Plain-English HVAC glossary.

AFUE is the percentage of fuel a gas furnace actually turns into usable heat over a heating season — the rest goes up the flue. A 95% AFUE furnace converts 95 cents of every dollar of gas into heat in your home. The U.S. minimum for new residential gas furnaces is 80% AFUE, and high-efficiency (condensing) furnaces typically land between 90% and 98%.

A heat pump's balance point is the outdoor temperature at which the heat the pump can move into your home equals the heat your home is losing — below that, the pump can't keep up by itself and either auxiliary heat (electric strip or gas furnace) has to take over, or efficiency drops sharply. The lower the balance point, the more hours per year the heat pump handles heating on its own.

A BTU (British Thermal Unit) is the energy needed to raise one pound of water by one degree Fahrenheit. In HVAC, we usually talk about BTU/h — BTUs per hour — to describe how much heat a furnace produces or how much heat an AC or heat pump removes from your home. 12,000 BTU/h of cooling equals one ton.

A capacitor in an HVAC system is a small cylindrical electrical component that stores and releases a burst of energy to start your compressor and fan motors and keep them running smoothly. Capacitors weaken over time — Oklahoma summer heat accelerates the breakdown — and a failed capacitor is the single most common cause of an AC that hums but won't start or trips its breaker. Capacitor replacement is a cheap, fast repair when caught early.

A cold-climate heat pump is a heat pump engineered to keep producing usable heat at outdoor temperatures well below freezing — typically down to 0°F or lower — by using variable-speed compressors, enhanced vapor injection, and refrigerants tuned for low-temperature performance. Older heat pumps lose meaningful capacity around 35°F; cold-climate models keep their capacity much further down.

The compressor is the pump at the heart of any refrigeration system — AC, heat pump, mini-split, walk-in cooler. It takes low-pressure refrigerant vapor from the indoor coil, squeezes it to high pressure and temperature, and sends it to the outdoor coil where the heat is rejected. A failed compressor is the most expensive single component to replace; in most cases the math tips toward replacing the whole outdoor unit at that point.

An AC or heat pump pulls humidity out of the air as it cools — the water condenses on the evaporator coil, drips into a pan beneath it, and flows out of the home through a 3/4-inch condensate drain line. If the drain line clogs (algae, dust, lint, biofilm), the pan overflows and you find water on the floor near the air handler.

A Daikin Authorized Dealer is an HVAC company that has met Daikin's training, business-practice, and warranty-registration requirements. Authorized dealers can register the full 12-year parts limited warranty on a new Daikin system in your home, and have factory-direct access to current product training, technical support, and warranty parts. Not every shop carrying a Daikin sign is an authorized dealer — ask for the certificate.

A ductless mini-split is a heat-pump system that pairs one outdoor compressor with one or more wall- or ceiling-mounted indoor units, connected by a refrigerant line set instead of ductwork. Each indoor unit (head) heats and cools its zone independently, which is why a mini-split is so good for additions, finished garages, bonus rooms, and older homes that never had ducts. VRF (variable refrigerant flow) is the larger, multi-zone commercial cousin of the residential mini-split.

EPA 608 is the federal certification required by Section 608 of the Clean Air Act for any technician who buys, handles, or recovers stationary refrigerant. It comes in four types — Type I (small appliances), Type II (high-pressure), Type III (low-pressure), and Universal (all three). A contractor without EPA 608 is legally barred from working on most HVAC and commercial refrigeration systems.

The evaporator coil is the indoor heat-exchanger inside an air handler or above a furnace. Low-pressure cold refrigerant flows through the coil; warm humid house air blows across it; the refrigerant absorbs the heat and the moisture, and the air leaves the coil cooler and drier. A dirty or frozen evaporator coil is one of the most common reasons an AC stops blowing cold.

Section 25C (Energy Efficient Home Improvement Credit) and Section 25D (Residential Clean Energy Credit) were federal tax credits homeowners could claim against qualifying HVAC and clean-energy upgrades. Both credits expired for property placed in service after December 31, 2025 under the One Big Beautiful Bill Act signed into law in July 2025. Equipment installed in 2026 and beyond is not eligible for 25C or 25D.

The heat exchanger is the metal chamber inside a gas furnace where combustion gases give up their heat to the air your blower pushes through the house. The combustion gases stay sealed inside the exchanger; the heated air on the outside of the exchanger walls is what reaches your rooms. A cracked heat exchanger is the failure mode that forces a furnace replacement rather than a repair, because it can let combustion byproducts (including carbon monoxide) into the supply air.

HSPF2 is the heating-mode efficiency rating for a heat pump, updated alongside SEER2 in 2023. It measures total seasonal heating output divided by the electricity needed to produce it, under the new external-static-pressure test conditions. Higher HSPF2 means the heat pump produces more heat per kilowatt-hour through your local winter.

Manual J is the residential-HVAC sizing methodology published by ACCA (Air Conditioning Contractors of America). It calculates the actual heating and cooling load of a specific home by accounting for square footage, insulation, window orientation, infiltration, occupancy, and local design temperatures — instead of just multiplying square feet by a rule-of-thumb tonnage. A correct Manual J is the difference between an HVAC system that's a comfortable fit and one that short-cycles or runs constantly.

MERV is a 1–16 scale (ASHRAE 52.2) that measures how well an air filter captures particles in your duct system. A higher MERV captures smaller particles — pollen, mold spores, smoke — but also pushes harder against your blower motor. The best filter for your home is the highest MERV your specific system can handle without choking airflow.

OG&E (Oklahoma Gas & Electric) runs an Energy Efficiency Program for residential customers that includes HVAC replacement and HVAC burnout-replacement rebates. As of June 25, 2026, OG&E's public rebates page lists incentives up to $3,000 per qualifying HVAC project — first-come-first-served until annual program funds are reserved.

Oklahoma Natural Gas runs efficiency rebates aimed at homeowners converting from electric resistance heat to high-efficiency natural-gas equipment, plus rebates on qualifying new gas furnaces and water heaters. Program rules, eligible equipment, and dollar amounts change year to year.

R-32 is a low-GWP (global warming potential) refrigerant that's replacing R-410A in many new residential AC and heat-pump systems starting in 2025. It is a single-component refrigerant (not a blend), which means it's easier to recharge correctly and recover cleanly. Daikin has used R-32 in international ductless systems for years and is now rolling it out across the U.S. residential lineup.

R-410A is a hydrofluorocarbon (HFC) refrigerant blend that has been the dominant residential AC and heat-pump refrigerant in the U.S. since around 2010. The EPA's AIM Act caps R-410A production starting in 2025, so new residential AC and heat-pump systems built in 2026 and later ship with lower-GWP refrigerants like R-32 or R-454B instead. Existing R-410A systems can still be serviced — the gas is still legal, just gradually getting more expensive.

R-454B is another low-GWP refrigerant rolling out in 2025–2026 to replace R-410A in residential and light-commercial HVAC systems. Like R-32, it carries an A2L mildly-flammable safety rating, which changes how technicians braze, recover, and leak-check the system but does not affect homeowners during normal operation. Different manufacturers picked R-32 vs R-454B; Sitton services both.

Reach-in refrigeration is the standalone or built-in commercial unit you operate by opening a door, instead of walking inside. Examples are restaurant prep tables with cold drawers, beverage merchandisers, deli display cases, and back-of-house reach-in freezers. Reach-ins use the same core refrigeration cycle as walk-ins, but service is usually faster and parts more standardized.

SEER2 is the U.S. Department of Energy's cooling-efficiency rating that replaced the older SEER metric on January 1, 2023. It measures how much cooling a system produces over a typical season divided by the electricity it consumes, but uses tougher external-static-pressure test conditions that better reflect real ducted homes. A higher SEER2 number means lower summer electric bills for the same amount of cooling.

Short-cycling is when an HVAC system turns on, runs for only a few minutes, shuts off, and starts again — often many times an hour. Each start-stop wears the compressor and contactor, drives up the electric bill, and never gives the system time to dehumidify or evenly cool the house. Causes include an oversized system, restricted airflow, low refrigerant, a frozen coil, or a failing control board.

A thermostat is the control that tells your HVAC system when to heat, cool, fan, or stay off. Conventional thermostats use 24-volt low-voltage wiring (R, C, Y, W, G). Communicating thermostats — like Daikin One+ — talk to a matched indoor and outdoor unit over a proprietary serial bus instead, and unlock features like inverter-staging and predictive defrost. Smart thermostats (Nest, Ecobee, Honeywell) work with conventional 24V wiring and add Wi-Fi, scheduling, and learning algorithms.

One ton of cooling equals 12,000 BTU/h — the rate of heat removal that would melt one ton of ice in 24 hours. Residential AC and heat-pump systems are sized in tons: a typical Stillwater home runs a 2-ton to 5-ton system. Tonnage is set by a Manual J load calc, not by square footage alone.

A walk-in cooler is a commercial refrigerated enclosure large enough to walk into — used by restaurants, c-stores, florists, and grocery operators to hold inventory above freezing (typically 33–41°F). The refrigeration system has a compressor (often roof-mounted), an evaporator coil inside the box, a door with a heated frame to prevent condensation, and a defrost cycle to clear coil frost.

Wisetack is a buy-now-pay-later financing platform built for home-service trades. It lets a homeowner break the cost of an HVAC repair or replacement into fixed monthly payments through a soft credit check that does not affect their credit score. Rates and term length depend on the homeowner's credit profile and the project size.