HVAC Glossary

HVAC terms, in plain English.

67 terms you'll hear from a technician, see on a quote, or find on a rebate form — each with what it means and why it matters in a SW Washington or Portland-metro home.

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Refrigerants A2L Refrigerant An A2L refrigerant is one classified under ASHRAE Standard 34 as lower toxicity (class A) and lower flammability (class 2L), meaning it is hard to ignite and burns slowly if it does. R-32 and R-454B, the main replacements for R-410A in new U.S. home systems, are both A2L. Equipment built for them follows the UL 60335-2-40 safety standard. What it means for you →
Money, Code & Permits AFUE AFUE (Annual Fuel Utilization Efficiency) is the percentage of a furnace's fuel energy that becomes usable heat in the home over a typical year. An 80% AFUE gas furnace loses about a fifth of its fuel energy, mostly up the vent, while a 95% AFUE condensing furnace captures extra heat from its exhaust and wastes far less. What it means for you →
Money, Code & Permits AHJ (Authority Having Jurisdiction) The Authority Having Jurisdiction (AHJ) is the government office legally responsible for permitting and inspecting construction work at a specific property, usually a city building department or, outside city limits, the county. It follows legal boundaries rather than mailing addresses, and it decides which permits apply, what they cost and whether finished work passes. What it means for you →
Money, Code & Permits AHRI Certificate (Matched System) An AHRI certificate is a record from the AHRI Certification Directory listing the certified efficiency ratings, such as SEER2, EER2 and HSPF2, for one specific combination of outdoor unit, indoor coil or air handler, and sometimes furnace. Each matched combination has its own AHRI reference number, so a mismatched pairing may not carry the advertised rating. What it means for you →
Pacific NW Home & Climate Air Changes per Hour (ACH) Air changes per hour (ACH) is the number of times a home's full volume of air is replaced with outdoor air in one hour. It describes both ventilation rate and airtightness: ACH50, measured with a blower-door test at 50 pascals of pressure, rates leakage, and lower ACH50 values mean a tighter house. What it means for you →
Airflow, Ducts & Air Quality Air Filter An air filter is the replaceable panel, pleated or fiberglass, that sits in a return grille or a filter slot at the furnace or air handler and catches dust, lint, pollen, and other particles before they reach the blower and coil. It protects the equipment first; its MERV rating describes how well it captures smaller particles. What it means for you →
Equipment Parts Air Handler An air handler is the indoor cabinet of a split heat pump or air conditioning system that houses the blower motor, the evaporator coil, controls and often the filter rack. It moves conditioned air through the supply ducts and pulls air back through the returns. Electric resistance heat strips inside many air handlers provide auxiliary and emergency heat. What it means for you →
Heating & Heat Pumps Auxiliary Heat Auxiliary heat is backup heat that a heat pump's thermostat turns on automatically when the heat pump alone cannot keep up, such as below the balance point, during a defrost cycle or after a large thermostat setback. In all-electric systems it usually comes from resistance heat strips in the air handler; in dual-fuel systems, from a gas furnace. What it means for you →

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Heating & Heat Pumps Gas Furnace A gas furnace is a heating appliance that burns natural gas or propane, passes the heat through a metal heat exchanger and blows warmed air through a home's ductwork. Combustion gases are vented outdoors through a flue. Furnace efficiency is rated by AFUE, the percentage of fuel energy delivered to the home as usable heat. What it means for you →

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Cooling Packaged Unit A packaged unit is a self-contained heating and cooling system with the compressor, condenser coil, evaporator coil and blower housed in one outdoor cabinet. Supply and return ducts run from the cabinet into the building, and the cabinet typically sits on a ground pad or roof. Common versions are packaged air conditioners, packaged heat pumps and gas-electric units. What it means for you →

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Money, Code & Permits Variable-Speed Compressor A variable-speed compressor, often called an inverter compressor, adjusts its output across a continuous range instead of running only fully on or off. A single-stage compressor has one output level and a two-stage compressor has two. Running longer at lower output keeps temperatures even and typically earns higher SEER2 and HSPF2 ratings. What it means for you →

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Want the bigger picture? The Learning Center has in-depth guides on heat pumps, sizing, ventilation, wildfire smoke and rebates.

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