HVAC Glossary
The trade's vocabulary, minus the mystery — 55 terms explained in plain counter English.
A2L Refrigerants
The 'mildly flammable' safety class covering R-454B and R-32. Ignition…AFUE
Annual Fuel Utilization Efficiency — the percentage of fuel a furnace …AHRI Match
A certified combination of outdoor unit, indoor coil/air handler, and …Auxiliary Heat Lockout
A thermostat setting that blocks electric strip heat above a chosen ou…B-Vent
Double-wall metal venting for draft-hood and 80%-class gas appliances.…Balance Point
The outdoor temperature where a heat pump's output exactly matches the…Black Iron Pipe
Threaded steel pipe, the traditional material for fuel-gas lines. Asse…Burner Orifice
The precisely drilled brass fitting that meters gas to each burner. Na…CFM
Cubic feet per minute — airflow volume. Residential cooling wants roug…Condensate Trap
The water-seal trap on an evaporator drain that stops air from blowing…Condensing Furnace
A 90%+ AFUE furnace with a secondary heat exchanger that wrings extra …Contactor
The relay that lets 24V thermostat wiring switch the condenser's line-…Core Charge
A deposit built into the price of certain parts — compressors, some mo…CSST
Corrugated Stainless Steel Tubing — flexible gas piping that snakes th…Defrost Cycle
In heating mode the outdoor coil runs below freezing and frosts over; …Design Temperature
The outdoor temperatures a system is sized against — typically the 99%…Draft Inducer
The small blower that pulls combustion air through the heat exchanger …Dual Fuel
A heat pump paired with a gas furnace: the heat pump handles mild weat…ECM Motor
Electronically commutated motor — a variable-speed DC blower motor tha…Encapsulated Crawlspace
A sealed, vapor-barriered, often conditioned crawlspace. It pulls duct…EPA Section 608
The federal certification required to purchase and handle regulated re…Flame Sensor
A rod that proves the burner lit by conducting a microamp current thro…Float Switch
A safety that breaks the cooling call when condensate backs up, before…GWP (Global Warming Potential)
How much heat a gas traps relative to CO₂ over a century. R-410A sits …Hard-Start Kit
A start capacitor plus relay that gives a compressor extra starting to…Heat Exchanger
The sealed metal passages that transfer combustion heat to household a…Heat Kit (Electric Heat Strips)
Resistance heating elements installed in an air handler, sized in kilo…Hot-Surface Igniter
The ceramic element that glows white-hot to light the burner, replacin…HSPF / HSPF2
Heating Seasonal Performance Factor — the heating-side efficiency rati…Hyper-Heat (H2i)
Mitsubishi's enhanced-vapor-injection cold-climate technology — heat p…Line Set
The pair of copper lines connecting outdoor and indoor units: insulate…Load Calculation
The math that determines a building's actual heating and cooling requi…Manifold Pressure
Gas pressure delivered to the burners, measured in inches of water col…Manual D
The ACCA duct-design method — sizing trunk and branch ducts to deliver…Manual J
The ACCA load-calculation procedure that determines how much heating a…MERV Rating
Minimum Efficiency Reporting Value — a filter's ability to capture par…MFD / Microfarad
The unit of capacitance printed on motor capacitors. Dual capacitors s…Mini-Split (Ductless)
A heat pump system connecting an outdoor unit to one or more indoor he…Multi-Zone System
One outdoor unit driving several indoor heads, each independently cont…Net-30 Terms
Trade credit where the invoice is due 30 days after billing. Supply ho…OEM Part
Original Equipment Manufacturer — the exact part the equipment shipped…R-Value
Resistance to heat flow — higher insulates better. On flex duct, the R…Reclaimed Refrigerant
Used refrigerant recovered, processed back to purity spec, and resold …Reversing Valve
The four-way valve that flips a heat pump between heating and cooling …Run Capacitor
The cylindrical component that phase-shifts power to keep compressor a…SEER2
Seasonal Energy Efficiency Ratio, version 2 — cooling efficiency measu…Short Cycling
Equipment starting and stopping in rapid cycles. Causes range from ove…Static Pressure
The resistance the blower pushes against, measured in inches of water …Subcooling
How many degrees the liquid refrigerant is below its condensing temper…Superheat
How many degrees the refrigerant vapor is above its boiling point at a…Two-Stage Equipment
Furnaces or ACs with a low and high output level. Low stage handles mo…TXV (Thermostatic Expansion Valve)
The metering device that regulates refrigerant flow into the evaporato…Variable-Speed / Inverter
Equipment that modulates output continuously instead of cycling on and…Vented Crawlspace
A foundation crawl with open vents to outdoor air — standard in older …Will-Call
Supply-house term for order-ahead counter pickup: call or submit the o…