How the correct boiler size is actually worked out
Choosing a boiler's output rating properly follows from heat loss and hot water demand data gathered at survey, rather than simply matching whatever the old boiler happened to be rated at. An oversized boiler cycles inefficiently, while an undersized one cannot keep up on the coldest days.
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.
Who should undertake the work
- Gas appliance work must be completed by a suitably qualified Gas Safe engineer
- Oil appliance work should be completed by a suitably qualified OFTEC technician
- Jamie Crawford Heating states both Gas Safe and OFTEC registration
Where we deal with this
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.
Get boiler sizing looked at properly
Put simply: Correct boiler sizing comes from heat loss and hot water demand data, not from matching whatever rating the old appliance happened to have.
Call Jamie Crawford Heating on 07516 118198 during the published weekday opening hours.
Related help pages
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- Oil running costsCutting oil heating running costs without cutting comfort
- Cost driversWhen scaffolding or extra access equipment is needed
- ProcessWhat happens when you call about an urgent heating fault
- Changeover schedulingUsing the changeover window for heating maintenance, not just cleaning
- Boiler technologyCondensing vs non-condensing boilers explained
Services that cover this
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