HVAC Glossary

Plain-English explainers for the technical terms, rebates, regulations, and acronyms that show up when you shop for HVAC systems and services. Each entry links the underlying source, lists the numbers that matter, and points to related concepts.

AFUE (Annual Fuel Utilization Efficiency)
The share of a furnace's fuel that becomes heat in your home — a 90% AFUE furnace turns 90 cents of every fuel dollar into warmth.
Blower Motor (Furnace / Air-Handler Fan)
The motor that pushes heated or cooled air through your ducts — its type sets how quiet, even, and efficient the whole system feels.
BTU (British Thermal Unit)
A small unit of heat energy — heating and cooling equipment is rated in thousands of BTUs per hour to show how much it can move.
Capacity Sizing Rule (BTU per Square Foot)
A quick estimate of how much cooling a space needs from its floor area — useful for a sanity check, but a Manual J calc is the real test.
Compressor Staging (Single, Two-Stage, Variable-Speed)
Whether your AC's compressor runs at one speed or can throttle down — it sets how evenly the system cools and how well it removes humidity.
Condenser (Outdoor Unit)
The outdoor box of a central AC or heat pump — it dumps the heat pulled from your house into the outside air.
Dual-Fuel System (Hybrid Heat Pump + Furnace)
Pairs an electric heat pump with a gas furnace and switches between them — the heat pump for mild days, the furnace for deep cold.
Ductless (No-Duct Heating & Cooling)
Heating and cooling that skips the ductwork — each room is conditioned by its own wall or ceiling unit, not air pushed through ducts.
ECM Motor (Electronically Commutated Motor)
An energy-efficient, electronically controlled blower motor that varies its speed — the standard in furnaces sold today.
EER vs SEER (Peak vs Seasonal Efficiency)
EER rates an air conditioner's efficiency on the hottest day; SEER rates its average over a whole summer — two scales you can't compare.
Evaporator Coil (Indoor Coil)
The indoor coil that absorbs heat and moisture from your home's air — the cold half of a central AC or heat pump.
COP (Coefficient of Performance)
How many units of heat a heat pump delivers per unit of electricity — a COP of 3 means three times more heat out than power in.
HSPF (Heating Seasonal Performance Factor)
Rates how efficiently a heat pump heats over a whole winter — the heating-season counterpart to SEER. Higher HSPF means lower heat bills.
MERV (Minimum Efficiency Reporting Value)
A 1-to-16 score for how well an air filter traps particles — higher MERV catches smaller particles but makes your blower work harder.
Mini-Split (Ductless Heat Pump System)
A heat pump with a small outdoor unit and one or more indoor wall units, linked by refrigerant lines — heating and cooling without ducts.
Refrigerant Transition (R-410A to R-454B / R-32)
The 2025–2026 switch to lower-emission refrigerants in new AC and heat pumps — new systems now use R-454B or R-32 instead of R-410A.
SEER (Seasonal Energy Efficiency Ratio)
Measures how efficiently an air conditioner cools over a full summer season — higher SEER means lower electric bills for the same cooling output.
SEER2 (Seasonal Energy Efficiency Ratio 2)
The 2023 update to the SEER cooling rating, tested under tougher conditions — a SEER2 number runs a bit lower than the old SEER.
Ton (of Cooling)
A measure of air-conditioner cooling power, not weight — one ton equals 12,000 BTU of heat removed per hour.
VRF (Variable Refrigerant Flow)
A large-building cousin of the mini-split — one outdoor unit feeds many indoor units and varies refrigerant flow to each zone independently.
ⓘ Educational content — not professional advice.

Glossary entries are provided "as is" for general educational and informational purposes only. They are not tax, legal, medical, engineering, or other professional advice and should not be relied on as such. We do not warrant that the information is accurate, complete, or current — rates, regulations, product specifications, rebate programs, and tax credits change frequently and may differ from what you read here.

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