How We Size Boilers for Homes (A Straightforward 2025 Guide)
After years of walking into cold houses in the middle of winter — usually because the boiler has been working far too hard for far too long — we’ve learned that picking the right size boiler isn’t as simple as choosing the next model up “just to be safe”. Bigger isn’t better. Smaller isn’t cheaper. It’s a bit like shoes: the wrong size feels wrong every single day.
At ARA Services Ltd we size boilers based on what we see in front of us: insulation that’s been added in strange patches, radiators that have clearly been moved around three times, loft conversions done in the 90s, the works. A proper sizing job takes the house itself into account — not just a rough rule of thumb.
If you're also comparing costs or trying to understand the price differences between combi, system or regular boilers, we’ve broken everything down in our 2025 installation cost guide.
The Energy Saving Trust also publishes good, no-nonsense advice if you're tightening up insulation or trying to reduce wasted heat before committing to a boiler size.
If you're comparing boiler types as well, you might like this too: Saving Money With a New Boiler • Gas vs Electric Boilers (Our Field Notes)
What Actually Affects Boiler Size
When we're stood in someone’s hallway, tape measure in hand, these are the details we look at — not because a manual told us to, but because after enough installations, you learn what genuinely matters:
- Property size: bedrooms, radiators, room shapes — some old homes leak heat like a sieve.
- Hot-water demand: if everyone showers at the same time, it matters.
- Water pressure: makes or breaks whether a combi is even an option.
- Insulation & heat loss: two houses the same size can behave completely differently.
- Number of bathrooms: often the deal-breaker between combi and system.
Ofgem offers reasonable guidance on pressure and system limitations if you like the more official side of things: Ofgem Advice.
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Rough Boiler Size Guide (What We Usually See)
These are not strict rules — they’re the ranges we’ve ended up using after hundreds of installs. You might fall outside them depending on insulation, pressure, pipework or layout.
| Home Type | Bedrooms | Recommended Output (kW) | Best Boiler Type |
|---|---|---|---|
| Flat / Smaller Home | 1 – 2 | 18 – 27 kW | Combi |
| Medium House | 3 – 4 | 24 – 34 kW | Combi or System |
| Large Property | 4+ | 35 – 42 kW + | System or Regular |
A Quick “Which Boiler” Shortcut
When we’re trying to give a customer the fast version, this is usually how we explain it:
- 🔥 If your mains pressure is strong → a combi usually works.
- 🚿 If you’ve got two or more bathrooms → go for a system boiler.
- 🏡 Older houses with low pressure → often need a regular (heat-only).
- 💧 Weak hot-water flow → system/regular with a good cylinder beats any combi.
Quick Boiler Size Calculator
This little calculator gives a rough estimate based purely on radiator count. It’s not perfect, but it’s surprisingly close in many average-sized homes. We still recommend a proper survey — houses rarely follow the rules.