BoilerBuying Guide2026-02-24
What Size Combi Boiler Do I Need?
Combi sizing balances heating output with peak hot-water flow — not just square footage.
Combi sizing is part heat output, part hot-water flow rate. Both must match your home.
Heating output
Output (BTU) should match your home's measured heat loss so the boiler isn't oversized (short-cycling) or undersized.
Hot-water flow
The combi must deliver enough flow (GPM) for your peak simultaneous taps and showers, accounting for cold BC inlet temperatures.
Key takeaways
- Match output to measured heat loss
- Match flow to peak hot-water use
- Account for cold winter inlet water
- Avoid oversizing
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