Rule-of-thumb boiler output sizing based on building area, type, and climate. Includes DHW load, piping loss, and sizing factor. Use for early-stage feasibility — final sizing requires a heat loss calc.
| Building Type | Mild | Moderate | Cold | Very Cold |
|---|---|---|---|---|
| Office / Commercial | 8–12 | 15–22 | 20–30 | 28–40 |
| School | 10–15 | 18–25 | 22–35 | 30–45 |
| Hospital / Healthcare | 12–18 | 20–30 | 28–40 | 35–50 |
| Hotel / Multifamily | 10–16 | 18–28 | 25–38 | 32–48 |
| Warehouse / Industrial | 5–10 | 10–18 | 15–25 | 20–35 |
| Residential | 12–18 | 20–30 | 28–42 | 35–55 |
This boiler size estimator gives a fast, rule-of-thumb heating capacity for early feasibility. Enter the conditioned floor area, building type, climate zone, heating system type, a domestic hot water allowance, and the boiler efficiency; the tool returns the space-heating load, DHW and piping additions, the boiler output and input in MBH, and the gas rate in CFH.
It is built for the concept and budgeting stage, not for final equipment selection. Treat the output as a starting point, then confirm with a full heat-loss calculation per ACCA Manual J or the ASHRAE Handbook — Fundamentals before specifying a boiler.
The heat-loss density is a benchmark midpoint chosen by building type and climate; the 1.25 multiplier is a 25% sizing/pickup margin. Dividing by 1,000 converts BTU/hr to MBH (thousands of BTU/hr). Because natural gas is about 1,000 BTU per standard cubic foot, the input in MBH is numerically close to the gas rate in CFH. Efficiency context follows ASHRAE 90.1 minimums — condensing mod/con boilers reach 92–98%, atmospheric units around 80%.
This calculator handles one step. AIM Works runs the complete MEP design workflow — thermal load calculations, duct & pipe networks, equipment selection, code compliance, and an AI design assistant — in one tool.