BTU, tons, SEER2 & AFUE, decoded

BTU, the unit of heat

One British Thermal Unit is the heat that warms a pound of water by 1°F, about as much energy as a kitchen match. On equipment, the number is really BTU per hour: how much heat the machine can move in or out of your house continuously. A 12,000 BTU air conditioner removes 12,000 BTU of heat per hour. All the sizing math on this site is about matching that rate to the rate your space gains (or loses) heat.

Tons: 12,000 BTU/h in a trench coat

A "ton" of cooling is 12,000 BTU/h, the melt rate of a literal ton of ice per day, a unit the industry kept from the ice age of refrigeration. Central AC is sold in half-ton steps: 1.5, 2, 2.5, 3, 3.5, 4, 5. Nothing mystical: a 3-ton unit is a 36,000 BTU/h unit.

SEER2, cooling efficiency

Seasonal Energy Efficiency Ratio: cooling delivered per electricity consumed across a simulated season. The "2" marks the 2023 test procedure with more realistic duct pressure, so SEER2 numbers read slightly lower than old SEER for the same hardware. Today's floor is roughly 14–15 SEER2 depending on region; good equipment runs 16–18; premium inverter systems into the 20s. Each point of SEER2 is roughly 5–7% off the cooling portion of your bill: worth paying for in Phoenix, marginal in Portland.

HSPF2, heat pump heating efficiency

The heating-season counterpart. The floor is about 7.5; cold-climate models run 9–10+. More useful than the seasonal average, though: the capacity table at 17°F and 5°F buried in the engineering data. Two heat pumps with identical HSPF2 can differ by 40% in how much heat they still deliver at 5°F; that table is what "cold-climate rated" actually means.

AFUE, furnace efficiency

Annual Fuel Utilization Efficiency: how much of the gas becomes heat in your house instead of going up the flue. The market splits into 80% conventional and 95–98% condensing models. Two traps: furnaces are advertised by input BTU (an 80,000 BTU / 80% furnace delivers 64,000), and the efficiency assumes decent ducts: a 96% furnace feeding 20%-leaky attic ducts is a 77% system. The heating calculator handles the input/output conversion.

Which numbers to pay for

• Size (BTU/tons): get it right, not big; see why bigger is worse.
• SEER2: pay for it in proportion to your cooling hours.
• HSPF2 / 5°F capacity: the one to scrutinize if a heat pump is your primary heat.
• AFUE: 95%+ almost always pays back in zones 3–5; in zone 1 the 80% unit may never earn the difference.