HVAC Glossary
Every HVAC term, in plain English.
62 terms from estimates, tune-up reports, and repair quotes — defined the way we'd explain them at your kitchen table, with the Georgia context that makes them matter.
HVAC has a vocabulary problem — and you shouldn't need a trade license to understand your own estimate. Each definition below includes the Georgia-climate context where it changes the answer, and links to the related Learning Center guide or service page when you want to go deeper. Spot a term we missed? Tell us and we'll add it.
A
- AFUE (Annual Fuel Utilization Efficiency)
- The percentage of a furnace's fuel that becomes usable heat over a year — a 96% AFUE furnace turns 96 cents of every gas dollar into warmth. Georgia's short, mild winters mean the payback on ultra-high AFUE is slower here than up north, so the right tier depends on your usage. Furnace Installation →
- Air Handler
- The indoor unit that moves air through your ducts — blower, evaporator coil, and controls in one cabinet, used with heat pumps and AC systems. In many two-story Metro Atlanta homes it lives in the attic, where 130°F summer heat ages components faster than a basement install. How long systems last in Georgia →
- Air Purifier (Whole-Home)
- A device installed in your ductwork that removes or neutralizes airborne particles, odors, or microbes as air circulates — beyond what a filter alone catches. Useful in Atlanta's long pollen season, but purifier types differ widely in what they actually remove. Air Purification →
- Airflow (CFM)
- Cubic feet per minute — how much air your system moves. Cooling typically needs around 350–400 CFM per ton; too little airflow (dirty filter, crushed flex duct, restrictive filter) causes frozen coils, weak rooms, and compressor strain.
B
- Blower Motor
- The motor that drives the indoor fan pushing air through your ducts. Older PSC blowers run at one speed; modern ECM (electronically commutated) blowers vary their speed, using less electricity and holding steadier comfort — a meaningful upgrade over a Georgia cooling season.
- BTU (British Thermal Unit)
- The basic unit of heating and cooling energy — the heat needed to raise one pound of water 1°F. Air conditioner capacity is rated in BTU/h; 12,000 BTU/h equals one ton of cooling.
C
- Capacitor
- A small electrical component that gives your compressor and fan motors the jolt they need to start and run. Capacitors are the most common summer failure our techs replace in Metro Atlanta — heat is their enemy, and a weak one makes the whole system struggle to start. AC Repair →
- Compressor
- The heart of your air conditioner or heat pump — it pressurizes refrigerant so the system can move heat out of (or into) your home. Compressor failure outside warranty is usually the event that ends a system's life, which is why maintenance that protects the compressor matters most. Repair or replace? The framework →
- Condensate Drain
- The pipe that carries away the water your AC wrings out of Georgia's humid air — often gallons per day in July. Algae loves our warm, damp summers; a clogged drain line is one of the most common causes of water stains under attic air handlers. AC Maintenance & Tune-Ups →
- Condenser (Outdoor Unit)
- The outdoor half of your cooling system, where refrigerant releases the heat collected inside. Its coil must breathe freely — Atlanta's spring pollen film and summer grass clippings coat condenser coils and force the compressor to work harder for the same cooling. Surviving Atlanta pollen season →
- Contactor
- A heavy-duty electrical switch that sends power to the compressor and condenser fan when the thermostat calls for cooling. Contacts pit and stick over thousands of cycles; a stuck contactor can keep the outdoor unit running nonstop, and a burned one leaves it dead.
- Crawlspace Encapsulation
- Sealing a vented crawlspace with a vapor barrier and conditioning the space to keep ground moisture out of the house. Over North Georgia's damp clay soil, a vented crawlspace pumps humidity into the home all summer — encapsulation attacks that moisture at the source. Controlling humidity in Georgia homes →
D
- Damper
- A movable plate inside ductwork that adjusts how much air flows to a branch. Manual dampers let a technician balance airflow between floors — often the first, cheapest step for a hot upstairs — while motorized dampers are the working parts of a zoning system. Why is my upstairs hotter? →
- Dehumidifier (Whole-Home)
- A duct-installed unit that removes moisture independently of cooling, holding indoor humidity in the healthy 40–55% range even when the AC isn't running. In Metro Atlanta's humid shoulder seasons (May, September) it solves the clammy-but-not-hot problem an AC alone can't. Humidity Control →
- DOAS (Dedicated Outdoor Air System)
- A system that brings in and conditions fresh outdoor air separately from the equipment that heats and cools the house. More common in commercial buildings, but the concept — treating ventilation air on purpose instead of relying on leaks — is the direction tight modern homes are headed. Ventilation Systems →
- Dual-Fuel System
- A heat pump paired with a gas furnace: the heat pump handles mild weather efficiently, and the furnace takes over on the coldest mornings. It's a strong fit for Georgia, where most winter hours are mild enough for the heat pump to carry the load cheaply. Heat Pump Installation →
- Duct Sealing
- Closing the joints, gaps, and disconnections in ductwork with mastic or aerosolized sealant so conditioned air reaches the rooms instead of the attic or crawlspace. Typical homes lose 20–30% of their airflow to leaks, so sealing is one of the highest-payback efficiency fixes in Georgia. Duct leakage: the hidden cost →
- Ductless Mini-Split
- A compact heat pump with one outdoor unit and one or more wall-mounted indoor heads, requiring no ductwork. Around here it's the go-to fix for bonus rooms over garages, sunrooms, and finished basements that the main system never conditioned well. Ductless Installation →
E
- ECM Motor
- An electronically commutated motor — a variable-speed, high-efficiency blower motor that ramps smoothly instead of slamming on at full speed. Longer, gentler run cycles move more air through the filter and wring more humidity from Georgia air.
- EER2 (Energy Efficiency Ratio 2)
- Cooling efficiency measured at a steady 95°F outdoor temperature — a peak-heat stress test, versus SEER2's whole-season average. For Georgia's long stretches of 90°+ afternoons, EER2 tells you how the system behaves exactly when your power bill is highest. SEER2 explained for Georgia →
- Emergency Heat (Auxiliary Heat)
- Electric resistance heat strips inside a heat pump's air handler that back up the heat pump in very cold weather or when it fails. They work, but cost several times more per hour of heat — if your thermostat shows AUX constantly on a mild Georgia day, something is wrong. Heat Pump Repair →
- ERV / HRV (Energy / Heat Recovery Ventilator)
- A ventilation appliance that exchanges stale indoor air for fresh outdoor air while transferring heat (and, in an ERV, moisture) between the streams. In humid climates like ours the ERV is the right pick — it brings in fresh air without importing the full muggy load. Ventilation Systems →
- Evaporator Coil
- The indoor coil where refrigerant absorbs heat and moisture from your home's air — it runs cold and wet all summer. Restricted airflow or low refrigerant can drop it below freezing, which is why an iced-over AC in July traces back to this coil. Why does my AC freeze up? →
F
- Filter (Media Filter)
- A 4–5 inch thick pleated filter in a dedicated cabinet at the air handler, versus the common 1-inch throwaway. The extra surface area filters better with less airflow penalty and lasts 6–12 months — a practical upgrade for pollen-season households. HEPA Filtration →
- Float Switch
- A small safety switch in the condensate drain pan that shuts the system off before an overflowing pan dumps water through your ceiling. Cheap insurance for the attic air handlers common in two-story Metro Atlanta homes.
- Freon (R-22)
- The refrigerant used in most systems installed before 2010, banned from production since 2020 for its ozone impact. Recovering and replacing R-22 is expensive enough that a significant leak on an R-22 system is usually a replacement conversation, not a repair. Repair or replace? The framework →
H
- Heat Exchanger
- The sealed metal chambers inside a gas furnace where combustion heat transfers to your home's air, keeping flue gases separate. A cracked heat exchanger can leak carbon monoxide and is the failure that retires most furnaces — it's the core of every fall safety inspection. Heating Maintenance →
- Heat Pump
- An air conditioner that runs in both directions — cooling in summer, and in winter pulling heat from outdoor air into your home. Georgia's mild winters are close to ideal heat-pump territory, which is why they're a growing share of Metro Atlanta replacements. Heat Pump Installation →
- HEPA (High-Efficiency Particulate Air)
- A filtration standard that captures 99.97% of particles at 0.3 microns. True HEPA is too restrictive to sit in a standard duct system directly, so whole-home HEPA units bypass a portion of airflow through the filter — a distinction worth understanding before buying. HEPA Filtration →
- HSPF2 (Heating Seasonal Performance Factor 2)
- The heating-season efficiency rating for heat pumps under the current (post-2023) test standard — higher is better, with 7.5 the federal floor. In Georgia's mild winters, a strong HSPF2 compounds into real savings because the heat pump covers nearly all your heating hours. Heat Pump Replacement →
- Humidistat
- A control that reads relative humidity and drives a dehumidifier, humidifier, or ventilation equipment the way a thermostat drives heating and cooling. Many smart thermostats now include one, which is how 'cool to 74° OR dry to 50%' control strategies work. Smart Thermostats →
- HVAC
- Heating, Ventilation, and Air Conditioning — the whole system of equipment, ductwork, and controls that conditions your home. The ventilation part is the one most homeowners overlook, and in a humid climate it deserves equal billing.
I
- IAQ (Indoor Air Quality)
- The overall healthfulness of the air inside your home — particulates like pollen and dust, humidity, biological growth, and chemical VOCs. Indoor air is commonly two to five times more polluted than outdoor air, and Georgia's sealed-up, air-conditioned summers concentrate the effect. Air Purification →
- Inverter Compressor
- A compressor that varies its speed continuously instead of switching between off and 100%. It sips power at part load, holds temperature within a fraction of a degree, and runs the long gentle cycles that dehumidify Georgia air best. Variable-Speed →
L
- Line Set
- The pair of insulated copper pipes carrying refrigerant between the outdoor condenser and indoor coil. Reusing an old line set on a new system is a common cost-cutting shortcut that can contaminate the new refrigerant circuit — ask how yours will be handled at replacement. What drives installation cost →
- Load Calculation
- The engineering math that determines how much heating and cooling your specific house needs, accounting for square footage, insulation, windows, orientation, and Georgia's design temperatures. It's the only defensible basis for sizing equipment — rules of thumb routinely oversize. HVAC sizing & Manual J explained →
M
- Manual J
- The industry-standard (ACCA) procedure for residential load calculation — the room-by-room worksheet behind a properly sized system. If a replacement quote skips Manual J and sizes 'like for like,' any sizing error in the original install gets copied forward another 15 years. HVAC sizing & Manual J explained →
- MERV (Minimum Efficiency Reporting Value)
- The 1–16 scale for how well a filter captures particles: MERV 8 handles dust and pollen basics, MERV 11 adds finer allergens, MERV 13 approaches smoke and virus-carrier sizes. Higher MERV also resists airflow more, so the right choice depends on what your blower and ducts can handle. Atlanta pollen season IAQ guide →
N
- NATE Certification
- North American Technician Excellence — the leading independent certification for HVAC technicians, earned by exam and maintained with continuing education. It's a fair proxy for whether the person diagnosing your system has verified, current knowledge.
P
- Plenum
- The sheet-metal box that connects your air handler or furnace to the duct system — supply plenum on the outlet side, return plenum on the inlet. Undersized plenums are a hidden cause of high static pressure and noisy, underperforming systems.
- PSC Motor
- Permanent split capacitor motor — the older, single-speed blower and fan motor design. Reliable but inefficient compared to ECM motors; when one fails on an aging system, the repair-versus-upgrade math is worth a minute of thought.
R
- R-410A
- The refrigerant standard in systems installed from roughly 2010 through 2024, now being phased down under federal HFC rules. Systems using it remain fully serviceable, but refrigerant costs are expected to climb through the phase-down — a factor in repair-or-replace decisions. Repair or replace? The framework →
- R-454B
- The lower-global-warming refrigerant replacing R-410A in new equipment from 2025 onward. It's mildly flammable (A2L class), which changed equipment design and handling rules — one reason new-system pricing shifted with the transition. What drives installation cost →
- Refrigerant
- The working fluid that absorbs heat at the indoor coil and releases it outdoors, cycling between liquid and vapor. A correctly charged, leak-free system never consumes refrigerant — if yours needs a 'top-off' every spring, it has a leak worth finding. AC Repair →
- Register / Grille / Diffuser
- The vents where air enters and leaves your rooms — supply registers deliver conditioned air, return grilles pull air back to the system. Closing supply registers to 'push air elsewhere' mostly just raises duct pressure and leakage; balance dampers are the right tool.
- Relative Humidity (RH)
- How much moisture the air holds compared to its maximum at that temperature, as a percentage. Indoors, 40–55% RH is the comfort-and-health sweet spot; Metro Atlanta summers push unmanaged homes into the 60s, where dust mites, mold, and that clammy feeling thrive. Controlling humidity in Georgia homes →
- Return Air
- The air pulled from your rooms back to the system to be filtered and reconditioned. Many 90s-era two-story homes around Atlanta were built with too little return on the second floor — a quiet contributor to the hot-upstairs problem. Why is my upstairs hotter? →
- Reversing Valve
- The component that lets a heat pump switch between heating and cooling by reversing refrigerant flow. When one sticks, a heat pump can blow cold air on a January morning — a classic heat-pump-specific diagnosis. Heat Pump Repair →
S
- SEER2 (Seasonal Energy Efficiency Ratio 2)
- The seasonal cooling-efficiency rating under the current test standard, which measures equipment against realistic duct static pressure. The federal minimum in the Southeast is 14.3 SEER2, and each point higher trims roughly 5–7% off cooling energy — meaningful across a 2,000+ hour Georgia cooling season. SEER2 explained for Georgia →
- Short Cycling
- When a system starts, runs briefly, and shuts down over and over instead of completing long cycles. Causes range from oversizing to a failing component; the costs are real either way — starts are the hardest wear events, and short runs never dehumidify Georgia air properly. HVAC sizing & Manual J explained →
- Smart Thermostat
- A connected thermostat that learns schedules, senses occupancy, and reports system behavior. Beyond convenience, the useful features in our climate are humidity readouts, runtime history (an early-warning sign of a struggling system), and multi-sensor averaging for uneven homes. Smart Thermostats →
- Split System
- The standard residential setup: an outdoor condenser and an indoor air handler or furnace-plus-coil, connected by a refrigerant line set. 'Split' just distinguishes it from packaged units where everything lives in one outdoor cabinet.
- Stack Effect
- The physics of warm air rising through a house and escaping high while pulling replacement air in low — like a slow chimney. In two-story Georgia homes it stacks heat upstairs all summer and is a core reason the second floor runs warmer than the thermostat floor. Why is my upstairs hotter? →
- Static Pressure
- The resistance your blower pushes against to move air through filters, coils, and ducts — HVAC's version of blood pressure. High static pressure (restrictive filters, undersized or crushed ducts) quietly cuts airflow, capacity, and equipment life; it's measurable in minutes with the right tools. Ductwork Repair →
- Superheat & Subcooling
- The two temperature measurements technicians use to verify a system's refrigerant charge is exactly right, rather than guessing by pressure alone. If your tune-up report includes superheat and subcooling numbers, the charge was actually checked — a quick way to judge service quality. What a real tune-up includes →
T
- Ton (of Cooling)
- A capacity unit equal to 12,000 BTU/h — most Metro Atlanta homes need between 2 and 5 tons. The right tonnage comes from a load calculation, not square footage alone; two same-size houses can need very different capacity depending on insulation, windows, and shade. HVAC sizing & Manual J explained →
- TXV (Thermostatic Expansion Valve)
- A precision valve that meters refrigerant into the evaporator coil, adjusting continuously to match load. A sticking TXV mimics low refrigerant — one of the diagnoses where a pressure-gauge-only look gets the answer wrong. AC Repair →
U
- UV Germicidal Light
- An ultraviolet lamp mounted at the indoor coil to prevent mold and bacterial growth on the coil's perpetually wet summer surface. Honest framing: it keeps the coil clean and helps with biological growth — it does not remove pollen, dust, or odors. UV Germicidal Lights →
V
- Variable-Speed System
- Equipment whose compressor and blower modulate output continuously (versus single-stage on/off or two-stage). The long, low, steady cycles suit Georgia's humidity exceptionally well — better moisture removal, quieter operation, steadier temperatures — at a higher upfront price. SEER2 explained for Georgia →
- Ventilation
- The deliberate exchange of stale indoor air for fresh outdoor air. Older leaky houses ventilate accidentally through gaps; tighter modern homes need mechanical ventilation — done thoughtfully in our climate so fresh air doesn't mean imported humidity. Ventilation Systems →
- VOCs (Volatile Organic Compounds)
- Airborne chemicals off-gassed by paints, new furniture, cleaners, and building materials. Filters don't capture gases — reducing VOCs takes source control, ventilation, and certain purification technologies designed for them. Air Purification →
Z
- Zoning (Zone Control)
- Dividing one HVAC system into independently controlled areas using motorized dampers and multiple thermostats — commonly one zone per floor. It directly targets the upstairs-downstairs temperature split in two-story homes, though it works best paired with variable-speed equipment. Why is my upstairs hotter? →
Keep learning
Definitions are the start — the Learning Center puts them to work: how long systems last here, what SEER2 means for a Georgia power bill, why oversizing ruins humidity control, and more. Or get quick answers in our FAQs.
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