Furnace size calculator
HeatingInput BTU/h and output capacity from load, climate zone and AFUE.
BTU/h · AFUE
Heating
3 tools, 5 guides, 4 comparisons
Furnaces, heat pumps and the efficiency ratings that price them — AFUE, HSPF2 and the cold-climate question.
Heating decisions come down to three questions: how much capacity the house needs at its design temperature, which fuel or machine delivers it, and what it costs to run. The furnace size calculator and the heat pump sizing calculator answer the first; the comparisons answer the second; the AFUE fuel cost calculator answers the third with the site's shared energy prices.
The ratings guides — AFUE for combustion equipment, HSPF2 for heat pumps — explain what the federal minimums mean on a nameplate. If the existing system is a boiler, start with boiler vs furnace; if it simply stopped heating, the troubleshooting guide comes before any sizing.
Tools
Federal minimums
| Product class | Minimum | Citation |
|---|---|---|
| Non-weatherized gas furnace | 80% AFUE | 10 CFR 430.32(e)(1)(ii)(A) |
| Non-weatherized gas furnace, from 2028-12-18 | 95% AFUE | 10 CFR 430.32(e)(1)(iii)(A) |
| Non-weatherized oil-fired furnace | 83% AFUE | 10 CFR 430.32(e)(1)(ii)(C) |
| Weatherized gas furnace | 81% AFUE | 10 CFR 430.32(e)(1)(ii)(E) |
| Electric furnace | 78% AFUE | 10 CFR 430.32(e)(1)(ii)(G) |
| Split-system heat pump | 7.5 HSPF2 | 10 CFR 430.32(c)(5)(ii) |
| Single-package heat pump | 6.7 HSPF2 | 10 CFR 430.32(c)(5)(iv) |
Sources: 10 CFR 430.32(e)(1)(iii) (read 2026-09-04)
Guides
How to size a furnace
HeatingFrom heating load to nameplate input, with the derating steps for altitude.
9 min read
AFUE rating explained
HeatingWhat annual fuel utilization efficiency measures, and the federal minimums.
8 min read
Furnace troubleshooting
HeatingBlowing cold, short cycling, no ignition — a decision list, question by question.
11 min read
Cold climate heat pumps
HeatingCapacity at 5°F, balance points, and when a dual-fuel setup wins.
11 min read
HSPF2 explained
HeatingThe 2023 metric, how it differs from HSPF, and the COP behind it.
8 min read
Comparisons
Heat pump vs furnace
CompareRunning cost, install cost and capacity at design temperature, side by side.
HSPF2 vs AFUE
Gas vs electric furnace
CompareFuel cost per delivered BTU, install cost and lifespan.
$/MMBtu
Two-stage vs single-stage furnace
CompareComfort, cycling and the price gap that decides it.
stages · CFM
Boiler vs furnace
CompareHot water or warm air: federal AFUE floors, cooling, air quality and service.
% AFUE
Other sections
Sizing & Load
Manual J load, BTU/h, tonnage, duct sizing and static pressure — the numbers that decide what equipment goes in.
8 tools, 4 guides
Cooling
Air conditioners, mini splits, refrigerant and SEER2 — the cool side of the manifold.
6 tools, 5 guides
Service & Costs
Replacement cost, maintenance, filters and the credits that move the price — what the job costs and what keeps it running.
6 tools, 6 guides