Boiler size is measured in kilowatts (kW), not physical dimensions. A higher kW output means more heating power. Get it right and your home heats evenly with efficient gas usage. Get it wrong and you either waste energy on an oversized boiler or shiver through winter with one that cannot keep up.
The right size depends on 3 things: how many radiators you have, how many bathrooms you use simultaneously, and which type of boiler suits your property. This guide gives you the numbers for Edinburgh’s most common property types.
How boiler size is measured
Boiler output is rated in kilowatts (kW). This is the amount of energy the boiler can produce per second. A 24kW combi boiler produces 24 kilowatts of heat energy. A 35kW boiler produces 35.
A rough sizing formula: allow 1.5kW per radiator and 3kW per bathroom. A home with 10 radiators and 1 bathroom needs approximately 18kW of heating output. For a combi boiler (which also heats hot water on demand), add extra capacity for the hot water side. This is why combi boiler kW ratings are higher than system boiler ratings for the same property.
Undersizing is worse than oversizing. An undersized boiler runs at maximum output constantly, wears out faster, and never fully heats your home. An oversized boiler wastes gas by cycling on and off too frequently. The goal is the right match.
Boiler size guide by property type
This table covers combi boilers, the most common type in Edinburgh homes. System and heat-only boilers use lower kW ratings because they do not heat hot water on demand.
Edinburgh property types and what they need
Tenement flats (Marchmont, Bruntsfield, Leith, Stockbridge)
Most Edinburgh tenement flats are 1 to 3 bedrooms with 1 bathroom and 6 to 10 radiators. A 24 to 28kW combi boiler is the standard fit. Space is limited, so a compact wall-mounted combi works best. The short pipe runs in a flat mean the boiler does not need to push water far, so a lower kW output delivers good performance.
Semi-detached and terraced houses (Corstorphine, Portobello, Craigentinny)
These are typically 3 bedrooms with 1 bathroom plus an en-suite, and 10 to 15 radiators. A 30 to 34kW combi handles this comfortably. If two showers run at the same time every morning, a system boiler with a hot water cylinder gives better flow rates than a combi.
Detached houses (Morningside, Colinton, Barnton)
4 bedrooms, 2 or more bathrooms, 15 to 20 radiators. A 35 to 42kW combi boiler is the minimum. For homes with 3 or more bathrooms or high simultaneous hot water demand, a system boiler with an unvented cylinder is the better choice. The cylinder stores 150 to 300 litres of pre-heated water, so multiple outlets run at full pressure without affecting each other.
New builds (Granton, Craigmillar, South Queensferry)
Modern insulation standards mean new builds retain heat more efficiently. A 24 to 28kW combi often provides enough output for a 3 bedroom new build that would need 30kW+ in an older property. Your installer factors in the building’s EPC rating when sizing the boiler.
3 mistakes to avoid when choosing boiler size
- Choosing the cheapest option without checking kW. A 24kW boiler costs less than a 35kW boiler, but if your home needs 35kW, the cheaper boiler will underperform, overwork, and fail sooner. The upfront saving costs more in the long run.
- Replacing like for like without reassessing. Your old boiler was sized for your home 15 years ago. If you have added an extension, a loft conversion, or extra radiators since then, the replacement needs to be larger. Always get a fresh survey.
- Ignoring mains water flow rate. Combi boilers heat water directly from the mains. If your mains flow rate is low (common in older Edinburgh buildings), a high-kW combi will not deliver the performance you expect. Your installer should test the flow rate before recommending a combi size.
Frequently asked questions :
What size boiler do I need for a 3 bed house?
A 3 bedroom house with 1 bathroom and 10 to 12 radiators suits a 28 to 30kW combi boiler. Add an en-suite and the requirement increases to 30 to 34kW. Your installer confirms the exact size after surveying the property and testing the mains water flow rate.
Is a 24kW boiler big enough for my home?
A 24kW combi boiler is sufficient for a 1 to 2 bedroom flat or small house with 1 bathroom and up to 10 radiators. It will struggle in a larger property with higher hot water demand. Check your radiator count and bathroom usage before committing to this size.
What is the difference between a combi and system boiler?
A combi boiler heats water on demand from the mains. A system boiler heats water and stores it in a separate cylinder. Combi boilers save space and suit smaller homes. System boilers deliver better flow rates for homes with 2 or more bathrooms running simultaneously.
Can I get a boiler that is too big?
Yes. An oversized boiler cycles on and off frequently, wasting gas and increasing wear on components. It also costs more upfront. The right size matches your home’s heating demand and hot water usage without excess capacity.
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