Glossary

The terms that actually matter

Every heat pump quote drops 10+ HVAC acronyms. 27 of them are below, defined in plain English — with what values are normal, where you see them, and the single most common way each is misread.

Efficiency ratings

Annual Fuel Utilization Efficiency — used for gas/oil furnaces. 95% AFUE means 95% of fuel BTUs end as useful heat. Direct comparison target when evaluating HP vs. furnace payback.

COP (Coefficient of Performance)

Ratio of heat output to electrical input at a specific temperature. COP 3.0 = 3 units of heat per unit of electricity. Instantaneous (unlike HSPF2 which is seasonal average).

Energy Efficiency Ratio 2 — cooling efficiency at peak conditions (95°F). Stricter than SEER2. Matters in hot-climate regions where peak-day output defines sizing.

Heating Seasonal Performance Factor 2 — updated 2023 DOE standard measuring heating efficiency across a full season. Higher = more heat per kWh. Modern HPs range 8.5 (entry) to 11.5+ (premium cold-climate).

Seasonal Energy Efficiency Ratio 2 — updated 2023 DOE standard measuring cooling efficiency. Higher = less kWh per BTU cooled. Modern HPs range 15 (code minimum) to 26+ (premium).

Climate + weather

99% winter design temperature

The temperature your location drops below only 1% of winter hours. Used by ASHRAE and NEEP to rate cold-climate HP capacity. Boston 9°F, Minneapolis -4°F, Fargo -20°F.

CDD (Cooling Degree Days)

Annual sum of degrees above 65°F. Houston ~2,800 CDD vs Boston ~650. Drives cooling load — matters for SEER2 weighting.

HDD (Heating Degree Days)

Annual sum of degrees below 65°F, day by day. Drives heating load estimates. Minneapolis ~7,500 HDD vs Atlanta ~2,800. Used as our climate-fit input.

IECC Climate Zone

Federal energy-code climate classification, 1 (hottest) to 8 (coldest) + humidity letter A/B/C. Determines HP sizing, insulation rules, rebate eligibility.

Equipment

Auxiliary / backup electric resistance heat

Resistance coils that kick in below the HP's rated minimum. Expensive to run — if yours cycles frequently, the HP is undersized or lockout temperature is misconfigured.

Central ducted heat pump

Heat pump that uses the existing or new ductwork of a forced-air home. Drop-in replacement for a gas furnace + AC combo.

Cold-climate heat pump (ccASHP)

A heat pump rated to maintain ≥ 70-100% capacity down to 5°F (or lower). Uses vapour-injection or enhanced vapour injection compressor tech. Essentials for zones 5+.

Automatic reversal of the refrigerant cycle to melt frost off the outdoor coil. Every 35–90 minutes in cold humid weather. Home cools briefly; most HPs activate backup heat during defrost.

Dual-fuel / hybrid system

Heat pump + gas furnace in one system. HP runs above ~30°F, gas below. Avoids cold-climate HP premium in zones 5–6 where gas is cheap.

Inverter-driven compressor

Variable-speed compressor that modulates output 25-100% based on load. Eliminates short-cycling, improves humidity control, tightens temperature swings.

Mini-split (ductless)

Refrigerant-only HP system with one outdoor condenser and 1–8 wall/ceiling-mounted indoor heads. No ductwork. Best for homes without existing forced-air systems.

R32 refrigerant

Lower-GWP (GWP 675) refrigerant used by Asian brands (Mitsubishi, Daikin, Fujitsu). A2L class — similar handling to R454B. Compatible with future U.S. regulations.

R454B refrigerant

Low-GWP (GWP 466) replacement for R410A, mandated by the 2025 AIM Act. Mildly flammable (A2L) — requires trained install. Most 2026+ new HPs ship with R454B.

Certifications

AHRI Certified Reference Number

Air-Conditioning Heating Refrigeration Institute match number proving the indoor + outdoor units test together at claimed efficiency. Required for the IRS §25C credit.

ENERGY STAR Cold Climate label

Federal label requiring HSPF2 ≥ 9.0 and specific low-temperature output. More accessible than NEEP ccASHP; required for some state rebates.

NEEP ccASHP list

The Northeast Energy Efficiency Partnerships cold-climate air-source heat pump product list. The industry standard for verifying a HP holds rated capacity at 5°F and below.

Sizing + design

Manual D duct design

ACCA-standard duct-sizing calc. HPs supply cooler air than gas furnaces — undersized ducts strangle HP performance. Critical in retrofits from high-temperature gas systems.

Manual J load calculation

ACCA-standard room-by-room heating and cooling load calc. Prevents oversized HPs (short-cycling, poor humidity) and undersized ones (lost comfort, aux heat reliance). Every quote should include one.

Manual S equipment selection

ACCA-standard method matching Manual J loads to specific HP model capacity at local design temperatures. The 'why this unit for this house' document.

Rebates + credits

§25C vs. HEEHRA — which do you use?

§25C is a tax credit (need tax liability), HEEHRA is a point-of-sale rebate (no tax liability needed). If you qualify for HEEHRA, usually take it instead of §25C.

HEEHRA / HEAR (Home Electrification & Appliance Rebate)

Income-qualified IRA program: up to $8,000 off a HP for households ≤80% AMI, 50% off for 80-150% AMI. Rolls out state-by-state 2024–2026. Mutually exclusive with §25C in most implementations.

IRS §25C federal tax credit

30% of HP equipment + install cost, max $2,000/year, non-refundable. Requires AHRI cert + manufacturer certification statement. Filed on IRS Form 5695 with your tax return.