How the correct boiler size is actually worked out

Boiler sizing — the facts that change the job

• Total property heat loss underpins the output range being considered
• Hot water demand or cylinder recovery needs are factored in separately
• Oversized boilers cycle inefficiently rather than running steadily
• Undersized boilers struggle to maintain temperature in colder conditions
• Matching the old boiler's rating is not treated as a reliable sizing method

The engineering behind boiler sizing

The total heat loss figure across the property, combined with an assessment of hot water demand for a combi, or cylinder recovery requirements for a system boiler, determines the output range that suits the property. Manufacturer model ranges are then matched against this figure, rather than simply selecting a popular size regardless of the specific building's actual requirements.

Where we deal with this

• Frome (BA11) · 7 mi
• Paulton (BS39) · 3 mi
• Pensford (BS39) · 7 mi

Questions we get asked

Is a bigger boiler always better?

No — oversizing causes inefficient cycling, so matching output to actual calculated demand is the correct approach.

Should I just replace like-for-like on output rating?

Not without checking — the original boiler may have been oversized or undersized to begin with.