HVAC Glossary

66+ residential HVAC terms defined in plain English — the exact language we use when explaining diagnoses, quotes, and service options to Baldwin County homeowners. Use this as a reference next to any quote you receive (from us or anyone else).

System Types (8)Equipment & Components (16)Efficiency Ratings & Sizing (9)Refrigerants (6)Indoor Air Quality (6)Process & Credentials (9)Coastal & Climate Terms (7)Incentives & Financing (5)

System Types

The major residential HVAC system types you'll see in Baldwin County homes — and when each one makes sense.

Central Air Conditioner(central AC): A whole-home cooling system with an outdoor condenser, an indoor evaporator coil sharing the air handler, and ductwork that distributes conditioned air. The most common cooling system in Baldwin County homes — paired with either a furnace or, more often, a heat pump in this climate.
Heat Pump: An air conditioner that runs in reverse during winter to provide heat — moving warmth from outdoor air into the home rather than burning fuel. Heat pumps are the dominant residential heating system in Baldwin County because Climate Zone 2A's mild winters let them deliver 2-3 units of heat per unit of electricity consumed.
Ductless Mini-Split(mini-split, ductless system): A heat pump with no ductwork — one small outdoor unit feeds refrigerant lines to one or more indoor wall, ceiling, or floor units that condition individual rooms. Ideal for additions, garages, sunrooms, historic homes, and any space where running ductwork is impractical.
Split System: Any HVAC system where the condenser sits outside and the air handler or furnace sits inside, connected by refrigerant lines. The opposite is a package unit. Most residential systems in Baldwin County are split systems.
Package Unit: A self-contained HVAC system housing the condenser, evaporator, and blower in a single outdoor cabinet — typically rooftop or ground-mounted. Common on manufactured homes and small commercial buildings; less common on single-family residential in Baldwin County.
Gas Furnace: A heating system that burns natural gas or propane to heat air, then circulates it through ductwork via a blower. Less common in Baldwin County than heat pumps because the mild winter heating season rarely justifies the equipment cost on top of a separate AC.
Electric Furnace: A heating system using resistance heat strips (basically large versions of toaster wires) to warm air for blower distribution. Common on manufactured homes and as auxiliary heat backup in heat pump systems. More expensive to operate than a heat pump in Baldwin County's climate.
Dual-Fuel System(hybrid system): A heat pump paired with a gas furnace, controlled by a thermostat that switches between fuels at a programmable balance-point outdoor temperature. The heat pump runs efficiently during mild Baldwin County winters; the furnace takes over only during the rare hard cold snap.

Equipment & Components

The parts that make up your HVAC system — the names you'll see on quotes, invoices, and diagnostic reports.

Condenser(outdoor unit, compressor unit): The outdoor unit of your AC or heat pump, containing the compressor, condenser coil, and condenser fan. In Baldwin County coastal homes, condensers face accelerated salt-air corrosion — coastal-grade units with treated coils and marine-grade fasteners last several years longer.
Evaporator Coil(indoor coil, A-coil): The indoor coil where refrigerant absorbs heat from your home's air. Sits inside or above the air handler. In humid Gulf Coast conditions the coil also condenses moisture, which drains out through the condensate line.
Compressor: The pump in your outdoor condenser that compresses refrigerant gas, raising its temperature so it can release absorbed heat to outside air. The most expensive single component in an AC system; failure usually pushes the system toward replacement rather than repair.
Air Handler(fan coil): The indoor unit housing the evaporator coil and blower fan, plus electric heat strips on heat-pump systems. Common in homes that don't have a gas furnace.
Blower Motor: The electric motor that drives the indoor fan, pushing conditioned air through your ductwork. Variable-speed (ECM) blower motors run more efficiently and quietly than older single-speed PSC motors.
Capacitor(run capacitor, start capacitor): A small cylindrical electrical component that stores a charge to help start and run motors (compressor and condenser fan). Capacitor failure is the single most common AC repair call we see in Baldwin County summers — usually a $150-$300 fix.
Contactor: An electrically controlled switch in the outdoor unit that closes when the thermostat calls for cooling, energizing the compressor and fan. Pitting, corrosion, or stuck contacts are common causes of "AC won't start" calls — especially in salt-air coastal areas.
Reversing Valve: The valve in a heat pump's outdoor unit that switches refrigerant flow direction between cooling and heating modes. A stuck reversing valve is the most common reason a heat pump won't switch to heat in fall — the valve hasn't moved since spring.
Defrost Board: The control board on a heat pump that periodically reverses the system to melt frost off the outdoor coil during cold-mode operation. Defrost board failure shows up as ice buildup on the outdoor unit during winter.
Heat Exchanger: On a gas furnace, the metal chamber where combustion gases transfer heat to the airstream while staying isolated from the air you breathe. Cracks in the heat exchanger leak carbon monoxide into the home — an immediate safety concern that requires furnace replacement.
Thermostat: The wall-mounted control that tells the HVAC system when to run. Modern smart thermostats (Ecobee, Nest, Honeywell T9) add humidity-aware setpoints, scheduling, and remote access — meaningful in humid Gulf Coast homes where humidity matters as much as temperature.
Condensate Line(condensate drain): The PVC drain that carries water condensed from your indoor coil to the outside. In Gulf Coast humidity these drains run constantly during cooling season and clog easily — biofilm and algae growth are the main culprits. A clogged drain trips the safety switch and shuts down cooling.
Expansion Valve(TXV, thermostatic expansion valve): The metering device that controls how much refrigerant flows into the evaporator coil. Modern systems use TXVs (thermostatic expansion valves) which adjust flow based on coil temperature; older systems used fixed-orifice pistons that don't adapt.
Filter Drier: An inline component on the refrigerant line that absorbs moisture and traps debris. Replaced any time the refrigerant circuit is opened — moisture in the line forms ice that blocks the metering device and acid that damages the compressor.
Igniter(hot surface igniter, HSI): The component on a gas furnace that heats up to ignite the gas-air mixture at the burner. Hot surface igniters are common today; they're a frequent gas furnace repair item because they're cycled every burn.
Flame Sensor: A safety component on a gas furnace that confirms a flame is present after ignition. If the sensor doesn't see flame, the gas valve closes within seconds. Cleaning the sensor (oxidation builds up) is one of the most common heating-season service calls.

Efficiency Ratings & Sizing

The numbers on the equipment sticker — what they actually mean for your monthly utility bill in Baldwin County.

SEER(Seasonal Energy Efficiency Ratio): Seasonal Energy Efficiency Ratio. Measures cooling output (BTU) divided by electricity input (watt-hours) over a typical cooling season. Higher SEER = more efficient. Federal minimum in the South is 14.3 SEER2; common Baldwin County installs run 15-18 SEER2 for the cost-efficiency sweet spot.
SEER2: The updated SEER metric (effective Jan 2023) that uses revised testing conditions reflecting real-world ductwork pressure drops. SEER2 numbers run roughly 4-5% lower than the same equipment's old SEER rating — same equipment, more honest measurement.
EER(Energy Efficiency Ratio): Energy Efficiency Ratio — the cooling efficiency at one specific point (95°F outdoor, 80°F indoor, 50% RH). Useful as a snapshot of peak-load efficiency. EER2 is the updated metric used alongside SEER2 today.
HSPF(Heating Seasonal Performance Factor, HSPF2): Heating Seasonal Performance Factor — the heat pump equivalent of SEER, measuring heating efficiency over a typical heating season. Climate Zone 2A (coastal Alabama) systems benefit most from HSPF2 ratings of 7.5+; higher numbers help less than they would in northern climates.
AFUE(Annual Fuel Utilization Efficiency): Annual Fuel Utilization Efficiency. The percent of fuel a gas furnace converts to usable heat. 80% AFUE units vent through a metal flue; 95%+ AFUE units (high-efficiency) condense exhaust water and vent through PVC. Federal minimum is 80% AFUE in the South.
BTU(British Thermal Unit): British Thermal Unit — the standard unit of heat. One BTU is roughly the heat from burning a kitchen match. Residential AC capacity is rated in BTU/hour; 12,000 BTU/hour equals one ton of cooling.
Ton: A unit of cooling capacity equal to 12,000 BTU/hour. Most Baldwin County homes need 2-5 tons of cooling — but "need" here means a Manual J load calculation, not a square footage rule of thumb. Oversizing is the most common mistake we see on residential replacements.
Manual J: ACCA Manual J is the industry-standard load calculation that determines correct AC and heating capacity for a specific home. Inputs include square footage, insulation, window orientation, ceiling height, and design temperatures. Skipping Manual J leads to oversized systems that short-cycle, fail to dehumidify, and wear out faster.
Balance Point: The outdoor temperature at which a heat pump's heating output equals the home's heat loss — below this temperature, auxiliary heat strips engage to make up the difference. Setting the balance point correctly is critical for heat pump efficiency in Baldwin County's variable winter weather.

Refrigerants

The fluids that move heat around inside your AC. The refrigerant a system uses determines what parts are available, what regulations apply, and whether replacement is on the table.

R-410A(Puron): The HFC refrigerant used in nearly all residential AC and heat pump installations from 2010 through 2024. Being phased out under the AIM Act starting 2025 in favor of lower-GWP alternatives like R-32. Existing R-410A systems remain serviceable; new installs increasingly ship with R-32.
R-32: The next-generation residential refrigerant replacing R-410A in new equipment from 2025 forward. Lower global warming potential, mildly flammable (A2L classification), and slightly more efficient. Common in Mitsubishi and Daikin equipment first; spreading across other major brands.
R-22(Freon): The old chlorofluorocarbon refrigerant phased out for new equipment in 2010 and discontinued for any use in 2020. Existing R-22 systems still operate but refrigerant top-offs are expensive and getting harder to source. R-22 systems over 15 years old are typically replacement candidates.
Refrigerant Recovery: The EPA-required process of capturing existing refrigerant from a system before opening the circuit, using certified recovery equipment. Releasing refrigerant to atmosphere is illegal and carries fines. EPA Section 608 certification is required to handle refrigerants legally.
Subcooling: The amount the liquid refrigerant has been cooled below its condensing temperature, measured at the outdoor unit. Used along with superheat to verify proper refrigerant charge. Improper charge is a leading cause of efficiency loss and component failure.
Superheat: The amount the gaseous refrigerant has been heated above its boiling temperature, measured at the indoor coil outlet. Manufacturers spec a superheat range; readings outside that range indicate undercharge, overcharge, or airflow problems.

Indoor Air Quality

Filtration, humidity, and ventilation — three things that matter more in Baldwin County than in drier climates.

MERV(Minimum Efficiency Reporting Value): Minimum Efficiency Reporting Value — a 1-16 scale rating filter efficiency. MERV 8 captures pollen, mold spores, and dust mite debris; MERV 11 catches pet dander and finer particulates; MERV 13 captures bacteria-sized particles. Above MERV 13 in standard 1-inch slots restricts airflow.
HEPA(High Efficiency Particulate Air): High Efficiency Particulate Air — a filter standard requiring 99.97% capture of 0.3-micron particles. True HEPA filters need their own dedicated airflow path in HVAC use; they cannot drop into a standard 1-inch filter slot without strangling the system.
UV-C Light: A coil-mounted ultraviolet lamp that sterilizes biological growth on the evaporator coil and drain pan. Genuinely useful in humid Baldwin County climates for controlling musty odors and biofilm. Marketing claims about killing airborne pathogens are overstated — airflow is too fast for meaningful kill rates.
Whole-House Dehumidifier: A separate appliance that ties into your HVAC ductwork to remove moisture from indoor air independent of the AC. Useful in tight new-construction homes, vacation rentals between stays, and homes with chronic humidity complaints despite good cooling. Typically $2,000-$3,500 installed.
Humidity Control(dehumidification): The HVAC function that removes moisture from indoor air. In Baldwin County's climate, getting indoor relative humidity into the 40-55% range is often more important than hitting an exact temperature. Right-sized AC, longer fan runtimes, and proper drainage all affect humidity control.
Ventilation(fresh air intake): The introduction of outdoor air into the home. Required in tight new-construction homes that don't leak naturally — code-compliant ERVs (energy recovery ventilators) bring in fresh air while pre-conditioning it against the outgoing exhaust airstream.

Process & Credentials

What a contractor should be doing on the install, and what credentials to look for on a license check.

AHRI Match: An AHRI (Air-Conditioning, Heating & Refrigeration Institute) certificate confirming a specific indoor + outdoor pair has been tested together. Required for full manufacturer warranty validity, Energy Star rebate eligibility, and federal 25C tax credit qualification.
Commissioning: The post-install procedure where the technician verifies refrigerant charge, airflow, temperature split, static pressure, and thermostat staging are all within manufacturer spec — then documents the readings. A real commissioning report is the difference between an install that lasts 15 years and one that fails at year 8.
Load Calculation(Manual J, load calc): The process of measuring a home's heating and cooling needs based on insulation, square footage, window orientation, ceiling height, design temperatures, and infiltration. The basis for sizing equipment correctly. ACCA Manual J is the residential standard.
EPA 608: The federal certification required for any technician handling refrigerants. Four levels (Type I, II, III, Universal) cover different equipment classes. Working on residential AC requires Type II or Universal certification at minimum.
Alabama HVAC License: Alabama requires HVAC contractors performing residential or commercial installation work to hold a Heating Air Conditioning Refrigeration Contractor license issued by the State Board. ACExperts holds Alabama HVAC license AL #16117 and carries general liability insurance through Progressive.
Tune-Up(preventive maintenance, PM visit): A scheduled HVAC service visit where the technician inspects, cleans, and tests system components — refrigerant pressures, electrical connections, capacitors, condensate drains, coils, blower motor, and thermostat calibration. Bi-annual cadence (spring AC, fall heat pump) is the Baldwin County standard.
Static Pressure: The air pressure in your duct system, measured in inches of water column. Spec range is typically 0.5 inches; readings above that mean restricted airflow (dirty filter, undersized return, kinked flex duct), which slowly damages the blower motor and shortens equipment life.
Leak Detection: The process of locating refrigerant leaks using electronic detectors, UV dye and lamp, nitrogen pressure tests, or soap bubbles. Required when a system is low on charge — adding refrigerant without finding the leak just delays the inevitable repair and wastes regulated refrigerant.
Evacuation(pulling vacuum): After installing or repairing a refrigerant circuit, the procedure of pulling deep vacuum (500 microns or below) to remove moisture and non-condensables before charging refrigerant. Skipping or shortening evacuation is a leading cause of premature compressor failure.

Coastal & Climate Terms

Terms specific to HVAC on the Alabama Gulf Coast — issues that don't matter inland but really do here.

Salt-Air Corrosion: Accelerated corrosion of outdoor unit components (aluminum coil fins, copper coil tubing, electrical contactors, cabinet fasteners) from airborne salt particles. Affects homes within several miles of Mobile Bay or the Gulf — outdoor unit lifespan can drop from 15-20 years to 8-12 without coastal-grade equipment and aggressive maintenance.
Coastal-Grade Equipment(seacoast package, marine-grade): Outdoor units factory-built with corrosion-resistant features: treated coil coatings, marine-grade stainless fasteners, sealed electrical compartments, and protective cabinet finishes. Adds 10-15% to equipment cost; adds 4-7 years of service life in salt-air conditions. Recommended for any home within a few miles of the bay or Gulf.
Climate Zone 2A: The U.S. Department of Energy climate zone covering coastal Alabama, including all of Baldwin County. Hot, humid summers; mild winters with occasional cold snaps below freezing. Heat pumps are the optimal residential heating system for this zone — gas furnaces are over-engineered for the heating load.
Latent Load: The portion of cooling capacity that goes to removing moisture from the air rather than reducing temperature. Baldwin County's climate is heavy on latent load — sometimes 40-50% of total cooling demand is humidity removal. Right-sized equipment with longer runtimes handles latent load far better than oversized equipment.
Sensible Load: The portion of cooling that reduces air temperature (the part you feel as "cooler"). The opposite of latent load. Drier climates have a higher sensible-to-latent ratio than Gulf Coast Alabama, where moisture removal often matters as much as temperature drop.
Design Temperature: The outdoor temperature at which equipment should be sized to handle peak conditions. Baldwin County's cooling design temperature is around 92°F; heating design is around 30°F. Sizing for averages instead of design temperatures produces undersized equipment that can't keep up on extreme days.
Humid Subtropical Climate: The climate classification (Köppen Cfa) covering Baldwin County. Long, hot, humid summers; short, mild winters; rain distributed across the year with summer thunderstorm peaks. This climate makes humidity control and corrosion management the two most important HVAC factors here.

Incentives & Financing

The federal credits, utility rebates, and financing options that change the math on a new install.

Federal 25C Tax Credit(heat pump tax credit): The federal Energy Efficient Home Improvement Credit covering up to $2,000 per year on qualifying high-efficiency heat pump installations (must meet specific SEER2/HSPF2/EER2 thresholds). Applies to equipment AND installation cost. Non-refundable; claimed on your federal return for the year of installation.
Energy Star: A joint EPA/DOE certification identifying HVAC equipment that meets specific efficiency thresholds. Some utility rebates and tax credits require Energy Star certification. Energy Star certification on AHRI-matched systems is verified at install time for any incentive claim.
Alabama Power Heat Pump Rebate: Alabama Power offers rebates of $200-$500 (depending on equipment efficiency tier and program year) for residential heat pump installations replacing electric resistance heating. Stack with the federal 25C credit and HVAC financing on the same install. Application paperwork is contractor-filed at install.
HVAC Financing: Equipment-specific consumer financing offered through HVAC lenders (Synchrony, Goodleap, EnerBank, others). Common terms: 0% promotional rates for 12-18 months, fixed APR for 7-10 year terms. Approval is consumer-credit based; disclosed at the in-home consultation alongside equipment quotes.
Section 179: Federal tax provision allowing businesses to expense the full cost of qualifying commercial HVAC equipment in the year of installation rather than depreciating over years. Applies to commercial installs only — not residential. Documented at install with equipment specifications and installation invoicing.

Still have questions?

ACExperts is a licensed Baldwin County HVAC contractor — we explain everything in plain English when we visit. No salesperson jargon, no surprise charges. Same-day service available in most cases.

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