HVAC Glossary
HVAC terms, in plain English.
74 terms you'll hear from a technician, see on a quote, or read on a permit — each with what it means and why it matters in a New Bern or Crystal Coast home.
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- Ratings & Efficiency AFUE AFUE (Annual Fuel Utilization Efficiency) is the percentage of a fuel's energy a furnace converts into usable heat over a year. An 80% AFUE furnace turns 80% of the gas it burns into heat in the house; the rest leaves through the flue. It applies to fuel-burning equipment, not heat pumps.
- Cooling & Heat Pumps Air Handler The air handler is the indoor cabinet containing the blower, the evaporator coil and, in many electric systems, backup heat strips. In a heat-pump installation it takes the place a furnace would occupy, moving conditioned air into the duct system.
- Gas Piping & Specialty Homes Appliance Connector An appliance connector is the short flexible line joining rigid gas piping to a movable appliance such as a range, dryer or grill. Connectors are rated for specific appliances and lengths, are not permitted to pass through walls or floors, and are not intended for reuse once disconnected.
- Cooling & Heat Pumps Auxiliary Heat Auxiliary heat is the backup heating source a heat pump calls on when its own output cannot meet demand — usually electric resistance strips in the air handler, or a furnace in a dual-fuel system. Emergency heat is a separate manual mode that runs the backup alone, bypassing the heat pump.
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- Cooling & Heat Pumps Balance Point The balance point is the outdoor temperature at which a heat pump's heating output exactly matches the building's heat loss. Above it the heat pump can hold the house on its own; below it, output falls short and backup heat has to make up the difference.
- Ratings & Efficiency BTU A BTU (British Thermal Unit) is the energy needed to raise one pound of water by one degree Fahrenheit. HVAC equipment is rated in BTU per hour — a rate, not a quantity — describing how fast a system adds or removes heat. Gas appliances are also rated in BTU/h for their fuel input.
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- Geothermal Closed-Loop System A closed-loop geothermal system circulates a sealed mixture of water and antifreeze through buried high-density polyethylene pipe. The same fluid recirculates indefinitely, exchanging heat with the surrounding earth through the pipe wall without ever contacting groundwater. Loop joints are heat-fused rather than mechanically clamped, so the circuit stays sealed underground.
- Coastal & Corrosion Coastal Humidity Load Humidity load — the latent portion of a cooling load — is the energy needed to remove moisture from indoor air, as distinct from the sensible load that lowers its temperature. In humid coastal climates latent load is a large share of the total, so a system must dehumidify as well as cool.
- Cooling & Heat Pumps Compressor The compressor is the pump at the centre of the refrigeration cycle. It raises the pressure and temperature of refrigerant vapour so the outdoor coil can release heat in cooling mode. It lives in the outdoor unit and is typically the single most expensive component in the system.
- Ductless & Mini-Split Condensate Pump A condensate pump moves the water a cooling coil produces to a drain when gravity alone cannot carry it there. Ductless heads mounted on interior walls, and equipment in basements or tight spaces, frequently need one because there is no downhill path to a drain.
- Cooling & Heat Pumps Condenser (Outdoor Unit) The condenser is the outdoor half of a split system: a coil, a fan and the compressor in a weatherproof cabinet. In cooling mode it rejects heat collected indoors to the outside air, which is why airflow across its fins and clear space around the cabinet matter to performance.
- Ratings & Efficiency COP COP (Coefficient of Performance) is the ratio of heat energy a system moves to the energy it consumes, measured at one operating condition. A COP of 4 means four units of heat delivered per unit of electricity used. Because heat pumps move heat rather than create it, their COP is greater than 1.
- Gas Piping & Specialty Homes Crossover Duct A crossover duct is the flexible duct connecting the two halves of a double-wide manufactured home's supply system, usually running underneath the floor between the sections. It carries conditioned air from the side with the furnace to the opposite side.
- Gas Piping & Specialty Homes CSST CSST (Corrugated Stainless Steel Tubing) is flexible stainless gas piping used in place of rigid black iron. It routes through framing more easily and with fewer joints, and it must be electrically bonded to the building's grounding system in the manner the manufacturer and code require.
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- Cooling & Heat Pumps Defrost Cycle The defrost cycle is a heat pump's routine for clearing frost from the outdoor coil in heating mode. The system briefly reverses to send warm refrigerant through the outdoor coil, melting accumulated frost, while backup heat usually covers the house for those few minutes.
- Geothermal Desuperheater A desuperheater is an auxiliary heat exchanger that captures surplus heat from a geothermal heat pump's hot refrigerant gas and transfers it to the domestic hot water tank. It makes use of heat the system is rejecting anyway, principally while the system is running in cooling mode.
- Gas Piping & Specialty Homes Downflow Furnace A downflow furnace takes return air in at the top and discharges heated air downward, into ductwork beneath the floor. Manufactured homes commonly use this configuration because their duct systems typically run in the floor rather than in an attic or basement.
- Heating Systems Dual-Fuel System A dual-fuel system pairs an electric heat pump with a fuel-burning furnace. The heat pump handles heating until the outdoor temperature reaches a set changeover point, below which the furnace takes over. One thermostat manages the handoff, so the house uses whichever source suits conditions.
- Air Quality & Ductwork Duct Leakage Duct leakage is conditioned air escaping from supply ducts, or unconditioned air being drawn into returns, through gaps at joints, boots and plenums. It is measured as leakage at a test pressure and is addressed by sealing with mastic or approved tape, not cloth duct tape.
- Ductless & Mini-Split Ductless Mini-Split A ductless mini-split is a heat pump that delivers conditioned air directly into a room through one or more indoor heads, with no ductwork. An outdoor compressor connects to each head by a refrigerant line set, and every head is controlled independently as its own zone.
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- Coastal & Corrosion E-Coated Coil An e-coated coil has been dipped and electrically charged so an epoxy coating deposits in a uniform film across the entire coil, including the interior fin surfaces a sprayed coating cannot reach. The coating acts as a barrier between corrosive deposits and the metal beneath.
- Ratings & Efficiency EER2 EER2 (Energy Efficiency Ratio 2) measures cooling efficiency at a single hot design condition — 95°F outdoors — rather than averaged across a season. It is cooling output in BTU/h divided by electrical input in watts at that one operating point, so it describes how a system behaves on the hottest afternoons.
- Heating Systems Electric Heat Strips Electric heat strips are resistance heating elements in an air handler that convert electricity directly into heat. They serve as backup for a heat pump, engaging when it cannot meet demand or during defrost. Their efficiency is fixed: one unit of electricity produces one unit of heat.
- Air Quality & Ductwork Energy Recovery Ventilator (ERV) An ERV brings in outdoor air and exhausts indoor air through a core that transfers both heat and a portion of the moisture between the two streams. Transferring moisture is what distinguishes it from an HRV, which exchanges heat only.
- Coastal & Corrosion Equipment Elevation (Flood Zone) Equipment elevation is mounting mechanical equipment above the anticipated flood level, on a stand or platform, rather than at grade. In flood-prone construction it applies to outdoor condensing units, air handlers and ductwork, and is governed by local floodplain requirements rather than by preference.
- Cooling & Heat Pumps Evaporator Coil The evaporator coil is the indoor coil where cold refrigerant absorbs heat from air passing over it. As the air cools below its dew point, moisture condenses on the coil and drains away — which is how an air conditioner dehumidifies as a by-product of cooling.
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- Heating Systems Flue (Vent) The flue is the pipe carrying combustion gases from a fuel-burning appliance to outside. Its material and routing depend on the appliance: conventional furnaces use metal venting into a chimney or vertical flue, while condensing furnaces vent cooler, acidic exhaust through plastic pipe.
- Heating Systems Furnace A furnace produces heat by burning fuel — natural gas or propane — and transfers it to indoor air through a heat exchanger, with combustion gases vented outside. It differs fundamentally from a heat pump, which moves existing heat rather than creating it by combustion.
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- Coastal & Corrosion Galvanic Corrosion Galvanic corrosion occurs when two dissimilar metals are in electrical contact with an electrolyte present — in coastal HVAC, typically copper tubing against aluminium fins with salt-laden moisture bridging them. The less noble metal corrodes preferentially, which is why aluminium fins fail around copper.
- Gas Piping & Specialty Homes Gas Leak Detection Gas leak detection covers the methods used to find escaping fuel gas: electronic detectors, leak-detection solution on joints, and pressure testing of a piping system. Fuel gas is odourised with a sulphur compound specifically so leaks can be noticed by smell.
- Gas Piping & Specialty Homes Gas Regulator A gas regulator reduces supply pressure to the level an appliance is designed to burn at and holds it steady as demand changes. Systems commonly have a regulator at the meter or propane tank and, where required, additional appliance-level regulation.
- Geothermal Geothermal Heat Pump (Ground-Source) A geothermal or ground-source heat pump exchanges heat with the ground rather than with outdoor air, circulating fluid through a buried loop. Because soil holds a far steadier temperature year-round than air does, the system works across a smaller temperature difference than an air-source heat pump.
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- Heating Systems Heat Exchanger The heat exchanger is the metal barrier inside a furnace that transfers combustion heat to indoor air while keeping exhaust gases separate from it. Its integrity is what keeps combustion products — including carbon monoxide — out of the air circulating through the house.
- Cooling & Heat Pumps Heat Pump A heat pump is a system that moves heat rather than generating it, using a refrigeration cycle that can run in both directions. In summer it moves heat out of the house; in winter it reverses and moves heat from outdoor air into the house. One piece of equipment therefore provides both heating and cooling.
- Air Quality & Ductwork Heat Recovery Ventilator (HRV) An HRV exchanges fresh outdoor air for stale indoor air while transferring heat between the two streams, recovering much of the energy that simple exhaust ventilation would waste. Unlike an ERV, it transfers heat only and does not move moisture between the streams.
- Air Quality & Ductwork HEPA Filtration HEPA (High-Efficiency Particulate Air) describes filters meeting a defined capture efficiency at the most-penetrating particle size, conventionally 99.97% at 0.3 microns. It is a far stricter specification than any residential MERV rating, and the dense media it requires carries much higher airflow resistance than a standard furnace filter.
- Geothermal Horizontal Loop Field A horizontal loop field buries loop pipe in trenches a few feet deep spread across an area of land, rather than in deep vertical boreholes. It trades land area for drilling depth, and is generally the lower-cost configuration where enough open ground is available.
- Ratings & Efficiency HSPF2 HSPF2 (Heating Seasonal Performance Factor 2) rates how much heat an air-source heat pump delivers per watt-hour of electricity across a heating season. It is the heating counterpart to SEER2 and, like SEER2, replaced its predecessor in 2023 under a higher-static-pressure test. Higher numbers mean less electricity for the same heat.
- Gas Piping & Specialty Homes HUD-Code Manufactured Home A HUD-Code manufactured home is built to the federal Manufactured Home Construction and Safety Standards (24 CFR Part 3280) rather than to a state or local building code. The standard covers construction, and specifically includes requirements for heating, cooling and fuel-burning systems.
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- Ductless & Mini-Split Line Set A line set is the pair of insulated copper refrigerant lines running between the outdoor unit and an indoor coil or head — a larger insulated suction line and a smaller liquid line. On ductless equipment it usually runs with the control wiring and condensate drain inside one covered chase.
- Geothermal Loop Antifreeze Loop antifreeze is the water-based heat-transfer fluid circulating in a closed loop, mixed with an antifreeze such as propylene glycol or methanol to keep it liquid below freezing. Concentration is set so the fluid cannot freeze at the lowest loop temperature the system will reach.
- Geothermal Loop Grout Loop grout is the thermally enhanced material pumped into a borehole around the loop pipe after installation. It serves two purposes: conducting heat between pipe and surrounding earth, and sealing the borehole so it cannot become a path between aquifers.
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- Code, Permits & Ownership Maintenance Agreement A maintenance agreement is an annual arrangement covering scheduled preventive service on a heating and cooling system, typically with defined inspection and cleaning tasks and some form of member benefit. Scope, visit count and benefits vary by provider and are set out in the agreement itself.
- Ratings & Efficiency Manual J Load Calculation Manual J is the ACCA standard procedure for calculating a building's heating and cooling loads. It accounts for floor area, orientation, insulation, window type and area, air leakage, occupancy and local design temperatures, and produces the BTU/h the house actually needs — separating sensible heat from moisture (latent) load.
- Coastal & Corrosion Marine-Grade Coil A marine-grade coil is an outdoor coil built or treated for corrosive environments — through protective coatings, all-aluminium construction that avoids dissimilar-metal contact, or both. Manufacturers market these under various seacoast or coastal designations, and the specification differs between them.
- Code, Permits & Ownership Mechanical Permit A mechanical permit is the authorisation a local inspections department issues before qualifying HVAC work begins, with inspection following to verify the installed work. Which jobs require one, and what is inspected, is set by the authority having jurisdiction for the property.
- Air Quality & Ductwork MERV Rating MERV (Minimum Efficiency Reporting Value) rates an air filter's ability to capture particles across defined size ranges, on a scale from 1 to 16 for residential-type filters under ASHRAE Standard 52.2. A higher number means a greater proportion of small particles captured on each pass.
- Gas Piping & Specialty Homes Mobile-Home-Approved Equipment Mobile-home-approved equipment is HVAC equipment specifically listed for installation in a manufactured home. The listing covers the differences that matter in that construction — sealed combustion and dedicated outside combustion air, the required venting arrangement, and clearances suited to the structure.
- Ductless & Mini-Split Multi-Zone System A multi-zone ductless system connects several indoor heads to a single outdoor unit, each head controlled separately. It contrasts with a single-zone system, where one outdoor unit serves one head, and lets several rooms be conditioned independently from shared outdoor equipment.
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- Cooling & Heat Pumps R-32 R-32 is a single-component lower-GWP refrigerant used in new air conditioners and heat pumps in place of R-410A. Like R-454B it is classified A2L — mildly flammable — but because it is a single compound rather than a blend, its composition does not shift if a system develops a leak.
- Cooling & Heat Pumps R-410A R-410A is the non-ozone-depleting HFC refrigerant blend that became the residential standard after R-22 was phased out. It is now itself being phased down under the AIM Act because of its high global warming potential, and manufacturers have moved new residential equipment to lower-GWP alternatives.
- Cooling & Heat Pumps R-454B R-454B is a lower-GWP refrigerant blend adopted by most residential manufacturers for new equipment as the R-410A phase-down took effect. It is classified A2L — mildly flammable — which brings different handling, leak-detection and storage requirements than the A1 refrigerants it replaces. It is not the only A2L in use: Daikin moved its residential lines to R-32 instead.
- Cooling & Heat Pumps Refrigerant Refrigerant is the working fluid that carries heat around the system, absorbing it where it evaporates and releasing it where it condenses. It is not consumed in normal operation — a system that is low on refrigerant has a leak, and topping it up without finding that leak treats the symptom only.
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- Coastal & Corrosion Salt-Air Corrosion Salt-air corrosion is the accelerated deterioration of metal caused by airborne salt (chloride) deposits combined with moisture. On HVAC equipment it attacks the outdoor coil's aluminium fins and copper tubing, along with cabinet fasteners and electrical contacts, progressing faster than ordinary weathering.
- Heating Systems Sealed Combustion Sealed combustion draws combustion air from outdoors through a dedicated pipe and vents exhaust through another, so the burner never uses indoor air. It contrasts with an atmospheric appliance, which takes air from the room it stands in and vents up a conventional flue.
- Gas Piping & Specialty Homes Sediment Trap A sediment trap is a short capped length of pipe fitted near a gas appliance's connection, positioned so debris and moisture carried in the gas stream drop out before reaching the appliance's control valve. Code requires one at appliances where the manufacturer specifies it.
- Ratings & Efficiency SEER2 SEER2 (Seasonal Energy Efficiency Ratio 2) is the U.S. Department of Energy rating for how much cooling an air conditioner or heat pump delivers per watt-hour of electricity across a typical cooling season. It replaced SEER in 2023 under a tougher test procedure that better reflects real duct resistance. Higher numbers mean less electricity for the same cooling.
- Cooling & Heat Pumps Short Cycling Short cycling is when a system starts and stops in rapid succession instead of running a full cycle. Causes include oversized equipment, restricted airflow, low refrigerant charge or a control fault. Each start draws peak current and produces the most mechanical wear of any part of the cycle.
- Cooling & Heat Pumps Split System A split system places the compressor and condenser coil outdoors and the evaporator coil and blower indoors, connected by refrigerant lines. It is the standard residential arrangement and is distinct from a packaged unit, which houses every component in one outdoor cabinet, and from ductless equipment.
- Air Quality & Ductwork Static Pressure Static pressure is the resistance a duct system presents to the air a blower is trying to move, measured in inches of water column. Total external static pressure is the standard diagnostic for whether a duct system is capable of carrying the airflow the equipment needs.
- Coastal & Corrosion Storm-Season HVAC Prep Storm-season preparation for HVAC covers the steps taken before and after a tropical system: securing the outdoor unit, shutting off power ahead of flooding or surge, keeping the unit clear of debris, and having submerged or wind-damaged equipment inspected before it is restarted.
- Cooling & Heat Pumps Superheat and Subcooling Superheat and subcooling are the two measurements used to verify refrigerant charge. Superheat is how much the refrigerant vapour has warmed past its boiling point leaving the evaporator; subcooling is how far the liquid has cooled below its condensing point leaving the condenser.
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- Cooling & Heat Pumps Thermostatic Expansion Valve (TXV) A thermostatic expansion valve is the metering device that controls how much refrigerant enters the evaporator coil, modulating flow in response to how much the refrigerant has superheated. It keeps the coil properly fed across changing conditions instead of at one fixed rate.
- Ratings & Efficiency Tonnage (Nominal Capacity) Tonnage is the nominal cooling capacity of a system, where one ton equals 12,000 BTU/h. A three-ton air conditioner is rated to remove about 36,000 BTU of heat per hour. The term comes from the cooling once produced by melting a ton of ice in a day, not from the weight of the equipment.
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- Ratings & Efficiency Variable-Speed (Inverter) A variable-speed or inverter-driven system varies its compressor and blower output continuously instead of switching between off and full power. It matches capacity to the load in small increments, so it runs for longer periods at lower output rather than cycling hard on and off.
- Geothermal Vertical Bore A vertical bore is a drilled borehole, typically several hundred feet deep, into which a U-shaped loop of pipe is inserted and then sealed in place with thermally conductive grout. Multiple bores are drilled at spaced intervals and manifolded together to form the complete loop field serving the system.
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- Code, Permits & Ownership Warranty Registration Warranty registration is the step of recording new equipment with its manufacturer after installation, within a stated window. Manufacturers commonly offer a longer parts warranty to registered owners than the base coverage that applies by default, with terms set by the manufacturer.
- Air Quality & Ductwork Whole-Home Dehumidifier A whole-home dehumidifier is a ducted appliance that removes moisture from household air independently of the cooling system. Because it is controlled by humidity rather than temperature, it can dry the house during mild weather when the air conditioner barely runs.
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