Heating output (kW)
This is what warms your radiators. It depends on heat loss, insulation, radiator demand, and how the property behaves. (Victorian terraces and drafty conversions love to complicate this.)
How to Choose the Right Size Boiler
without guessing (or buying “bigger just in case”)
Boiler sizing sounds like a boring numbers thing… until you’re standing in a kitchen in Forest Gate, someone’s got a towel on the radiator “to make it work better” (it doesn’t), and the shower goes lukewarm the moment the heating kicks in.
We’re ARA Services Ltd — Gas Safe engineers based in E7, usually moving along Romford Road / Woodgrange Road, up towards Stratford and across North & East London most weeks. If you want a quick sanity-check on what size you actually need, call +44 7727 154 746. Phone first. No finance plans. Just a straight answer.
Worcester • Vaillant • Ideal
Combi • System • Regular
East & North London
Before we start: boiler “size” means two different things
Most people think boiler size = how big your house is. Sort of… but not really.
Hot water performance
For combis, hot water output is often the real driver — especially if you want a strong shower. Two boilers can be “similar” on heating but wildly different for hot water delivery.
If you’re also trying to choose boiler type (combi vs system vs regular), we’ve got a plain-English guide here: a simple combi/system/regular breakdown.
TL;DR — the fast way we size boilers on the phone
This isn’t the full heat loss survey (we do that properly when needed). But if you want a fast, practical steer, this is the stuff that gets you 80–90% right without guessing.
If you’ve got 1 bathroom + decent pressure
A combi can be a great fit — the “right size” is often driven by hot water demand, not your square metres. Oversizing can make the boiler cycle on/off too much (wastes energy and wears parts).
If you want real-world cost context after sizing, start here: what boiler fitted cost actually includes.
If you’ve got 2 bathrooms (or a chaotic morning house)
Don’t force a combi just because it’s popular. A system boiler with a cylinder can be calmer and more reliable for back-to-back showers. It’s not “old fashioned” — it’s practical.
Want pricing examples for swaps/changes? Try: London boiler change price examples.
If you’re not sure, stop shopping by kW alone
Tell us: bathrooms, property type, boiler location, and whether pressure feels strong or weak at the kitchen tap. That gets you a better recommendation than “I’ve got 3 bedrooms”.
The fastest route is a few photos here: send your setup photos.
If you want the full install flow (survey, checks, cleaning, commissioning), this is the main page we build around: our boiler installation process (no fluff).
A quick story about “bigger is safer” (it isn’t)
We did a job not far from Wanstead Park where the customer had been told, confidently, to “go big”. New boiler went in, and for the first couple days they were happy… then the complaints started: radiators getting hot too fast then cooling, the boiler firing up constantly, and the house never feeling settled.
The boiler wasn’t “bad”. It was just oversized for the heating demand, and it was short-cycling. Once we adjusted settings and corrected the approach, it calmed down — but honestly, the better move would’ve been choosing the right output in the first place. That’s why we’re a bit obsessive about sizing, especially in London homes where layouts are weird and pipework isn’t always friendly.
If you’re comparing brands while you’re at it, we’ve written the honest version here: Worcester vs Vaillant vs Ideal — what we see in real installs.
What we actually check when sizing (the “engineer” bits, explained normally)
Proper sizing is a combination of heat loss and lifestyle. Here’s what changes the decision in the real world. If you want a wider cost picture for 2025, this is the clean breakdown page: new boiler cost & installation guide.
Bathrooms + simultaneous hot water
“We’ve got one shower but it’s used like a gym changing room” matters. For combis, hot water output is what you feel daily.
- One bathroom, one shower at a time: combi often fits.
- Two showers back-to-back: system + cylinder starts making sense.
- Weak mains: combi can disappoint even if the kW looks impressive.
Heat loss, not just bedroom count
A 2-bed flat above a takeaway with heat loss like a fridge can need more radiator support than a well-insulated 3-bed. Windows, insulation, drafts, and exposed walls matter a lot.
Property type really does change outcomes — we’ve covered it here: how the building affects the quote and setup.
Radiator demand + balancing
If radiators are undersized or badly balanced, people blame the boiler. Sometimes the boiler is fine — the system needs sorting.
If you’re unsure whether you even need replacement, start with: boiler repairs and honest fault advice.
Control strategy (quietly the biggest money-saver)
Correct commissioning, sensible flow temps, and decent controls can make a “normal” boiler feel brilliant. Bad controls make a great boiler feel annoying.
If you want a clean way to get a proper quote without chasing, use: our quick quote checklist.
Want us to size it properly for your home?
Call +44 7727 154 746 and tell us: bathrooms, property type (flat/house), and where the boiler sits now. If you’re near Forest Gate station / Romford Road and you’re mid-renovation, say that — we’ll work around access and timing.
Call — quick sizing guidance
Or send photos (boiler + controls + cylinder if you have one) here: message us your setup.
The sizing mistakes that cause the most regret
You can feel these mistakes in day-to-day life. Not “maybe”. You’ll feel them when it’s cold, when you’re late, or when the shower goes moody for no reason.
Buying “bigger” to be safe
Oversizing can lead to short cycling (on/off constantly), which is noisy, inefficient, and hard on parts. It also doesn’t magically fix weak water pressure.
Choosing combi when you actually need stored hot water
If you’ve got multiple bathrooms and everyone wants water at once, a combi can become a daily compromise. Sometimes a cylinder is the grown-up answer.
Forgetting the “job around the boiler”
Boiler sizing isn’t separate from the install. Pipework condition, system cleanliness, and commissioning decide whether it behaves. If you’re looking at costs, this keeps it grounded: costs explained without guesswork.
What people say after we’ve done it
Most reviews aren’t about the kW number. They’re about the feeling: “it works”, “they explained it”, “they didn’t mess me about”.
“My boiler went wrong… Abdul and his team came and fitted a new one. They worked without taking a break, cleaned up when…”
Alison Louis“Brilliant Service! Identified the issues immediately and fixed the problem. Very professional.”
Rachel Boston“Great service prompt and efficient. Fitted a new boiler at a very competitive price… took away old boiler and all other waste.”
Grant Hodges (Local Guide)If you’re comparing quotes and want to know whether the suggested boiler size actually makes sense, call +44 7727 154 746. We’d rather you get it right first time than pay twice.
Boiler sizing FAQs (real questions, not filler)
What size combi boiler do I need for a 2 bedroom house?
It depends more on hot water demand and pressure than the bedroom count. If it’s one bathroom and you’re not running multiple showers at once, a combi can work well — but we still check pressure, radiator demand, and heat loss. If you want a quick steer, call us and we’ll ask the right questions (takes about 5 minutes, not an hour).
Is it bad to oversize a boiler?
It can be. Oversized boilers can short-cycle (rapid on/off), which wastes energy and can wear components faster. Bigger also doesn’t fix weak mains pressure, which is where a lot of “my shower’s rubbish” complaints come from.
Does boiler size affect my bills?
Indirectly, yes. Correct sizing plus sensible setup (flow temps, controls, commissioning) usually reduces waste. The install quality matters too — that’s why our main service hub is here: boiler installation done properly.
I’ve got two bathrooms — should I avoid a combi?
Not always, but it’s a warning flag. If two showers are used back-to-back (or at the same time), a system boiler with a cylinder can be a much calmer setup. If you want to understand the difference without the sales talk: here’s our combi vs system vs regular guide.
How do I get a sizing recommendation without a survey?
Either call +44 7727 154 746 or send photos of: boiler location, controls, any cylinder/tank, and a quick note on bathrooms + pressure. Use the contact form and we’ll respond with a realistic direction.
Will you install Worcester, Vaillant, or Ideal once we’ve sized it?
Yes — we fit all three regularly. After sizing and boiler type are confirmed, we can advise on brand and model suitability for your property and usage. If you’re already comparing brands, this page helps: Worcester vs Vaillant vs Ideal.