Learning Center

HVAC Glossary

Every heating, cooling, and air-quality term we use across the Learning Center — defined in plain English for the Phoenix homeowner. No jargon, no sales pitch.

25C tax credit: The federal Energy Efficient Home Improvement Credit — ENDED for equipment placed in service after December 31, 2025 (July 2025 federal tax law). Systems installed by that date could claim 30% of cost, up to $600 for a high-efficiency central AC or $2,000 for a heat pump, on the 2025 return. Confirm details with your tax professional.
AFUE: Annual Fuel Utilization Efficiency — the percentage of fuel a gas furnace turns into usable heat. A 95% AFUE furnace wastes only 5% of its fuel. It matters less in Phoenix's short heating season than cooling efficiency does.
AHRI-matched system: An outdoor and indoor unit certified by the Air-Conditioning, Heating, and Refrigeration Institute to perform together at their rated efficiency. Rebates and warranties generally require an AHRI-matched system — mixing mismatched parts can void both.
APS HVAC rebate (ended): Arizona Public Service ended its residential HVAC rebate program effective January 1, 2026. If you're an APS customer, there is no APS AC rebate to claim in 2026 — be wary of any company that promises one. Federal incentives still apply.
AZ ROC license: An Arizona Registrar of Contractors license — required to legally perform HVAC work in Arizona. RM III AC & Heating holds AZ ROC #332513. Always verify a contractor's ROC number is active before hiring; it's your protection if something goes wrong.
BTU: British Thermal Unit — the basic unit of heat. AC capacity is rated in BTU/hour; 12,000 BTU/hour equals one ton of cooling.
Capacitor: An electrical component that gives the compressor and fan motors the jolt they need to start and run. Capacitors are a frequent, inexpensive failure point in the desert because extreme heat shortens their life — a common cause of an AC that hums but won't start.
Compressor: The heart of the system — the pump inside the outdoor condenser that circulates refrigerant. Compressors are the most expensive single component to replace, which is why a failed compressor on an older system often tips the decision toward replacement.
Condensate drain line: The line that carries water (condensation) away from the indoor coil. In Phoenix's monsoon humidity it can clog with algae and overflow, tripping a safety switch that shuts the AC off — a common 'AC stopped working' call.
Condenser (outdoor unit): The outdoor part of a split-system AC or heat pump. It houses the compressor and condenser coil and rejects heat from your home to the outside air. In Phoenix it sits in brutal sun and is the component most stressed by extreme heat.
Ductless mini-split: A system with an outdoor unit connected to one or more wall- or ceiling-mounted indoor heads, with no ductwork. Ideal for additions, casitas, garages, converted rooms, and zoning a single hard-to-cool space in a Phoenix home.
EER2: Energy Efficiency Ratio 2 — efficiency measured at a single high outdoor temperature (95°F). Because Phoenix spends so much of the year near or above that mark, EER2 is arguably a more honest efficiency number for the desert than the seasonal SEER2 average.
Evaporator coil (indoor coil): The indoor coil where refrigerant absorbs heat from your home's air. A dirty, iced-over, or low-refrigerant evaporator coil is one of the most common reasons an AC runs but won't cool.
Furnace / air handler: The indoor unit that moves conditioned air through your ducts. A gas furnace burns fuel for heat; an air handler paired with a heat pump moves air for both heating and cooling. Its blower is what pushes cold air through the house.
HEAR / Efficiency Arizona: The federal Home Electrification and Appliance Rebates program, administered in Arizona as 'Efficiency Arizona.' It offers income-qualified households up to $8,000 toward a qualifying heat pump, with funding available through September 30, 2031.
Heat pump: An all-electric system that both cools and heats by moving heat rather than burning fuel. A heat pump cools exactly like an AC in summer and reverses to provide efficient heat in Phoenix's mild winters — one system instead of a separate AC and furnace.
HEPA filtration: High-Efficiency Particulate Air filtration that captures 99.97% of particles down to 0.3 microns. True HEPA usually requires a dedicated bypass or portable unit because it's too restrictive to drop straight into most return grilles.
HSPF2: Heating Seasonal Performance Factor 2 — the heating-efficiency rating for a heat pump. It matters less in mild Phoenix winters than SEER2/EER2 cooling efficiency, but it's worth knowing if you're choosing a heat pump for year-round comfort.
Inverter compressor (variable-speed): A variable-speed compressor that ramps up and down to match the cooling load instead of cycling fully on and off. Inverter-driven systems — standard in quality ductless mini-splits and premium central units — run longer at lower speed, which improves efficiency and comfort during Phoenix's long run-hours.
Load calculation: The process of determining how much heating and cooling a specific home actually needs. A proper load calculation (Manual J) is why two 2,000 sq ft homes can need different-sized systems, and why a free in-home assessment beats any online estimate.
Manual D duct design: The industry-standard method for sizing a home's supply and return ducts so every room gets the right amount of air. Paired with a Manual J load calculation, a Manual D design is what prevents starved, hot rooms at the end of long duct runs — a common cause of a hot upstairs in Phoenix.
Manual J load calculation: The industry-standard method for sizing an HVAC system. A Manual J accounts for your home's square footage, insulation, windows, orientation, and the Phoenix design temperature to calculate the exact cooling and heating load — so the system isn't oversized or undersized.
MERV rating: Minimum Efficiency Reporting Value — how well an air filter captures particles, from MERV 1 to 16. For Phoenix dust, MERV 8–13 is the practical sweet spot: high enough to catch fine desert dust, low enough not to choke airflow. Higher isn't always better without the right system.
Metering device (TXV): A valve (often a thermostatic expansion valve, or TXV) that controls how much refrigerant enters the evaporator coil. A failing metering device can cause the coil to freeze or the system to lose capacity on the hottest days.
Refrigerant: The fluid that carries heat through the system. Older systems use R-410A; new equipment is transitioning to lower-global-warming refrigerants such as R-454B. Refrigerant should never need 'topping off' — if it's low, there's a leak that needs to be found and fixed.
SEER2: Seasonal Energy Efficiency Ratio 2 — the current rating for how efficiently a central AC or heat pump cools over a season. Higher is more efficient. As of 2023 the federal minimum for new split-system ACs in the hot-dry Southwest (including Arizona) is 14.3 SEER2; SRP's Cool Cash rebate requires 15.2 SEER2 or higher.
Short cycling: When an AC turns on and off in rapid, short bursts instead of running full cooling cycles. It wastes energy, strains the compressor, and often points to an oversized system, low refrigerant, or a control problem.
SRP Cool Cash: Salt River Project's rebate for SRP electric customers who install qualifying high-efficiency equipment (15.2 SEER2 or higher). It pays up to $1,125, with an additional variable amount based on system tonnage, for installs completed by April 30, 2027. RM III files the paperwork for you.
Static pressure: The resistance air meets as it moves through your ducts and filter. High static pressure — from undersized ducts or a too-restrictive filter — makes the blower work harder, reduces airflow, and can freeze the coil.
Ton (of cooling): A measure of an air conditioner's cooling capacity. One ton equals 12,000 BTU/hour of heat removal. Phoenix homes are commonly sized around 1 ton per 400–600 square feet, but the only correct way to size a system is a Manual J load calculation — not a rule of thumb.
UV germicidal light: An ultraviolet light installed at the coil or in the air handler to kill mold, bacteria, and biofilm that thrive on a cool, damp evaporator coil. Helpful for indoor air quality and for keeping the coil clean in humid monsoon months.
Zoning: Dividing one ducted HVAC system into independently controlled areas (zones), each with its own thermostat and motorized dampers in the ducts. Zoning targets the classic Phoenix two-story problem — a hot upstairs and a cold downstairs — from a single system instead of adding a second one.

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