HVAC & Indoor-Air Glossary
47 plain-English definitions for Charleston homeowners. SEER2, heat pumps, MERV, R-454B, IRA tax credits — explained without the jargon.
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- Efficiency & Climate
AFUE
The efficiency rating for gas furnaces — a 90% AFUE furnace converts 90% of fuel energy into heat, 10% goes up the flue.
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Air Handler
The indoor portion of a split HVAC system — contains the evaporator coil and blower fan, conditions air for the duct system.
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Berkeley Electric Cooperative
A member-owned utility serving Berkeley County and parts of Charleston / Dorchester — runs its own HVAC efficiency incentive programs.
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Blower Motor
The fan motor inside the air handler or furnace that pushes conditioned air through your duct system — ECM blowers are variable-speed and quieter.
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BTU
The unit measuring heating or cooling capacity — 12,000 BTU/hr equals 1 ton of cooling.
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Capacitor
A small electrical component that gives the compressor or fan motor the extra jolt of energy it needs to start — common failure point.
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Central Air Conditioning
A whole-home cooling system with an outdoor condenser and indoor air handler connected by refrigerant lines and ducted to every room.
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CFM (Cubic Feet per Minute)
The volume of air the blower moves through the duct system per minute — proper CFM matters as much as proper tonnage.
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CO Detector (Carbon Monoxide)
A safety device required by SC code in any home with combustion appliances — alerts to dangerous levels of colorless, odorless carbon monoxide.
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Compressor
The motor in the outdoor unit that pressurizes refrigerant — the most expensive single component to replace in any AC or heat pump.
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Condenser Coil
The outdoor coil where compressed refrigerant releases its heat to outside air — the part most affected by salt-air corrosion in coastal homes.
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Contactor
An electrical switch inside the outdoor unit that turns the compressor and condenser fan on and off — wears out with cycle count.
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Dominion Energy SC
The investor-owned utility serving most of coastal SC — runs residential HVAC incentive programs for qualifying high-efficiency equipment.
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Ductless Mini-Split
An HVAC system with individual indoor 'heads' connected to one outdoor compressor — no ductwork required.
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Ductwork
The network of sheet-metal or flex tubes that distribute conditioned air through your home — leaks here waste up to 30% of HVAC output.
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EER2
An efficiency rating that measures cooling output at a single high-temperature test point — more relevant to hot climates than SEER2's seasonal average.
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ERV (Energy Recovery Ventilator)
A whole-home ventilation device that brings in fresh outdoor air while exchanging both heat AND moisture — preferred for humid Charleston climate.
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Evaporator Coil
The indoor coil where refrigerant absorbs heat from your home's air — also where humidity is removed by condensation.
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Furnace
A gas-fired or electric heating appliance that warms air and pushes it through ductwork via a blower fan.
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Heat Exchanger
The metal chamber inside a furnace that transfers combustion heat to your home's air while keeping flue gases separate — cracks here are dangerous.
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Heat Pump
An HVAC system that both cools and heats by reversing the refrigerant flow — usually the lowest operating cost option in the Lowcountry.
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HEPA
A filter standard capturing 99.97% of particles at 0.3 microns — true HEPA in residential HVAC requires a dedicated bypass system, not an inline filter.
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HERS Rating
A 0-150 scale where lower numbers mean more efficient — used in new construction; an HVAC replacement can move the rating by 5-15 points.
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HRV (Heat Recovery Ventilator)
A whole-home ventilation device that exchanges heat between incoming fresh and outgoing stale air — but NOT moisture, so less suited to Lowcountry summers.
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HSPF2
The federal efficiency rating for heat pump heating — the higher, the more heat output per unit of electricity over an entire heating season.
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IAQ (Indoor Air Quality)
A broad term covering particulate, humidity, biological growth, VOCs, and CO levels inside your home — multiple HVAC accessories address different parts.
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IRA 25C Tax Credit
The federal Inflation Reduction Act tax credit for qualifying HVAC upgrades, claimed on IRS Form 5695 — stacks with utility rebates.
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Line Set
The pair of insulated copper tubes connecting the outdoor condenser to the indoor evaporator coil — carries refrigerant between them.
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Manual J Load Calculation
The ACCA-standardized calculation for sizing residential HVAC equipment based on insulation, windows, orientation, and climate — not square footage.
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MERV Rating
A 1-16 scale measuring how small a particle a filter can capture — MERV 13 catches bacteria and smoke, MERV 8 catches dust and pollen.
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Packaged Unit
An HVAC system with all components (compressor, coils, blower) in one outdoor cabinet — common on commercial buildings and homes without indoor mechanical space.
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R-410A
The refrigerant in most residential AC and heat pumps installed 2010-2024 — being phased out for new equipment starting January 2025.
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R-454B
A lower-GWP refrigerant replacing R-410A in new residential systems starting 2025 — mildly flammable (A2L class) but performance-equivalent.
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Return Air
The path air takes back to the air handler — undersized returns are the #1 hidden cause of comfort complaints in older Charleston homes.
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SEER2
The current federal efficiency rating for AC and heat pump cooling — replaced the older SEER standard in 2023 with stricter testing conditions.
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Smart Thermostat
A Wi-Fi thermostat (Nest, Ecobee, Honeywell T-Series) that learns schedules, integrates with utility programs, and reports runtime.
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Static Pressure
The resistance air encounters as the blower pushes it through the duct system — measured during commissioning; high values shorten equipment life.
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Thermostat
The controller that tells your HVAC system when to run — programmable, smart, and Wi-Fi-connected versions all exist.
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Tonnage
The capacity rating of an AC or heat pump in 'tons' (one ton = 12,000 BTU/hr) — sized via Manual J, not by square footage rule-of-thumb.
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Two-Stage
A compressor or furnace with two output levels (low and high) — middle ground between single-stage and fully variable, better humidity removal.
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TXV (Thermostatic Expansion Valve)
The component that regulates refrigerant flow into the evaporator coil based on cooling load — modern systems use TXVs for efficiency.
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UV-C Light
An ultraviolet lamp installed near the indoor coil or in the duct stream that kills biological growth — particularly valuable in our humid climate.
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Variable-Speed
A compressor or blower that runs at infinite speeds rather than just on/off — better humidity control, quieter, more efficient in our climate.
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Whole-Home Air Purifier
A duct-mounted device (electronic, photocatalytic, or HEPA-bypass) that treats every cubic foot of air the system circulates.
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Whole-Home Dehumidifier
A dedicated appliance (Ultra-Aire, Aprilaire, Santa Fe) that removes humidity independent of the AC — solves persistent moisture the AC can't.
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Whole-Home Humidifier
A device added to the duct system that adds moisture to dry winter air — rarely needed in coastal SC where winter humidity stays moderate.
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Zoning
Dividing one HVAC system into multiple temperature zones using motorized dampers and multiple thermostats — uneven Charleston homes benefit.
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Don’t see the term you’re looking for?
Call our Charleston team — we’ll explain it in plain English and tell you whether it applies to your system.