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HVAC glossary

Every term you'll meet in a quote, an inspection report, or one of our guides — defined in plain English, with the part that actually matters: what it means for your home and your money.

In brief

A plain-English HVAC glossary from Best Owner Direct HVAC & Electrical — 44 terms from AFUE and SEER2 to CCB licensing and Energy Trust rebates, every one grounded in the same service FAQs and Learning Center guides we quote from across the Tualatin Valley & Portland metro. No invented figures: money terms link to the sourced, dated rebate page rather than repeating a number that could drift.

Browse the full list

Heating

AFUE (Annual Fuel Utilization Efficiency) AFUE is the percentage of the fuel a gas furnace burns in a full heating season that actually becomes heat inside the home — the rest escapes up the flue as exhaust. A furnace rated 95% AFUE turns 95 cents of every gas dollar into warmth; an older 80% AFUE unit sends 20 cents up the flue. It's the efficiency rating the U.S. Department of Energy requires on every gas furnace sold. Heat Exchanger The heat exchanger is the sealed metal chamber inside a gas furnace where combustion happens — it transfers heat from burning gas into the air your ducts circulate, without ever letting combustion byproducts mix with that air. It's the one furnace part where a failure is a safety issue, not just a comfort one. Condensing Furnace & Venting A condensing furnace burns gas so completely (95%+ AFUE) that the exhaust gas cools enough to condense into water before it leaves the house — cool enough that it is no longer safe to send up a hot metal flue. Instead it vents sideways through PVC pipe and drips condensate into a drain, the way a standard 80% AFUE furnace's hot exhaust never has to. Single-Stage / Two-Stage / Variable-Speed Furnace These describe how a gas furnace ramps its burner and blower. Single-stage is full-blast on or fully off. Two-stage runs a lower flame most of the time and only jumps to full output on the coldest days. Variable-speed continuously adjusts flame and airflow to match exactly the heat the house needs. Short-Cycling Short-cycling is when a heating or cooling system turns on, runs for only a few minutes, and shuts off again — repeatedly — instead of running a normal, longer cycle. It's usually a symptom of an oversized system or a control/airflow problem, not normal behavior. Dual-Fuel (Hybrid) System A dual-fuel (or hybrid) system pairs a heat pump with a gas furnace in the same ductwork. The heat pump handles heating and cooling most of the year, since it costs less to run in mild weather; the system automatically switches to the gas furnace only when outdoor temperatures drop low enough that gas becomes the more efficient or reliable choice.

Air Conditioning

SEER2 (Seasonal Energy Efficiency Ratio 2) SEER2 rates how much cooling an air conditioner or heat pump delivers per unit of electricity over a typical season, tested under a revised, more realistic 2023 DOE procedure that replaced the older SEER rating. Higher SEER2 means lower cooling cost for the same comfort. Refrigerant Charge Refrigerant charge is the exact amount of refrigerant an air conditioner or heat pump needs to move heat correctly — set by the manufacturer to a specific weight, not "topped off" like motor oil. Too little or too much both hurt performance and can damage the compressor over time. R-410A vs. R-22 Refrigerant R-22 (Freon) was the standard AC refrigerant for decades until U.S. production was phased out under the Montreal Protocol; R-410A replaced it as the standard for new equipment. The two aren't interchangeable — a system designed for one is not simply refilled with the other. Capacitor & Contactor The capacitor stores and releases the extra jolt of electricity an AC compressor and fan motor need to start spinning; the contactor is the electrically-controlled switch that turns the outdoor unit's power on and off each time the thermostat calls for cooling. Both are common failure points because they cycle every single time the system runs. Condenser (Outdoor Unit) The condenser is the outdoor unit of a split AC or heat pump system — it contains the compressor, the outdoor coil, and the fan that rejects heat pulled from inside the house (or, for a heat pump in winter, absorbs heat from outside air). It's the half of the system most exposed to weather and debris.

Heat Pumps & Mini-Splits

Heat Pump A heat pump is a single system that both heats and cools by moving heat rather than generating it — pulling warmth from outside air (even cold air) into the home in winter, and reversing to pull heat out in summer, the way a refrigerator moves heat out of its cabinet. One piece of equipment replaces both a furnace and a separate AC. COP (Coefficient of Performance) COP is the ratio of heat a heat pump delivers to the electrical energy it consumes to deliver it — a COP of 3 means three units of heat output for every one unit of electricity in. Unlike a furnace's AFUE, which tops out under 100%, COP is routinely well above 1 because the heat pump is moving heat, not manufacturing it from fuel. HSPF2 (Heating Seasonal Performance Factor 2) HSPF2 is the standardized seasonal efficiency rating for a heat pump's heating mode, tested under the same 2023 DOE update that produced SEER2 — replacing the older HSPF rating. Higher HSPF2 means more heat delivered per dollar of electricity across a full heating season. Cold-Climate (Extended-Capacity) Heat Pump A cold-climate (extended-capacity) heat pump is engineered to keep delivering meaningful heat output at outdoor temperatures well below freezing, where a standard heat pump's capacity falls off sharply. It typically meets a specific efficiency tier (CEE Tier 1 Path A) and pairs with lockout controls that manage when backup heat engages. Defrost Mode Defrost mode is a heat pump's automatic process for melting frost that builds up on the outdoor coil during cold, humid weather — it briefly reverses to cooling mode to warm the coil, which is why you may see steam rising from the outdoor unit or feel a short burst of cool air indoors during winter. Auxiliary/Backup Heat & Lockout Controls Auxiliary (or backup/emergency) heat is a secondary heat source — usually electric-resistance strip heat, sometimes a gas furnace in a dual-fuel setup — that supplements or replaces the heat pump when outdoor temperatures drop below its efficient range. Lockout controls set the exact outdoor temperature at which that switch happens. Ductless Mini-Split A ductless mini-split is a heat pump that conditions a room or zone directly through a wall- or ceiling-mounted indoor unit connected by a small refrigerant line to an outdoor compressor — no ductwork required. It's the standard fix for additions, garage conversions, and homes heated by inefficient electric baseboard or wall heaters. Reversing Valve The reversing valve is the component that lets a heat pump switch between heating and cooling mode — it physically redirects the flow of refrigerant so the same outdoor and indoor coils can either absorb heat from outside (winter) or reject heat outside (summer). It's a part standard air conditioners simply don't have. Matched System (Indoor/Outdoor Pairing) A matched system pairs an outdoor condenser/heat pump unit and indoor coil or air handler that the manufacturer specifically designed and rated to work together — same manufacturer, compatible capacity and airflow specs. Mixing an old indoor coil with a new outdoor unit ("mismatched") is possible but sacrifices efficiency and can void warranties.

Water Heaters

Tank vs. Tankless Water Heater A tank water heater stores and continuously reheats 40-80 gallons of hot water; a tankless (on-demand) unit heats water only as it flows through, with no storage tank, at the cost of a higher upfront price and specific gas-line or electrical requirements. Heat Pump Water Heater A heat pump water heater uses the same heat-moving technology as a space-heating heat pump — pulling ambient heat from the surrounding air to heat water — instead of a standard electric-resistance element, using roughly a third of the electricity of a conventional electric tank for the same hot water output. Dip Tube The dip tube is the internal pipe that carries incoming cold water down to the bottom of a tank water heater, so it can be heated from the bottom up rather than mixing with already-hot water at the top. A cracked or degraded dip tube lets cold water mix in near the top, where the outlet draws hot water from. Seismic Strapping (Oregon Code) Seismic strapping is a code-required bracing system — typically two metal straps anchored into wall studs — that keeps a tank water heater from tipping over during an earthquake, which could shear off the gas line or water connections and create a fire or flood hazard. Expansion Tank An expansion tank is a small secondary tank installed on a water heater's supply line to absorb the extra volume water creates as it heats and expands, relieving pressure that would otherwise build up in a closed plumbing system (one with a check valve or pressure-reducing valve preventing backflow to the street).

Indoor Air Quality

MERV Rating MERV (Minimum Efficiency Reporting Value) is a 1-16 scale rating how effectively an air filter captures particles of different sizes — higher MERV traps smaller particles (like wildfire smoke and fine dust) but also restricts airflow more, which matters for a system not designed to handle it. Duct Sealing (Mastic / Aerosolized Sealant) Duct sealing closes gaps, loose joints, and holes in ductwork using mastic paste or an aerosolized sealant injected and cured inside the duct system, stopping conditioned air from leaking into unconditioned attics, crawlspaces, or wall cavities before it ever reaches a room. Whole-Home Filtration vs. Portable Air Purifier Whole-home filtration treats air as it circulates through your existing HVAC ductwork, conditioning every room the system serves from one central point; a portable air purifier treats only the air in the single room it sits in, independent of the HVAC system. UV / Electronic Air Cleaner A UV air cleaner uses ultraviolet light installed inside the ductwork or air handler to disrupt mold, bacteria, and viruses as air passes by; an electronic air cleaner instead charges particles so they're attracted to a collector plate, capturing finer particles than a standard filter alone.

Money, Code & Licensing

CCB License (Oregon Construction Contractors Board) A CCB license is the credential the Oregon Construction Contractors Board issues to legally perform construction work, including HVAC and electrical, in the state — it requires a surety bond, liability insurance, and a clean complaint history to stay active, and is separate from the trade-specific licenses electricians must also hold. Mechanical & Electrical Permit In Oregon, replacing a furnace, heat pump, AC, or water heater requires a mechanical permit (plumbing too, for water heaters) with a final inspection, and the dedicated circuit a heat pump or heat-pump water heater needs takes a separate electrical permit — sometimes issued by two different city offices for the same address. AHJ (Authority Having Jurisdiction) The AHJ is whichever government office actually has legal authority to issue permits and conduct inspections for a given address — in the Portland metro this varies by both city and permit type, so the AHJ for a mechanical permit and an electrical permit on the same house is sometimes two different offices. Energy Trust of Oregon Energy Trust of Oregon is the nonprofit that administers cash incentives for energy-efficient equipment (heat pumps, water heaters, furnaces, insulation) on behalf of Oregon's investor-owned utilities — PGE, Pacific Power, NW Natural, Cascade Natural Gas, and Avista — funded by a public purpose charge on customer bills. Trade Ally (Enrolled Contractor) A Trade Ally is a contractor formally enrolled with Energy Trust of Oregon to install qualifying equipment and apply certain incentives directly, on the spot, without the homeowner filing separate rebate paperwork — required specifically for Energy Trust's instant and income-qualified incentive tiers. Savings Within Reach (Income-Qualified Incentives) Savings Within Reach is Energy Trust of Oregon's income-qualified incentive tier, offering a higher rebate amount than the standard tier for the same equipment — for example, $3,000 instead of $1,000 on a qualifying ducted heat pump — for households, rentals, and attached homes that meet the income threshold. Federal 25C / 25D Tax Credit (ended 2025) The federal 25C (Energy Efficient Home Improvement) and 25D (Residential Clean Energy) tax credits offered up to $2,000 for qualifying heat pumps and heat pump water heaters, plus other home-efficiency credits — until the July 2025 federal budget act terminated both for equipment placed in service after December 31, 2025. Oregon HOMES & HEAR Rebates Oregon HOMES and HEAR are federally-funded, state-administered rebate programs (created under the Inflation Reduction Act) intended to fund whole-home efficiency upgrades and electrification for income-qualified Oregon households, separate from Energy Trust's utility-funded incentives. AHRI Certificate An AHRI certificate documents that a specific combination of outdoor and indoor HVAC equipment (the matched system) has been tested and rated together by the Air-Conditioning, Heating, and Refrigeration Institute — confirming the efficiency rating on the label actually applies to that exact equipment pairing, not just each unit individually. EPA Section 608 Certification EPA Section 608 certification is the federal credential required to legally purchase, handle, and work with refrigerants under the Clean Air Act — technicians must pass an EPA-administered exam covering safe refrigerant handling and leak prevention before they can service equipment containing regulated refrigerants. Seller Disclosure (ORS 105.464 / 105.475) Oregon's seller disclosure statement (ORS 105.464) requires a home seller to answer, to their actual knowledge, whether work was done on the property, whether a permit was required, and whether a final inspection was obtained — disclosed on a standardized state form before closing. Oregon 811 / Dig Safely Oregon 811 (Dig Safely Oregon) is the no-cost locate-request service that sends utility companies out to mark the location of buried gas, electric, and other lines before any digging — legally required at least two full business days before breaking ground, with marks staying valid for 45 days. Load Calculation (Manual J) A load calculation is a room-by-room engineering assessment — square footage, insulation, window area and orientation, air-tightness, duct condition — that determines the actual heating and cooling capacity a home needs. "Manual J" is the ACCA-published industry-standard method most contractors use to run it. It replaces "tonnage per square foot" folklore with math specific to your house. Owner-Direct Pricing Owner-direct pricing means a family-owned company with no sales quotas: the technician who diagnoses a repair quotes it on the spot, the project manager designs and quotes replacements, and both answer to the owners — not to a private-equity rollup or a franchise paying a percentage of every invoice up the chain. HVAC Financing HVAC financing is a loan or payment plan — offered through the contractor's financing partner rather than paid in cash upfront — that spreads the cost of a furnace, AC, heat pump, or major electrical project over monthly payments, usually with a specific promotional rate or term.
Learning Center Vocabulary is step one — the longer answers, buying guides, and comparisons live here. Rebates & incentives The current, sourced dollar figures behind every money term on this page.

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