AFUE (Annual Fuel Utilization Efficiency)
How efficiently a furnace converts fuel into heat over a year, expressed as a percentage. A 96% AFUE furnace turns 96% of the fuel into useful heat. Current minimum for new gas furnaces is 80% AFUE; high-efficiency systems are 90-98%.
SEER2 (Seasonal Energy Efficiency Ratio)
Cooling efficiency rating for air conditioners and heat pumps. Higher SEER2 = more efficient. As of 2023 the federal minimum in the North is 14 SEER2; high-efficiency systems are 16-22 SEER2. SEER2 replaced SEER in 2023 with a tougher test procedure.
HSPF2 (Heating Seasonal Performance Factor)
Heating efficiency rating for heat pumps. Higher HSPF2 = more efficient. As of 2023 the federal minimum is 7.5 HSPF2; cold-climate heat pumps are 8.5+.
Manual J load calculation
The ACCA industry-standard method for sizing an HVAC system. Accounts for square footage, insulation R-values, window types and orientation, infiltration, and occupancy. Manual J is the difference between a system that runs at design conditions and one that short cycles all summer.
Refrigerant (R-410A, R-454B, etc.)
The chemical that absorbs and releases heat as it cycles through your AC or heat pump. R-410A has been the dominant residential refrigerant since 2010 but is being phased down for environmental reasons. Newer systems use R-454B or R-32. They are not interchangeable.
Plenum
The large rectangular metal box that sits on top of a furnace or air handler. The supply plenum sends conditioned air into the duct system; the return plenum collects air coming back.
Dual-fuel system
A heat pump paired with a gas furnace. The heat pump heats the home efficiently above ~30-35°F; the gas furnace takes over below that point when the heat pump becomes less economical. Common in cold climates.
Static pressure
The resistance air encounters as it moves through ductwork. Too high (often from dirty filters, undersized returns, or restrictive transitions) and the system works harder and wears out faster.
BTU (British Thermal Unit)
The unit of heat used to size HVAC equipment. 12,000 BTU/hour = 1 ton of cooling capacity. A typical Western Mass single-family home needs 24,000 to 48,000 BTU/hour of cooling (2 to 4 tons).
Ton (of cooling)
HVAC slang for 12,000 BTU/hour of cooling capacity. A "3-ton system" cools at 36,000 BTU/hour.
Heat exchanger
The component inside a furnace that transfers heat from combustion gases to the air your blower pushes through the house. A cracked heat exchanger is dangerous (carbon monoxide risk) and almost always means furnace replacement.
Condenser unit
The outdoor part of a central AC or heat pump. Contains the compressor and the outdoor coil. The big square or cylindrical box outside your house.
Evaporator coil
The indoor part of a central AC or heat pump that absorbs heat from your home's air. Lives inside the air handler or on top of the furnace.
Air handler
The indoor blower-and-coil cabinet for systems that do not have a furnace. Common with all-electric heat pumps and ductless mini-split central installations.
Mini-split (ductless)
A heat pump system that does not use ductwork. An outdoor condenser connects to one or more wall-mounted (or ceiling-mounted) indoor heads via refrigerant lines. Excellent for additions, finished basements, and homes without existing ductwork.
Two-stage / Variable-speed
Equipment that can operate at less than 100% capacity. A two-stage furnace runs at low or high; a variable-speed furnace or AC can modulate between 30-100%. Quieter and more efficient than single-stage.
Hot-surface igniter (HSI)
The component in a modern gas furnace that glows red-hot to ignite the burners. Replaced electronic spark ignition and standing pilots in most furnaces built after 1995. Typical lifespan 3-7 years.
Flame sensor
A small metal rod next to the burners that confirms the flame is actually lit. Gets coated with combustion residue over time, which causes intermittent ignition failures. Cleaning is part of every annual tune-up.
Mass Save
The statewide energy-efficiency program in Massachusetts. Offers rebates and 0% financing for qualifying heat pumps, insulation, and other efficiency upgrades. Rebate amounts and rules change annually. masssave.com.
ECM blower motor
Electronically commutated motor. The variable-speed blower technology in modern high-efficiency furnaces and air handlers. Quieter and uses 50-70% less electricity than older PSC motors.
NATE certification
North American Technician Excellence. Industry credentialing for HVAC technicians. A NATE-certified tech has passed standardized tests on specific equipment categories.
Last updated: May 13, 2026