Heating capacity by airside sensible heat, space rule of thumb, or boiler and furnace sizing.
| Space Type | Rule-of-Thumb (BTU/hr/ft², envelope + infiltration) | Notes |
|---|---|---|
| Residential (new) | 8–15 | Well-insulated, sealed construction |
| Residential (older) | 20–35 | Pre-1980, minimal insulation |
| Commercial office | 10–18 | Depends on glazing ratio and climate |
| Warehouse / industrial | 8–20 | Large volume, high infiltration potential |
| Retail | 15–25 | High infiltration from frequent door openings |
| School / gym | 12–20 | High OA requirement inflates heating load |
Typical screening ranges at a moderate (≈60°F) design ΔT. Ventilation air is extra: a whole-building rule of thumb that includes it runs higher — see the Quick Load Estimator.
Doing this for a whole building? See this step inside a connected AIM Works project — loads, ventilation, equipment, and Design Review in one place.
Book a Demo →This heating BTU calculator covers three common ways to size a heating load. The airside tab applies the sensible heat equation to a supply airflow and temperature rise with altitude correction. The space tab gives a quick rule-of-thumb estimate from floor area, construction quality, and design temperature difference. The boiler/furnace tab converts a required output into fuel input, an oversized capacity, and a fuel consumption rate.
Together these cover the three questions that come up early in a heating design: how much capacity an air handler delivers, roughly how much a space loses, and what fuel input a boiler needs. The rule-of-thumb and quick methods are for screening; final sizing should come from a detailed heat-loss calculation.
| Method | Equation |
|---|---|
| Airside sensible | Q (BTU/hr) = 1.08 × CFM × (LAT − EAT) |
| Altitude factor | 1.08 × (1 − 6.8754×10⁻⁶ · elev)⁵⋅²⁵⁵⁹ |
| Space rule-of-thumb | Q = Area × base × (ΔT ÷ 60) |
| Boiler sizing | Sized output = Output × (1 + oversizing); Input = Output ÷ Efficiency |
The airside method is the standard-air sensible heat equation: 1.08 = specific heat (0.24 BTU/lb·°F) × density (0.075 lb/ft³) × 60 min/hr, scaled by the barometric pressure ratio for elevation (ASHRAE Fundamentals Ch. 1). Fundamentals Ch. 18 uses 1.10 (moist-air cp 0.244), about 2% higher. The space rule-of-thumb base is BTU/hr per ft² at a 60°F design ΔT for envelope conduction plus infiltration, scaled linearly to your ΔT; it excludes ventilation (outdoor-air) heating. Boiler input is the output divided by efficiency (AFUE for residential-size units, thermal efficiency for commercial boilers); the input rating of the sized unit is the sized output divided by the same efficiency. Fuel use at design load divides the design-load fuel input (BTU/hr) by the fuel heating value — natural gas ≈1,020 BTU/ft³, propane ≈91,600 BTU/gal, #2 fuel oil ≈138,700 BTU/gal, electric 3,412 BTU/kWh.
AIM Works brings peak loads, ASHRAE 62.1 ventilation, equipment and plant sizing, compliance checks, Design Review, and engineering deliverables into one connected mechanical design workspace.
Results are design estimates for preliminary sizing. Verify final designs against applicable codes and standards — engineering judgment and a licensed professional engineer’s review are required.