HVAC Glossary — Plain-English Definitions

Every trade has jargon. Here's the plain-English translation for the HVAC terms you'll hear from contractors, on service reports, and in equipment marketing.

Efficiency ratings

SEER / SEER2

Seasonal Energy Efficiency Ratio (v2). Cooling efficiency over a season, higher = more efficient. Federal minimum in Southwest region: 14.3 SEER2 for split AC. See What Is SEER2?

EER / EER2

Energy Efficiency Ratio at a single hot design point (95°+). More relevant than SEER2 in the Coachella Valley — we spend hundreds of hours above 110°F. Look for EER2 ≥ 12 for Valley applications.

HSPF / HSPF2

Heating Seasonal Performance Factor (v2). Heat-pump heating efficiency over a season. Matters more at higher elevations than on the Valley floor.

AFUE

Annual Fuel Utilization Efficiency. Gas furnace efficiency percentage. 80% AFUE = 80% of the fuel becomes useful heat; the rest goes up the flue. 96% AFUE is "condensing" — captures much more heat but requires PVC venting.

COP

Coefficient of Performance. Heat pump heat output ÷ electrical input. COP 3 = 3× the heat output per unit of electricity. Higher outdoor temperature = higher COP for heat pumps in heating mode.

Sizing and load

BTU / BTU/hr

British Thermal Unit. Heat measurement. HVAC capacity is measured in BTU/hr. 1 ton of cooling = 12,000 BTU/hr.

Ton

HVAC capacity unit. Comes from the era of ice-based cooling: 1 ton = the cooling delivered by melting 1 ton of ice per day. Coachella Valley homes are commonly 2.5–5 tons.

Refrigeration cycle

Compressor

The heart of the system. Compresses refrigerant to increase temperature/pressure. Failure = major repair or replacement decision.

Evaporator coil

Indoor coil where refrigerant absorbs heat from house air. Also called the "A-coil" or "cased coil."

Condenser coil

Outdoor coil where refrigerant releases heat to outside air. What you see when you look at the outdoor unit.

Refrigerant / R-410A / R-454B / R-32

The working fluid. R-410A was the standard 2010–2024; R-454B and R-32 are the lower-GWP replacements as of 2025+. See R-454B vs R-410A.

TXV (thermostatic expansion valve)

Meters refrigerant into the evaporator coil based on load. Common failure item on older systems.

Superheat / Subcool

Diagnostic numbers that reveal whether refrigerant charge is correct. Set at commissioning; measured at every tune-up.

Airflow and ductwork

CFM

Cubic Feet per Minute. Airflow measurement. Standard target: ~400 CFM per ton of cooling.

Static pressure

Air pressure across the air handler, measured in inches of water column ("WC). High static = restricted airflow (dirty filter, undersized duct, closed dampers). Should be under 0.5" WC total across the air handler.

Plenum

The large duct box directly on the air handler. Supply plenum distributes conditioned air; return plenum brings return air back.

MERV

Minimum Efficiency Reporting Value. Filter rating from 1 (loose) to 20 (HEPA-adjacent). See Air Filters & MERV.

Controls

Contactor

Relay that switches 240V power to the compressor and outdoor fan when the thermostat calls for cooling. Common failure item, cheap to replace.

Capacitor

Component that stores the jolt required to start the compressor and outdoor fan. Fails more in high heat. Most common single failed part in Valley systems.

Float switch (drain safety)

Cuts thermostat power when the condensate drain pan fills. Prevents attic water damage. Trips = clogged drain.

Reversing valve

Heat-pump-specific component that switches the refrigerant flow direction to change between cooling and heating.

Regulatory / installation

AHRI Certified

Air-Conditioning, Heating & Refrigeration Institute certificate confirming a specific outdoor + indoor combination has been tested as a matched system. Required for warranty and most rebates.

Title 24

California's building energy code. HVAC installs must comply; some require HERS field verification.

HERS Verification

Home Energy Rating System field verification by an independent third party — confirms duct sealing, refrigerant charge, and airflow meet code. Required on many California HVAC installs.

Terms that mostly don't matter

• "Two-stage compressor" — real feature but marketed heavily. Modest benefit in the Valley.
• "Variable-speed" — real feature. Worth the money in some applications, overkill in others.
• "iSEER" / "Super Efficient" — marketing terms. Not standardized. Ask for actual SEER2 / EER2 / HSPF2.

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