Free HVAC & Hydronic Tool
Boiler Sizing Calculator
Size a boiler from actual heat loss instead of the old nameplate. Set climate zone and envelope quality, apply domestic hot water priority, then check whether the existing baseboard can even deliver the load at condensing temperatures.
Built by SubcontractorHub — the software HVAC contractors use to quote, finance, and run every job.
Book a DemoDesign heat loss
92,000 BTU/hr
46 BTU per sq ft
Required net output
105,800 BTU/hr
includes 15% pickup
Boiler input to quote
117,556 BTU/hr
at ~90% efficiency
Optional — can the existing emitters deliver it?
Heat loss here is a square-foot factor by climate zone and envelope, not a room by room calculation; it does not account for window orientation, actual infiltration rate, ceiling height above eight feet, or unheated adjacent spaces. Emitter output varies by manufacturer and by flow rate. All calculations are estimates based on historical information and should be verified by the user with a licensed HVAC contractor. This tool is provided as a free service for planning purposes only and is not a substitute for a professional assessment, a supplier quote, or applicable local code requirements.
Typical hydronic loads by home
| Home | Design heat loss | Boiler input |
|---|---|---|
| 1,200 sq ft, Zone 4, average envelope | 45,600 BTU | 60,000 BTU |
| 1,800 sq ft, Zone 5, average envelope | 82,800 BTU | 100,000 BTU |
| 2,400 sq ft, Zone 5, tight envelope | 86,100 BTU | 110,000 BTU |
| 2,400 sq ft, Zone 6, older envelope | 158,100 BTU | 175,000 BTU |
| 3,200 sq ft, Zone 6, average envelope | 172,800 BTU | 199,000 BTU |
Why Replacement Boilers Are Usually Oversized
The fastest way to size a replacement is to read the old nameplate and match it. It is also the most reliable way to install a boiler two to three times larger than the house needs — because the unit being replaced was very likely sized the same way, from a boiler installed before the house had insulation or replacement windows.
An oversized boiler short cycles. It satisfies the thermostat fast, shuts down, and restarts a few minutes later. Every one of those cycles spends fuel on purge and standby loss, wears the igniter and heat exchanger, and on a modulating condensing unit keeps return water too warm to ever condense — which forfeits the efficiency the homeowner paid a premium for.
The emitter check matters just as much on a condensing retrofit. Fin-tube baseboard puts out roughly 570 BTU per foot at 180°F but only about 270 at 140°F. Drop the supply temperature far enough for the boiler to condense and a system with just enough baseboard at 180°F will not hold temperature on a design day. Run that check before you promise condensing efficiency.
For forced-air equipment use the HVAC sizing calculator, and for a fuller room by room load see the Manual J calculator. Once the equipment is picked, HVAC contractor software turns the load numbers into a quotable proposal without re-entry.
Frequently Asked Questions
- What size boiler do I need for my house?
- Most homes need between 25 and 55 BTU per square foot of conditioned space depending on climate zone and envelope quality. A 2,000 square foot house in a Zone 5 climate with average insulation lands near 92,000 BTU per hour of heat loss, which is typically served by a 100,000 to 110,000 BTU input boiler. The single biggest sizing error in hydronic work is sizing to the old boiler's nameplate rather than to the actual heat loss — replacement boilers are very often two to three times larger than the house needs.
- Why is oversizing a boiler a problem?
- An oversized boiler short cycles: it satisfies the thermostat quickly, shuts off, then restarts minutes later. Every cycle wastes fuel on purge and standby loss, accelerates wear on the igniter and heat exchanger, and on a condensing boiler prevents the return water from ever staying cool enough to condense — which forfeits the efficiency the homeowner paid for. Correct sizing plus outdoor reset produces longer, lower-fire runs and materially better seasonal efficiency.
- Should you add domestic hot water load to the heating load?
- It depends on the control strategy. With domestic hot water priority — where the boiler diverts fully to the indirect tank and pauses space heating — you size to the larger of the two loads rather than the sum, because they never run at once. Without priority, or on a combi unit serving high simultaneous draw, the domestic load has to be added. Priority control is the usual approach precisely because it avoids oversizing the boiler for a load that lasts a few minutes a day.
- How do you check whether existing baseboard can deliver the load?
- Multiply the installed feet of fin-tube by its output at your design water temperature — roughly 570 BTU per foot at 180°F, 420 at 160°F, and about 270 at 140°F. If the installed emitter output is below the calculated heat loss, dropping to condensing water temperatures will leave the house short on a design day. That check decides whether a condensing boiler can run low enough to condense or whether emitters have to be added first.
- Is a heat loss calculation the same as a Manual J?
- No. A square-foot factor like the one used here is a fast planning estimate suitable for budgeting and equipment shortlisting. A Manual J is a room by room calculation using actual wall assemblies, window U-values, infiltration rates and orientation, and it is what should govern the final equipment selection and the emitter layout on any job where you are committing to a condensing design or a radiant system.
Quote, Finance, and Install Every System in One Platform
SubcontractorHub is the platform HVAC contractors use to turn sizing numbers like these into a branded system proposal, financing the homeowner can accept at the kitchen table, and a job the install team can schedule — without re-entering anything. This tool is free to use; the platform is here if you want a closer look.
Book a Free DemoBoiler sizing output on this page is a planning estimate. Actual design load varies with construction assemblies, infiltration, glazing area and orientation, and final equipment selection should follow a room by room Manual J or equivalent heat loss calculation performed by a licensed professional. All calculations are estimates based on historical information and should be verified by the user. This tool is provided as a free service for planning purposes only and is not a substitute for a professional assessment, a supplier quote, or applicable local code requirements.