor area is guessing.
Design Temperature — The outdoor temperature a system is sized to handle, chosen so it is exceeded only a small percentage of hours per year. Sizing for the worst day ever recorded produces an oversized system that performs badly the rest of the time.
Balance Point — The outdoor temperature at which a heat pump’s output exactly matches the building’s heat loss. Below it, supplementary heat is needed. Knowing yours tells you how often backup heat will actually run.
Short Cycling — Equipment switching on and off too frequently, usually because it is oversized. It wastes energy, leaves humidity in the air, and wears out compressors and igniters early.
Airflow and Air Quality
CFM (Cubic Feet per Minute) — Volume of air moved per minute. Used to size exhaust fans, ventilation systems and duct runs. A bathroom fan rated 110 CFM moves that much only against the static pressure it was tested at.
Static Pressure — The resistance a duct system presents to the blower, measured in inches of water column. High static pressure starves the system of airflow and is one of the most common reasons a correctly sized system underperforms.
MERV (Minimum Efficiency Reporting Value) — Filter effectiveness from 1 to 16. MERV 8 catches dust and pollen; MERV 13 catches finer particles including many bacteria and smoke. Higher is not automatically better — a dense filter in a system not designed for it raises static pressure and cuts airflow.
HEPA — A filter removing at least 99.97% of particles at 0.3 microns. True HEPA needs a dedicated fan; it cannot simply be dropped into a standard residential return.
Sone — A measure of perceived loudness used for ventilation fans. One sone is roughly a quiet refrigerator. Anything at or below 1.0 sone reads as quiet in a bathroom.
HRV (Heat Recovery Ventilator) — Brings in fresh air while recovering heat from the outgoing stale air. Transfers heat only, not moisture.
ERV (Energy Recovery Ventilator) — As an HRV, but transfers moisture as well as heat. Generally the better choice in humid climates, where an HRV would bring the humidity straight in with the fresh air.
ACH (Air Changes per Hour) — How many times the full volume of air in a space is replaced each hour.
Equipment and Operation
Single-Stage — The equipment is either fully on or fully off. Cheapest, and the least comfortable, because every call for heat or cooling runs at 100%.
Two-Stage — A high and a low output setting. Runs on low most of the time, which means longer, gentler cycles and better humidity control.
Modulating / Variable-Speed — Output varies continuously to match demand, often down to 20–40% of capacity. The most comfortable and usually the most efficient in practice, and the most expensive to buy and repair. “Modulating” generally refers to the burner; “variable-speed” to the blower or compressor.
Inverter — The drive electronics that let a compressor run at variable speed. Standard on mini-splits and increasingly common on ducted equipment.
Zoning — Dividing a building so areas can be conditioned independently, using motorised dampers on a ducted system or separate indoor heads on a multi-zone mini-split.
Line Set — The pair of insulated copper refrigerant lines connecting an outdoor unit to an indoor coil. Length, diameter and routing all affect performance, and a reused line set must be properly cleaned or replaced.
Refrigerant — The working fluid that moves heat. R-410A is being phased down under global-warming-potential rules; R-32 and R-454B are the current replacements. A system is designed for one refrigerant and cannot simply be switched to another.
Condensate — Water produced when warm humid air passes over a cold coil, and also by condensing furnaces. It needs a drain, a trap and often a pump. Blocked condensate drains are among the most common summer service calls.
Heat Exchanger — The component separating combustion gases from the air you breathe. A cracked one is a carbon monoxide risk and means replacement, not repair.
Enthalpy — Total heat content of air, both temperature and moisture. It is why ERVs are described as transferring enthalpy while HRVs transfer only sensible heat.
Sensible vs Latent Heat — Sensible heat changes temperature; latent heat changes moisture. An air conditioner does both, and a system sized only for sensible load will leave a house cold and clammy.
AHRI Directory — The certification database where you verify the rated performance of a specific indoor and outdoor equipment combination. If a rating is not in there for the exact model pairing you are quoted, treat the number with suspicion.
Put These Terms to Work
Now that the vocabulary is out of the way, our guides walk through the actual decisions — what to buy, how to size it, and what a fair quote looks like.