HVAC Glossary
HVAC terms, in plain English.
61 terms you'll hear from a technician, see on a quote, or find on a rebate form — each with what it means and why it matters in a Greater Houston home.
A
- Refrigerants A2L Refrigerant An A2L refrigerant is one classified under ASHRAE Standard 34 as lower toxicity (class A) and lower flammability (class 2L), meaning it is hard to ignite and burns slowly. R-32 and R-454B, the main replacements for R-410A in new U.S. home systems, are both A2L. Equipment designed for them follows the UL 60335-2-40 safety standard. What it means for you →
- Money, Code & Permits AFUE AFUE (Annual Fuel Utilization Efficiency) is the percentage of a furnace's fuel energy that becomes usable heat in the home over a typical year. An 80% AFUE gas furnace turns about 80 cents of every fuel dollar into heat, while a 95% AFUE condensing furnace captures extra heat from its exhaust and loses far less up the vent. What it means for you →
- Money, Code & Permits AHJ (Authority Having Jurisdiction) The Authority Having Jurisdiction (AHJ) is the government office legally responsible for permitting and inspecting construction work at a specific address, usually a city building department or, in unincorporated areas, the county. It follows legal boundaries rather than mailing addresses, and it decides which permits apply, what they cost and whether finished work passes. What it means for you →
- Airflow, Ducts & Air Quality Air Filter An air filter is the replaceable pleated or fiberglass panel, installed in a return grille or a filter slot at the air handler or furnace, that traps dust, lint, and other airborne particles before they reach the blower and evaporator coil. Its first job is protecting the equipment; its MERV rating describes how well it captures smaller particles. What it means for you →
- Equipment Parts Air Handler An air handler is the indoor cabinet of a split air-conditioning or heat pump system that contains the blower motor, the evaporator coil, controls and, on many models, the filter rack. It pushes conditioned air through the supply ducts and draws return air back. Electric heat strips can be installed inside for supplemental or emergency heat. What it means for you →
- Heating & Heat Pumps Auxiliary Heat Auxiliary heat is the backup heat a heat pump thermostat switches on automatically when the heat pump alone cannot keep up, such as in very cold weather, after a large thermostat setback, or during a defrost cycle. In most all-electric systems it comes from heat strips in the air handler, which use considerably more electricity per unit of heat. What it means for you →
B
C
- Equipment Parts Capacitor A capacitor is an electrical component that stores and releases energy to start and run the motors in an air conditioner or heat pump. A run capacitor keeps the compressor and fan motors operating efficiently, while a start capacitor gives the compressor an extra boost at startup. A dual run capacitor serves the compressor and condenser fan together. What it means for you →
- Heating & Heat Pumps Carbon Monoxide (CO) Carbon monoxide (CO) is a colorless, odorless, poisonous gas produced when fuels such as natural gas, propane or gasoline burn. Household sources include leaking furnaces, blocked chimneys and flues, and portable generators. EPA notes exposure can cause headaches, dizziness, confusion and nausea and can be fatal at high concentrations; a working CO alarm is the reliable warning. What it means for you →
- Cooling Central Air Conditioner A central air conditioner is a whole-home cooling system that removes heat and humidity from indoor air and distributes cooled air through ductwork. It pairs an outdoor condensing unit with an indoor evaporator coil, connected by copper refrigerant lines, and is rated for efficiency in SEER2 and for capacity in tons or Btu per hour. What it means for you →
- Cooling Compressor A compressor is the motor-driven pump in an air conditioner or heat pump that pressurizes refrigerant vapor, raising its temperature so heat can be released outdoors. Located in the outdoor condensing unit, it drives refrigerant circulation through the whole system. Common types are single-stage, two-stage and variable-speed, and it is typically the system's costliest component. What it means for you →
- Airflow, Ducts & Air Quality Condensate Drain Line A condensate drain line is the pipe, usually PVC, that carries water removed from indoor air by the evaporator coil away from the air handler to a drain, the outdoors, or a condensate pump. Algae, sludge, and debris can clog it, causing water to back up into the drain pan, trip a float switch, or overflow into ceilings. What it means for you →
- Cooling Condenser Coil A condenser coil is the finned outdoor heat exchanger in an air conditioner's condensing unit where hot, high-pressure refrigerant releases heat absorbed from inside the home. The condenser fan pulls outdoor air across the coil's aluminum fins and tubing, and the refrigerant condenses from vapor to liquid as it gives up that heat. What it means for you →
- Cooling Condensing Unit A condensing unit is the outdoor half of a split air-conditioning system. It contains the compressor, the condenser coil, a condenser fan motor, and electrical parts such as the capacitor and contactor. It receives refrigerant vapor carrying heat from the house, compresses it, and releases that absorbed indoor heat to the outdoor air. What it means for you →
- Equipment Parts Contactor A contactor is an electrically controlled switch in an air conditioner's or heat pump's outdoor unit that connects line voltage to the compressor and condenser fan. When the thermostat calls for cooling, a 24-volt signal energizes the contactor's coil and closes its contacts; when the call ends, the contacts open and the unit shuts off. What it means for you →
- Houston Home & Climate Cooling Degree Days Cooling degree days (CDD) measure how much cooling a climate demands: each day contributes the number of degrees its average temperature exceeds 65°F, summed over a year. Heating degree days (HDD) count the opposite. NOAA's 1991–2020 normals for Houston Intercontinental Airport (IAH) show about 3,288 cooling versus 1,266 heating degree days annually. What it means for you →
D
- Heating & Heat Pumps Defrost Cycle A defrost cycle is the routine a heat pump runs in heating mode when frost builds up on its outdoor coil. A defrost control board briefly reverses the system into cooling to warm the coil and melt the ice, often switching on backup heat indoors. During defrost the outdoor fan usually stops and the unit may release visible steam. What it means for you →
- Houston Home & Climate Dehumidification Dehumidification is the removal of moisture from indoor air, done by an air conditioner's evaporator coil as warm air passes over it and water condenses, or by a dedicated whole-home dehumidifier. Engineers call it latent cooling, as opposed to sensible cooling, which lowers air temperature; comfort in humid climates requires both. What it means for you →
- Airflow, Ducts & Air Quality Ductwork Ductwork is the network of supply and return ducts that carries conditioned air from the air handler or furnace to each room and brings room air back for reconditioning. Residential ducts are usually insulated flexible duct, sheet metal, or duct board, and leaks, kinks, crushed runs, or undersized sections reduce airflow, comfort, and efficiency. What it means for you →
E
- Money, Code & Permits EER2 EER2 (Energy Efficiency Ratio 2) is the U.S. Department of Energy rating for an air conditioner's cooling output per watt of electricity at a single hot test condition, 95°F outdoors, rather than across a season. Federal EER2 minimums apply only in the Southwest region, so Texas installations must meet SEER2 minimums but have no EER2 requirement. What it means for you →
- Heating & Heat Pumps Emergency Heat Emergency heat is a thermostat setting on heat pump systems that shuts off the outdoor heat pump and heats the home with backup heat only, usually electric heat strips in the air handler. Unlike auxiliary heat, which turns on automatically, emergency heat is selected manually. It is meant for a failed or damaged heat pump, not everyday use. What it means for you →
- Refrigerants EPA Section 608 Certification EPA Section 608 certification is the federal credential, required under Section 608 of the Clean Air Act, for technicians who maintain, service, repair or dispose of equipment that could release refrigerants. EPA offers Type I, Type II, Type III and Universal certification; Type II covers high-pressure equipment, including residential split air conditioners and heat pumps. What it means for you →
- Cooling Evaporator Coil An evaporator coil is the indoor heat exchanger in an air conditioner or heat pump, mounted on the air handler or above the furnace. Cold refrigerant flowing through its tubes absorbs heat from air the blower pushes across its fins, and moisture condenses on the coil and drains away, which is how central air dehumidifies a home. What it means for you →
F
- Money, Code & Permits Federal 25C Tax Credit The Federal 25C tax credit, the Energy Efficient Home Improvement Credit, was a federal income tax credit for qualifying heat pumps, central air conditioners, furnaces and other efficiency upgrades. Under Public Law 119-21, enacted July 4, 2025, it is not allowed for property placed in service after December 31, 2025, so 2026 installations do not qualify. What it means for you →
- Airflow, Ducts & Air Quality Float Switch A float switch is a safety device, mounted in an air conditioner's secondary drain pan, the primary drain line, or a condensate pump, that rises with backed-up water and shuts off the cooling system before water overflows. When it trips, the system stops cooling until the drain is cleared and the water level drops. What it means for you →
- Houston Home & Climate Flood-Damaged HVAC Equipment Flood-damaged HVAC equipment is any outdoor condensing unit, air handler, furnace, ductwork, or control that floodwater has submerged or soaked. Water and silt corrode electrical components, motors, controls, and gas valves and contaminate ducts and insulation, so flooded equipment must be inspected by qualified professionals before it is powered up or used again. What it means for you →
G
H
- Heating & Heat Pumps Heat Exchanger A heat exchanger is the metal chamber or set of tubes inside a gas furnace that keeps burning fuel and its exhaust separate from the air circulating through your home. Heat passes through the metal walls into the airflow while combustion gases vent outdoors. A cracked heat exchanger can let those gases, including carbon monoxide, into the airstream. What it means for you →
- Heating & Heat Pumps Heat Pump A heat pump is an electric heating and cooling system that moves heat instead of burning fuel. In summer it pulls heat out of the house like an air conditioner; in winter a reversing valve runs the refrigerant cycle backward to bring outdoor heat inside. Its cooling efficiency is rated in SEER2 and its heating efficiency in HSPF2. What it means for you →
- Heating & Heat Pumps Heat Strips Heat strips are electric resistance heating elements installed in an air handler that warm passing air the way a toaster's coils do. They serve as the main heat source in an all-electric air conditioner system, or as auxiliary and emergency heat in a heat pump system. Their capacity is rated in kilowatts, and they convert electricity directly into heat. What it means for you →
- Houston Home & Climate HOA and Covenant Equipment Rules HOA and covenant equipment rules are deed-restriction requirements, enforced by a homeowners association or community authority, that govern where outdoor HVAC equipment may sit, how it looks, and whether replacing it needs approval. In The Woodlands, Section 2.2 of the Township's 2024 Residential Development Standards addresses outdoor condensing unit placement, color, and replacement. What it means for you →
- Money, Code & Permits HSPF2 HSPF2 (Heating Seasonal Performance Factor 2) is the U.S. Department of Energy rating for how much heat a heat pump delivers per watt-hour of electricity over a typical heating season, measured under the M1 test procedure adopted in 2023. Since January 1, 2023, new split-system heat pumps nationwide must be rated at least 7.5 HSPF2. What it means for you →
- Money, Code & Permits HVAC Financing HVAC financing is a loan or credit plan that lets a homeowner pay for heating and cooling equipment, installation or major repairs over time instead of all at once. Rates, terms and approval are set by the lender, not the contractor. Applications are usually started through the contractor and decided by the lending partner. What it means for you →
- Money, Code & Permits HVAC Maintenance Plan An HVAC maintenance plan is a service agreement in which a contractor performs scheduled tune-ups on a home's heating and cooling equipment, typically once before cooling season and once before heating season, often with added member terms. Visits usually cover cleaning coils, checking electrical components and refrigerant performance, clearing the condensate drain and inspecting airflow. What it means for you →
M
- Money, Code & Permits Manual J Load Calculation A Manual J load calculation is the Air Conditioning Contractors of America (ACCA) method for working out how much heating and cooling a specific house needs, room by room. It accounts for insulation, windows, orientation, air leakage, duct location and local design temperatures, and ACCA describes it as the ANSI-recognized standard for sizing residential HVAC equipment. What it means for you →
- Money, Code & Permits Mechanical Permit A mechanical permit is written approval from a local building department to install, replace or alter heating, cooling and ventilation equipment, usually followed by an inspection of the finished work. In Greater Houston, cities such as Houston, Sugar Land, Pearland, Missouri City and Katy issue their own, while unincorporated areas follow state code standards. What it means for you →
- Airflow, Ducts & Air Quality MERV Rating MERV (Minimum Efficiency Reporting Value) is a 1-to-16 rating, defined by ASHRAE Standard 52.2, for how well an air filter captures particles from 0.3 to 10 microns. Higher numbers capture more small particles such as smoke and fine dust, but denser media can restrict airflow, so the right MERV depends on what your system can handle. What it means for you →
P
R
- Refrigerants R-22 (Freon) R-22, also called HCFC-22 and widely known by the brand name Freon, is an ozone-depleting refrigerant used in most U.S. home air conditioners and heat pumps installed before 2010. Under the Clean Air Act phaseout, U.S. production and import of R-22 ended January 1, 2020. Existing systems may keep running on recovered or stockpiled supply. What it means for you →
- Refrigerants R-32 R-32, or difluoromethane, is a single-component refrigerant used in many new residential air conditioners and heat pumps as a lower-global-warming replacement for R-410A, of which it is already one ingredient. It is classified A2L, meaning lower toxicity and mild flammability, so equipment built for it follows the UL 60335-2-40 safety standard. Goodman's current residential split systems use R-32. What it means for you →
- Refrigerants R-410A R-410A, sold under trade names including Puron, is the HFC refrigerant used in most U.S. home air conditioners and heat pumps installed from about 2010 through 2024. Under the AIM Act, manufacturing and import of new R-410A split systems ended January 1, 2025. Existing systems can still be repaired, and new equipment now mostly uses A2L refrigerants. What it means for you →
- Refrigerants R-454B R-454B is a refrigerant blend of R-32 and R-1234yf used in many new residential air conditioners and heat pumps as a lower-global-warming replacement for R-410A under the EPA's HFC phase-down. It is classified A2L, meaning lower toxicity and mild flammability, and equipment built for it follows the UL 60335-2-40 safety standard. What it means for you →
- Refrigerants Refrigerant Refrigerant is the fluid sealed inside an air conditioner or heat pump that carries heat out of the home, absorbing it as it evaporates in the indoor coil and releasing it as it condenses in the outdoor coil. It is not consumed in normal operation. Common residential refrigerants include R-22, R-410A, R-32 and R-454B. What it means for you →
- Refrigerants Refrigerant Leak A refrigerant leak is an escape of refrigerant from an air conditioner or heat pump's sealed circuit, usually through corroded coils, worn joints or vibrating line connections. Because refrigerant is not used up, a system that is low on refrigerant has a leak. Common symptoms include weak cooling, longer run times, ice on the indoor coil and hissing sounds. What it means for you →
- Houston Home & Climate Relative Humidity Relative humidity (RH) is the amount of water vapor in air expressed as a percentage of the maximum that air can hold at its current temperature. Indoors, the U.S. Environmental Protection Agency recommends keeping relative humidity below 60 percent, ideally between 30 and 50 percent, to limit mold growth and the clammy feel of damp air. What it means for you →
- Money, Code & Permits Repair vs. Replace ($5,000 Rule) The $5,000 rule is a repair-or-replace rule of thumb for HVAC systems: multiply the system's age in years by the repair quote in dollars, and if the result exceeds 5,000, price a replacement before approving the repair. A related 50% rule flags replacement when a repair would cost more than half the price of a new system. What it means for you →
- Heating & Heat Pumps Reversing Valve A reversing valve is the solenoid-operated valve in a heat pump's outdoor unit that switches the direction of refrigerant flow, turning the system from cooling to heating and back. It also shifts the system into cooling briefly during a defrost cycle. When it sticks or loses its control signal, a heat pump may blow cool air in heat mode. What it means for you →
- Cooling Rooftop Unit (RTU) A rooftop unit (RTU) is a packaged heating, ventilation and air-conditioning system mounted on a building's roof, usually on a curb that connects it to ductwork below. Each cabinet contains the compressor, coils, blower and controls, often with gas or electric heat and an outdoor-air damper or economizer. RTUs are standard on light commercial buildings. What it means for you →
S
- Money, Code & Permits 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 over a typical cooling season. It replaced SEER in 2023 under a tougher test procedure. In Texas, new split air conditioners must reach 14.3 SEER2 below 45,000 Btu/h. What it means for you →
- Equipment Parts Service Disconnect A service disconnect is the weatherproof electrical switch box mounted within sight of an outdoor air conditioner or heat pump that lets a technician cut power to the unit at the equipment. It may contain a pull-out block, fuses or a breaker, and electrical codes generally require one so equipment can be safely de-energized for service. What it means for you →
- Cooling Short Cycling Short cycling is when an air conditioner, heat pump or furnace turns on and off in rapid, frequent cycles without completing a normal run. Common causes include an oversized system, a dirty filter, a frozen evaporator coil, low refrigerant, and a failing or poorly placed thermostat. It adds wear to the compressor and reduces dehumidification. What it means for you →
- Cooling Split System A split system is an air conditioner or heat pump whose components are divided between two cabinets: an outdoor condensing unit holding the compressor and condenser coil, and an indoor air handler or furnace holding the evaporator coil and blower. Insulated copper refrigerant lines and low-voltage control wiring connect the two halves. What it means for you →
- Airflow, Ducts & Air Quality Static Pressure Static pressure is the resistance to airflow inside an HVAC system's ducts, filter, and coil, measured in inches of water column (in. w.c.). Total external static pressure combines the readings entering and leaving the air handler or furnace; when it exceeds the equipment's rated limit, the blower moves less air, efficiency falls, and components work harder. What it means for you →
- Refrigerants Superheat and Subcooling Superheat and subcooling are temperature measurements technicians use to judge whether an air conditioner or heat pump holds the correct refrigerant charge. Superheat is how far refrigerant vapor leaving the evaporator sits above its boiling point; subcooling is how far liquid leaving the condenser sits below its condensing point. Each reading is compared against manufacturer targets. What it means for you →
- Airflow, Ducts & Air Quality Supply and Return Vents Supply and return vents are the two halves of a home's air path: supply registers deliver conditioned air from the ductwork into rooms, while return grilles draw room air back to the air handler for filtering and reconditioning. Supply registers usually have adjustable louvers; return grilles are larger, often fixed, and sometimes hold the system's air filter. What it means for you →
T
- Money, Code & Permits TDLR Contractor License A TDLR contractor license is the Texas air conditioning and refrigeration (ACR) contractor license issued by the Texas Department of Licensing and Regulation, required for businesses that install, repair or maintain HVAC systems in Texas. Each license has a public number, and homeowners can confirm its status on TDLR's online license search. What it means for you →
- Equipment Parts Thermostat A thermostat is the wall-mounted control that senses indoor temperature and signals heating and cooling equipment to start or stop. Types include manual, programmable, Wi-Fi smart and communicating models. Heat pump thermostats also manage auxiliary and emergency heat, and some models control humidity and fan settings or report system alerts. What it means for you →
- Money, Code & Permits Tonnage (BTU/h) Tonnage is the cooling capacity of an air conditioner or heat pump, where one ton equals 12,000 BTU/h (British thermal units per hour) of heat removed. Residential systems commonly range from 1.5 to 5 tons. Tonnage measures capacity, not efficiency, and the right size for a home comes from a Manual J load calculation. What it means for you →
- Equipment Parts TXV (Thermostatic Expansion Valve) A TXV (thermostatic expansion valve) is a metering device at the evaporator coil inlet that controls how much liquid refrigerant enters the coil. A sensing bulb on the suction line opens or closes the valve to hold a target superheat, keeping the coil fully used as heat loads change. A fixed-orifice piston is the simpler alternative. What it means for you →
U
V
Have a question we didn't cover?
Ask a Houston-area technician.
Tell us what's going on and our Houston team will reach out — usually within the hour during business hours. For emergencies, call us directly anytime.
Call us 24/7(832) 447-3233- 24/7 Emergency Service
- Licensed & Insured
- Reviewed on Google
- Serving the Greater Houston area
23210 Birnam Wood Blvd, Spring, TX 77373
Fast response
Request your service
No obligation. We never sell your info.