Heating & cooling efficiency, explained

The ratings behind every running-cost number on this site.

Four ratings decide how cheap a system is to run. AFUE is a furnace's combustion efficiency (e.g. 95% = 95 cents of every fuel dollar becomes heat). HSPF/HSPF2 rates a heat pump's seasonal heating efficiency; COP is the instantaneous heat-out per energy-in (a COP of 3 = 300% efficient); SEER/SEER2 rates cooling. Pick a topic below.

AFUE

Furnace/boiler efficiency — how much of the fuel becomes heat.

HSPF & HSPF2

A heat pump's seasonal heating efficiency rating.

COP

Coefficient of performance — heat out per unit of electricity in.

SEER & SEER2

The cooling-efficiency rating (and why it matters in summer).

Climate zones & COP

Why seasonal COP falls in colder climates, and the presets we use.

Then run the numbers

Heat pump vs furnace calculator
Heat pump vs gas / propane / oil / electric
Your state

Last updated: 2026-06-29