Rule-of-thumb boiler output for early feasibility — final sizing needs a heat loss calc.
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Building & Climate
0 if DHW has its own heater. Recovery MBH ≈ peak GPH × 8.33 × temp rise ÷ 1,000 (e.g. 100 GPH at 70°F rise ≈ 58 MBH).
One margin for morning pickup and distribution losses. 25% typical for hot water; steam often ~33%.
Condensing mod/con: 92–98% (needs return water below ~130°F); non-condensing: 80–85%
Above 2,000 ft the rated gas input is derated (4%/1,000 ft; ~2% for condensing).
Boiler Sizing Results
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Est. Heat Loss (MBH)
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DHW Load (MBH)
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Pickup Margin (MBH)
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Boiler Output (MBH)
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Rated Boiler Input (MBH)
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Gas Rate (CFH)
Heat Loss Benchmarks (BTU/hr·ft²)
Building Type
Mild
Moderate
Cold
Very Cold
Office / Commercial
10–14
15–22
20–30
28–40
Retail
12–18
17–24
24–32
32–44
School
12–18
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
6–10
10–18
15–25
20–35
Residential
12–18
20–30
28–42
35–55
Whole-building screening densities (envelope, infiltration and ventilation air) for typical code-era construction; high-performance envelopes with energy recovery can run well below them. The value the tool uses sits inside each range and inside the heating-intensity bands the AIM Works app benchmarks a load calc against.
Inputs, results, and method — as an Excel workbook or plain CSV.
Doing this for a whole building? See this step inside a connected AIM Works project — loads, ventilation, equipment, and Design Review in one place.
This boiler size estimator gives a fast, rule-of-thumb heating capacity for early feasibility. Enter the conditioned floor area, building type, climate zone, any domestic hot water load the boiler serves, a pickup margin, the boiler efficiency, and the site elevation; the tool returns the space-heating load, the margin, the boiler output and rated 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 screening benchmark chosen by building type and climate. One margin (default 25%) covers morning pickup and distribution losses; it is not stacked on a separate piping-loss allowance. Dividing by 1,000 converts BTU/hr to MBH. Above 2,000 ft the rated input is raised for the altitude derate — 4% per 1,000 ft of elevation (the NFPA 54 / IFGC default), or 2% per 1,000 ft for condensing boilers (efficiency 90% or higher); confirm with the manufacturer. The gas rate uses 1,020 BTU per standard cubic foot. Condensing mod/con boilers reach 92–98% with low return water; non-condensing units typically run 80–85%.
Frequently Asked Questions
How do I estimate boiler size for a building?
Multiply the conditioned floor area by a heat-loss density in BTU/hr per square foot that fits the building type and climate, and add any domestic hot water load the boiler also serves. Apply one pickup and distribution margin, about 25 percent, to set the boiler output, divide by the boiler efficiency for the input, and raise the rated input for altitude above 2,000 ft. This tool does that chain and shows each step.
What heat-loss density should I use per square foot?
It depends on building type and climate. Lightly loaded warehouses can be under 10 BTU/hr per square foot in mild climates, while hospitals or residential buildings in very cold climates can reach 40 to 55. The tool stores a benchmark for each combination and shows the value it used, but these are screening numbers, not a substitute for a room-by-room heat-loss calculation.
Why apply a 25 percent pickup margin?
The margin covers morning pickup after a night setback, distribution losses, and the fact that benchmark densities are approximate. Apply it once: adding a separate piping-loss allowance on top double counts. About 25 percent is a common rule of thumb for hot-water plants; steam systems often use about 33 percent. If you select from net (I=B=R) ratings, a piping and pickup allowance is already deducted from the gross output, so compare the net rating with the load before this margin. Mod/con boilers with good turndown tolerate generous sizing; single-stage atmospheric boilers should not be grossly oversized, or they short-cycle and lose efficiency.
How is boiler gas input and CFH calculated?
Boiler input equals the required output divided by the boiler's thermal efficiency, so a 92 percent boiler needs about 9 percent more input than output. The tool converts that input to a gas rate at 1,020 BTU per standard cubic foot of natural gas; use your utility's heating value if it differs. Above 2,000 ft, NFPA 54 and the IFGC reduce gas input ratings 4 percent per 1,000 ft of elevation unless the appliance is listed otherwise. Many sealed-combustion condensing boilers derate about 2 percent per 1,000 ft, so the tool raises the required rated input at those rates.
Connect the calculation to the rest of the project.
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.
Peak loads (RTS or CLTD/CLF) and ASHRAE 62.1-2019 ventilation
Equipment and plant sizing against ASHRAE 90.1-2022 efficiency floors
Design Review that cites the check behind every finding
An AI co-pilot that drafts changes — you approve each one