Home Boiler Size Calculator
Boiler Size Calculator
Find the right residential boiler size in BTU/hr for baseboard or panel radiator heating — adjusted for insulation, climate, and efficiency — and compare combi vs. system/regular boiler types.
Planning-grade whole-home estimate. Sizing for radiant floor heating specifically? See the radiant floor heating calculator instead.
Quick reference — boiler output by home size (average insulation, moderate climate, condensing)
1,000 sq ft: ~50,000 BTU/hr · 1,500 sq ft: ~70,000 BTU/hr · 2,000 sq ft: ~90,000 BTU/hr · 2,500 sq ft: ~110,000 BTU/hr · 3,000 sq ft: ~140,000 BTU/hr
Based on a 40 BTU/hr per sq ft rule of thumb at 95% condensing efficiency, system/regular boiler (not combi). Combi boilers typically need to size up further for hot water demand. Use the calculator below for a number adjusted to your home.
Ready for real numbers?
On this page: Calculator · Combi vs. system · Efficiency tiers · Sizing table · Circulator & expansion tank · Related calculators · FAQ
Size your boiler
Whole-home heated square footage.
Combi boilers size up further for hot water demand.
Auto-filled from your region where possible.
$/therm (gas/propane), $/gallon (oil), or ¢/kWh (electric).
Planning-grade estimate. A contractor’s Manual J heat-loss calculation is the only way to know your exact number — compare quotes from local pros before buying.
Combi vs. system/regular boilers
| Combi boiler | System / regular boiler | |
|---|---|---|
| Hot water | On-demand, no storage tank | Needs a separate hot water tank/cylinder |
| Space needed | Compact — no tank | More space for the tank |
| Sized by | Hot water flow rate and heating load | Heating load only |
| Best for | Smaller homes, 1–2 bathrooms | Larger homes, homes with high simultaneous hot water demand |
A combi boiler with no storage tank must heat water instantly at whatever rate the household draws — two showers running at once can demand more output than a well-insulated home’s space heating alone. For 2+ bathroom homes, compare combi models by hot water flow rate (gallons per minute), not BTU/hr alone.
Boiler efficiency tiers
| Tier | AFUE | Notes |
|---|---|---|
| Standard (non-condensing) | ~80% | Older technology, higher flue gas temperature, simpler venting |
| Condensing | 90–96% | Recovers heat from flue gas; needs low return water temperature to condense |
| Premium condensing | 96%+ | Modulating output, best efficiency at partial load, pairs well with outdoor reset |
Condensing boilers only hit their top efficiency when return water stays cool enough to condense flue gas — radiant floor and oversized baseboard runs (lower supply temperatures) favor condensing efficiency more than older high-temperature baseboard systems. An outdoor reset controller helps any condensing boiler stay in its efficient range more of the year.
Boiler size by home size
Recommended output for a system/regular boiler at 95% condensing efficiency, average insulation, moderate climate, baseboard/radiator distribution — the same defaults as the calculator above.
| Home size | Recommended output | Standard boiler size |
|---|---|---|
| 1,000 sq ft | ~42,000 BTU/hr | 50,000 BTU/hr |
| 1,500 sq ft | ~63,000 BTU/hr | 70,000 BTU/hr |
| 2,000 sq ft | ~84,000 BTU/hr | 90,000 BTU/hr |
| 2,500 sq ft | ~105,000 BTU/hr | 110,000 BTU/hr |
| 3,000 sq ft | ~126,000 BTU/hr | 140,000 BTU/hr |
General rule of thumb of 40 BTU/hr per sq ft (average insulation, moderate climate) — see the BTU per square foot page for rates by climate zone and insulation quality. Combi boilers typically need to size up further; use the calculator above to adjust for your actual home and boiler type.
Circulator pump & expansion tank
Two components come with nearly every hydronic boiler installation, separate from the boiler itself:
- Circulator pump — moves hot water through the distribution loop to baseboard or radiators. Sized to the system’s flow rate and piping length, not to the boiler’s BTU output directly.
- Expansion tank — absorbs the pressure increase as water heats and expands in a closed system, protecting against relief-valve discharge or damage. Sized to total system water volume.
Both are relatively inexpensive compared to the boiler itself, but sizing them wrong can cause circulation or pressure problems — have your contractor confirm the specs during installation.
Frequently asked questions
What size boiler do I need for my house?
As a rough starting point, whole-home heat loss for an averagely insulated home in a moderate climate runs around 30–40 BTU/hr per square foot — a 2,000-square-foot home lands near 70,000–80,000 BTU/hr of output, before adjusting for efficiency and climate. A well-insulated home needs less, a drafty or older home needs more. Use the calculator above for a number adjusted to your insulation, climate zone, and boiler type.
How many BTUs per square foot does a boiler need?
General guidance for whole-home hydronic heat loss runs roughly 30 BTU/hr per square foot for a well-insulated home in a mild climate up to 60 BTU/hr per square foot or more for an older, poorly insulated home in a cold climate, with about 40 BTU/hr per square foot as a typical average-climate, average-insulation benchmark. See the BTU per square foot page for the full rate table by climate zone and insulation quality.
What’s the difference between a combi boiler and a system or regular boiler?
A combi boiler heats the home and produces hot water on demand from a single unit, with no storage tank — simpler to install and space-saving, but it must be sized for hot water flow rate as much as heating load. A system or regular boiler handles space heating only (or pairs with a separate hot water cylinder), which usually allows a smaller, less expensive unit sized purely to the home’s heat loss.
Is boiler sizing different for radiant floor heating?
Yes. Radiant floor systems are limited by how much heat the floor surface can safely emit — capped by floor covering and a comfortable surface temperature — which can require a larger boiler and different supply water temperatures than the same home would need with baseboard or panel radiators. If you’re sizing specifically for radiant floor, use the radiant floor heating calculator, which accounts for floor covering and per-zone output capacity.
Why do combi boilers need to be sized for hot water, not just heating?
A combi boiler has no storage tank, so it must heat water instantly at whatever flow rate the household draws — running two showers at once demands far more instantaneous output than a well-insulated home’s space heating alone. For homes with two or more bathrooms, comparing combi models by hot water flow rate (gallons per minute) matters as much as comparing BTU/hr output.
Does a boiler need a separate circulator pump and expansion tank?
Yes, on most systems. The circulator pump moves hot water through the distribution loop to radiators or baseboard, and the expansion tank absorbs the pressure increase as water heats and expands, protecting the system from relief-valve discharge or damage. Both are standard, relatively inexpensive components in any hydronic heating installation.