Learning Center

The HVAC Glossary

Every term a technician, quote or manual will throw at you — defined in plain English, with the South Sound context that matters.

Equipment & Systems Efficiency & Ratings Refrigeration Cycle Airflow & Ductwork Indoor Air Quality Installation & Service

Equipment & Systems

Air Handler #: An air handler is the indoor unit that moves air through your home when your system does not include a furnace. It contains the blower fan, the indoor coil, and often electric backup heat strips. In an all-electric heat pump system, the air handler does the job a furnace would otherwise do: circulating conditioned air through the ducts.
Condenser #: The condenser is the outdoor unit of an air conditioner or heat pump. Its job is to release the heat your system has collected: refrigerant flows through the outdoor coil while a fan pulls air across it, dumping heat outdoors in cooling mode. Keeping a couple of feet of clearance around the condenser, and keeping it clear of leaves and debris, helps it run efficiently.
Dual-Fuel System #: A dual-fuel system pairs an electric heat pump with a gas furnace on the same ductwork. The heat pump handles most of the heating season efficiently, and the furnace takes over automatically when outdoor temperatures drop low enough that gas becomes the more practical heat source. In the South Sound's mild marine climate, the heat pump typically carries the large majority of the heating load. Heating services
Ductless Mini-Split #: A ductless mini-split is a heat pump system that delivers heating and cooling through one or more wall-mounted indoor units instead of ductwork. Each indoor unit conditions the room it serves, so you can set different temperatures in different areas. Mini-splits are a common choice for older South Sound homes that were built without ducts, additions, garages, and ADUs. Ductless systems
Evaporator Coil #: The evaporator coil is the indoor coil where refrigerant absorbs heat from your home's air. As warm air passes over the cold coil, the air cools down and moisture condenses out of it, which is why air conditioning also dehumidifies. A dirty or frozen evaporator coil is a common cause of weak cooling.
Gas Furnace #: A gas furnace burns natural gas or propane to heat air, which a blower then pushes through your ducts. Furnaces heat quickly and deliver warmer air at the registers than a heat pump does. Many homes in our area pair a furnace with a heat pump or air conditioner on the same ductwork. Heat pump vs. gas furnace in the PNW
Heat Exchanger #: The heat exchanger is the metal component inside a gas furnace that transfers heat from the burner flames to your home's air while keeping combustion gases separated from the air you breathe. A cracked heat exchanger can allow carbon monoxide into the airstream, which is why furnace safety inspections focus heavily on this part. Heat exchanger condition is often the deciding factor in whether an aging furnace is worth repairing.
Heat Pump #: A heat pump is a single system that both heats and cools your home by moving heat rather than creating it. In winter it pulls heat from the outdoor air and moves it inside; in summer it runs in reverse and works like an air conditioner. Because the Puget Sound area rarely sees extreme cold, heat pumps are an especially good match for South Sound homes, where they can handle heating duty almost all year. Heat pump services
Packaged Unit #: A packaged unit combines all heating and cooling components in a single outdoor cabinet, usually installed on a roof or a ground pad, instead of splitting equipment between indoor and outdoor units. Ducts connect directly to the cabinet. Packaged units are more common on commercial buildings and manufactured homes than on typical site-built houses.
Thermostat C-Wire #: The C-wire, or common wire, is the thermostat wire that supplies continuous low-voltage power so a smart thermostat can run its screen and Wi-Fi. Many older homes were wired without one, which is a frequent snag when upgrading to a smart thermostat. Solutions include pulling a new wire, using a manufacturer's adapter, or repurposing an unused wire in the existing cable.
Zoning System #: A zoning system divides one ducted heating and cooling system into separately controlled areas using motorized dampers and multiple thermostats. Instead of one temperature for the whole house, each zone gets its own setting, useful for two-story homes where upstairs and downstairs never agree. Zoning must be designed carefully so airflow stays healthy when only some zones are calling.

Efficiency & Ratings

AFUE #: AFUE (Annual Fuel Utilization Efficiency) is the percentage of a furnace's fuel that becomes usable heat over a year. A 95% AFUE furnace turns 95 cents of every fuel dollar into heat, with the rest lost up the flue. Modern condensing furnaces typically rate 90% AFUE or higher, while older units may be 80% or below.
COP (Coefficient of Performance) #: COP compares the heat a heat pump delivers to the electricity it consumes at a specific outdoor temperature. A COP of 3 means the system delivers three units of heat for every one unit of electricity, which is why heat pumps beat electric resistance heat, whose COP is always 1. In the South Sound's mild winters, heat pumps spend most of the season operating at favorable COPs.
EER2 #: EER2 (Energy Efficiency Ratio 2) measures cooling efficiency at one fixed, hot condition (95°F outdoors) rather than averaged over a season like SEER2. It is a useful indicator of how equipment performs during a genuine heat wave. For our area's occasional hot spells, EER2 tells you how the system will behave on exactly the days you need it most.
ENERGY STAR #: ENERGY STAR is a U.S. EPA certification identifying equipment that meets efficiency levels above the federal minimum. For heat pumps and air conditioners, certification is based on ratings like SEER2 and HSPF2. Utility rebate programs and tax incentives often use ENERGY STAR certification (or specific efficiency tiers) as an eligibility requirement, so it is worth checking before you buy. Rebates and incentives
HSPF2 #: HSPF2 (Heating Seasonal Performance Factor 2) measures how efficiently a heat pump heats over a full season; a higher number means lower heating bills for the same warmth. Because South Sound winters are long, damp, and mild rather than bitterly cold, heating efficiency matters more here than cooling efficiency, making HSPF2 the more important number for most local buyers. SEER2 and HSPF2 ratings explained
SEER2 #: SEER2 (Seasonal Energy Efficiency Ratio 2) measures how efficiently an air conditioner or heat pump cools over an entire season; higher numbers mean less electricity used for the same cooling. It replaced the older SEER rating in 2023 with a test procedure that better reflects real ductwork conditions, so SEER2 numbers run slightly lower than the old SEER numbers for the same equipment. Compare SEER2 to SEER2, not to older SEER figures. SEER2 and HSPF2 ratings explained
Two-Stage Equipment #: Two-stage equipment can run at two output levels: a lower stage for mild days and full capacity for cold snaps or heat waves. It spends most of its time in the low stage, which means longer, gentler cycles, more even temperatures, and less noise than single-stage equipment. Two-stage sits between basic single-stage and fully variable-speed systems in both performance and cost.
Variable-Speed (Inverter) Technology #: A variable-speed or inverter system continuously adjusts its compressor and fan speed to match what your home needs, instead of switching fully on and off. The result is steadier temperatures, quieter operation, better humidity control, and higher efficiency, because the system spends most of its time cruising at low speed. Most premium heat pumps and ductless systems sold today use inverter technology.

Refrigeration Cycle

Compressor #: The compressor is the pump at the heart of the refrigeration cycle, pressurizing refrigerant vapor so it can release heat in the condenser coil. It lives in the outdoor unit and is usually the most expensive single component in the system. Because compressor replacement often costs a large fraction of a new system, compressor health is central to any repair-versus-replace decision.
Defrost Cycle #: The defrost cycle is a normal heat pump function that periodically melts frost off the outdoor coil by briefly running in cooling mode. You may notice steam rising from the outdoor unit and a whooshing sound; both are expected. The South Sound's wet, near-freezing winter weather is prime frost-forming territory, so local heat pumps defrost fairly often, but ice that builds up and never clears is a problem worth a service call. Heat pump ice buildup explained
R-410A Phaseout #: R-410A, the refrigerant used in most systems installed from the mid-2000s through 2024, is being phased down under federal law because of its high global-warming potential. New equipment now ships with lower-impact refrigerants such as R-454B or R-32. Existing R-410A systems remain legal to run and service, though the refrigerant itself is expected to get scarcer and more expensive over time — a factor worth weighing when repairing an older system.
Refrigerant #: Refrigerant is the working fluid that carries heat through an air conditioner or heat pump, absorbing heat in one coil and releasing it in the other as it changes between liquid and vapor. It circulates in a sealed loop and is never consumed, so a system that is low on refrigerant has a leak, not a maintenance need. Handling refrigerant legally requires EPA certification.
Refrigerant Charge #: The refrigerant charge is the precise amount of refrigerant a system is designed to hold. Too little or too much both hurt capacity and efficiency and can shorten compressor life. Verifying the charge with gauges and temperature measurements is a core step in a proper installation and in diagnosing weak cooling or heating.
Reversing Valve #: The reversing valve is the component that lets a heat pump switch between heating and cooling by reversing the direction refrigerant flows through the system. It is the one part that makes a heat pump different from a plain air conditioner. A stuck reversing valve is a classic cause of a heat pump blowing cold air in heating mode or warm air in cooling mode.
TXV (Thermostatic Expansion Valve) #: A TXV is a metering device that controls exactly how much refrigerant flows into the evaporator coil, adjusting continuously as conditions change. Compared to a fixed orifice, a TXV keeps the coil fed correctly across a wider range of temperatures, improving efficiency and protecting the compressor. A stuck or misadjusted TXV can mimic the symptoms of a refrigerant leak.

Airflow & Ductwork

CFM (Cubic Feet per Minute) #: CFM measures how much air your system moves. Every system has a target airflow (commonly around 350–400 CFM per ton of cooling), and rooms each need a share of it to stay comfortable. Rooms that never feel right often trace back to too little CFM reaching them, not to the equipment itself.
Duct Sealing #: Duct sealing closes the gaps and joints in ductwork so conditioned air reaches your rooms instead of leaking into the crawlspace or attic. Leaky ducts waste heating and cooling you have already paid for and can pull crawlspace air into the house. In our region, where ducts commonly run through damp, vented crawlspaces, sealing is one of the more cost-effective comfort upgrades available.
Return Air #: Return air is the air your system pulls back from the house to be filtered and reconditioned; return grilles are the vents that suck rather than blow. A system needs enough return capacity to breathe — undersized or blocked returns starve the blower, raise static pressure, and make whole rooms stuffy. Keep furniture and rugs clear of return grilles.
Short Cycling #: Short cycling is when a system starts, runs only a few minutes, and shuts off, over and over. It wears out components quickly, wastes energy, and never runs long enough to mix air or control humidity properly. Common causes include an oversized system, a clogged filter, a refrigerant problem, or a thermostat mounted in a bad spot. Why your AC is short cycling
Stack Effect #: Stack effect is the natural tendency of warm air to rise and escape through a home's upper leaks while cold air is pulled in through lower ones, creating a slow chimney-like flow through the house. It is strongest in winter and in multi-story homes, and it explains chilly floors and drafty basements even when the heat is running. Air sealing at the attic and crawlspace levels reduces it.
Static Pressure #: Static pressure is the resistance your blower has to push against to move air through ducts, filters, and coils — think of it as blood pressure for your HVAC system. High static pressure, often caused by undersized ducts or restrictive filters, makes systems loud, inefficient, and short-lived. Measuring it takes minutes and reveals problems a visual inspection misses.
Supply Plenum #: The supply plenum is the sheet-metal box attached to the outlet of your furnace or air handler where conditioned air first collects before branching into individual duct runs. Its size and layout affect how evenly air distributes to the house. The plenum is also where accessories like high-capacity media filters and UV lights are commonly mounted.

Indoor Air Quality

ERV / HRV (Energy / Heat Recovery Ventilator) #: ERVs and HRVs bring fresh outdoor air into a tightly sealed home while exhausting stale indoor air, transferring heat between the two airstreams so you are not throwing away the energy you paid for. An HRV exchanges heat only; an ERV also exchanges some moisture. As newer homes get tighter, mechanical ventilation like this is increasingly required by code rather than optional. Air filters, MERV, UV, and ERV explained
HEPA Filter #: HEPA (High-Efficiency Particulate Air) filters capture at least 99.97% of particles at 0.3 microns, the hardest size to catch. True HEPA filters are too restrictive to drop into a standard furnace filter slot; in homes they are typically used in portable air purifiers or dedicated bypass units. For whole-house filtration through your ducts, a high-MERV media filter is usually the practical choice.
Media Filter #: A media filter is a thick (typically 4–5 inch) pleated filter installed in a dedicated cabinet at the air handler or furnace, in place of a standard 1-inch filter. The deep pleats provide far more surface area, so it can filter at a high MERV rating without choking airflow, and it usually only needs changing about once or twice a year. It is a common, practical upgrade for allergy sufferers and for smoke season.
MERV Rating #: MERV (Minimum Efficiency Reporting Value) rates how well an air filter captures particles, on a scale from 1 to 16 for residential filters; higher numbers catch smaller particles. Most homes do well with MERV 8–13, but a filter must be matched to what your blower and duct system can handle — a too-restrictive filter causes airflow problems of its own. Air filters, MERV, UV, and ERV explained
PM2.5 #: PM2.5 refers to airborne particles smaller than 2.5 microns — small enough to bypass your nose and throat and reach deep into the lungs. Wood smoke and wildfire smoke are major sources, and during the South Sound's late-summer smoke events outdoor PM2.5 can reach unhealthy levels for days. Running your system's fan with a quality high-MERV filter is one of the most effective ways to keep indoor levels down during smoke season. Indoor air quality services
Relative Humidity #: Relative humidity is the amount of moisture in the air compared to the maximum it could hold at that temperature, expressed as a percentage. Indoors, roughly 30–50% is the comfortable, healthy range: too high encourages mold and dust mites, too low dries out skin and woodwork. Western Washington's damp climate means local homes more often fight excess moisture — especially in crawlspaces and poorly ventilated bathrooms — than dry air.
UV Coil Light #: A UV coil light is an ultraviolet lamp mounted inside the system to shine on the indoor coil, preventing mold and biofilm from growing on its perpetually damp surface. It keeps the coil clean and helps with musty odors, but it is not a substitute for filtration — UV treats surfaces, while filters remove particles from moving air. Bulbs lose strength over time and need periodic replacement.

Installation & Service

Commissioning #: Commissioning is the final verification stage of an installation: measuring airflow, checking refrigerant charge, confirming electrical values, and testing every operating mode against the manufacturer's specifications. It is the difference between equipment that is merely running and equipment that is performing as designed. Skipped or rushed commissioning is a leading reason two identical systems can deliver very different comfort and bills. Install day: what to expect
Condensate Drain #: The condensate drain carries away the water that forms on the indoor coil during cooling and, for high-efficiency furnaces, the water produced by condensing combustion. A clogged drain is one of the most common service calls: most systems have a safety switch that shuts everything down before the pan overflows. Algae growth in the drain line is the usual culprit, and periodic cleaning prevents it.
Line Set #: The line set is the pair of insulated copper pipes that carry refrigerant between the outdoor unit and the indoor coil — a larger insulated suction line and a smaller liquid line. Correct sizing, routing, and insulation of the line set directly affect system performance. When replacing equipment, the existing line set must be evaluated: it may be reused after flushing, or need replacement, especially when the refrigerant type changes.
Manual J Load Calculation #: A Manual J load calculation is the industry-standard method for determining exactly how much heating and cooling a specific home needs, based on its size, insulation, windows, orientation, and local climate. It replaces rule-of-thumb sizing, which routinely produces oversized equipment that short cycles and controls humidity poorly. A contractor who sizes your system without a load calculation is guessing. HVAC sizing and Manual J explained
Mechanical Permit #: A mechanical permit is the authorization your city or county requires before HVAC equipment is installed or replaced, followed by an inspection to verify the work meets code. In Pierce, Thurston, and King counties, permits are required for furnace, heat pump, and ductless installations. Unpermitted work can complicate a home sale and may void certain rebates, so confirm your contractor is pulling the permit. Pierce County HVAC permits explained
Refrigerant Recovery #: Refrigerant recovery is the process of capturing the refrigerant from a system before repair or disposal, using dedicated recovery equipment, rather than venting it to the atmosphere. Venting refrigerant is illegal under federal law, and technicians must hold EPA Section 608 certification to handle it. Proper recovery is a baseline marker of a professional operation when your old system is removed.

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