What Size Boiler Do I Need? A Derby Homeowner's Guide to kW
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Get a free quoteShort answer: The right boiler size depends on how much heat your home loses and how much hot water you use at once. As a guide, a typical 3-bed home needs a 24 to 30kW combi boiler. Bigger homes, extra bathrooms and low mains water pressure can change that, which is why a proper survey should always come before a quote.
What size boiler do I need? It is one of the first questions Derby homeowners ask when they start looking at a new boiler, and it matters more than most people expect. Boiler size affects how warm your home gets, how fast hot water comes through and how much the boiler costs to run.
This guide explains what boiler kW actually means, the factors that decide the right size and why bigger is not always better. It is written by engineers who have been sizing and fitting boilers in Derby since 1997.
What does boiler kW mean?
Boilers are sized by their output in kilowatts, written as kW. The number tells you how much heat the boiler can produce. A higher kW boiler can heat more radiators and, in a combi, heat more hot water at the same time.
On a combi boiler, you will often see two figures. One is the output for central heating. The other, usually higher, is the output for hot water. The hot water figure is what decides how quickly water flows from your taps and shower.
What size boiler do I need? A general guide
This table gives a starting point for combi boilers in a typical Derby home. Treat it as a guide only. The final size should always come from a survey of your home.
| Home | Typical combi size | Things that might change it |
|---|---|---|
| Flat or 1 to 2 bed, one bathroom | Around 24 to 27kW | Usually straightforward |
| 3 bed, one bathroom | 24 to 30kW | Insulation, number of radiators, older property |
| 3 to 4 bed, one bathroom and an en-suite | Around 30kW or more | Showers used at the same time, mains water pressure |
| 4 bed or more, two or more bathrooms | A larger combi, or a system boiler with a cylinder | Hot water demand may suit stored hot water better |
If your home has two or more bathrooms in regular use at the same time, a combi may not be the best choice at any size. Our guide to combi vs system vs regular boilers explains the difference.
The five things that decide your boiler size
When we survey a Derby home for a new boiler, these are the things we look at before recommending a size.
1. How much heat your home loses
The boiler has to replace the heat your home loses on a cold day. A well-insulated modern house loses far less heat than an older solid-walled terrace or a large detached house with high ceilings. Insulation, windows, wall type and the size of each room all play a part.
2. The number of radiators
More radiators, or larger ones, need more heat. Your engineer will count them and look at their size. If you are planning an extension or adding radiators soon, mention it at survey so the new boiler can cope.
3. How many bathrooms and showers you use at once
A combi boiler heats water on demand. If two showers run at the same time, the boiler has to heat both. A boiler with too little hot water output will leave someone with a lukewarm shower.
4. Your mains water pressure and flow rate
A combi can only deliver hot water as fast as the mains supplies cold water. If your incoming flow is low, a bigger combi will not make your shower stronger. We check your water pressure and flow rate before recommending a combi in a Derby home.
5. Your current heating system
If you have a hot water cylinder and tanks in the loft now, switching to a combi is not always the right move. Sometimes keeping stored hot water with a system or regular boiler is the better fit. We will talk through the options rather than assume.
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View our price listWhy bigger is not always better
It is tempting to think a more powerful boiler will be better. In practice, an oversized boiler causes its own problems.
| Boiler too small | Boiler too big |
|---|---|
| Struggles to heat the home on cold mornings | Switches on and off frequently, known as short cycling |
| Weak or lukewarm hot water when two taps run | Extra wear on parts from constant starting and stopping |
| Works flat out for long periods | Can run less efficiently and cost more to run |
| Rooms furthest from the boiler stay cool | You may pay more upfront for power you do not need |
The aim is a boiler that matches your home. That is why we assess sizing before quoting, so the price you see is based on your house, not a guess.
Can I just replace my old boiler with the same size?
Not always. Many older boilers were oversized when they were fitted. Homes also change over the years, with new insulation, double glazing, extensions or extra bathrooms. A like-for-like swap can mean you pay for a bigger boiler than you need, or end up with one that cannot keep up.
A survey takes those changes into account. If your current boiler is the right size, we will say so. If it is not, we will explain why and what we would recommend instead.
Not sure what your home needs?
Talk it through with a Gas Safe engineer. Call 01332 204533, Monday to Friday, 8am to 5pm, or send us a message.
Speak to our teamWhat happens at a boiler sizing survey in Derby
Every boiler quote we give starts with a visit, not a guess over the phone. Here is what to expect:
- A look at your current boiler and system. Type, age, flue position and pipework.
- A walk round your home. Room sizes, radiators, insulation and windows.
- A check of your hot water needs. Bathrooms, showers and how your household uses them.
- Mains water pressure and flow. Essential if a combi is on the table.
- A recommendation and a fixed price. We give fixed-price quotes wherever possible, with no hidden charges.
Quotes are free and no obligation, with evening appointments available. Once you go ahead, most boiler swaps are completed within a single day, and every installation includes a central heating power flush.
Sizing oil and LPG boilers in off-gas Derbyshire homes
The same principles apply if your home runs on oil or LPG, which is common in villages around Derby that are off the mains gas network. The boiler still needs to match your home's heat loss and hot water use.
There are a few extra things to check. Oil and LPG homes are often older, larger or more exposed rural properties, which can lose more heat. The fuel supply, tank position and flue route also affect which boilers will work. Our engineers are OFTEC registered (C100278) for oil and Gas Safe registered for LPG, so we can size and fit either.
See our oil, natural gas and LPG page for more on each fuel.
Does boiler size affect the price?
Yes, but it is only one part of the cost. Larger boilers tend to cost more, and the type of boiler, the brand, any changes to pipework or flue, and whether the boiler is moving all affect the final figure. Our 2026 new boiler cost guide for Derby explains what goes into a quote.
Getting the size right also pays off over time. Modern condensing boilers are up to 40% more efficient than older models, and a correctly sized boiler gives you the best chance of seeing that efficiency in practice. New boilers come with up to 12 years' manufacturer's warranty.
Boiler sizing across Derby and Derbyshire
We size and fit gas, oil and LPG boilers across Derby, Amber Valley and Erewash, South Derbyshire, Uttoxeter and Ashbourne and Coalville. Visit our boiler installations and replacements page to find out more.
What size boiler do I need: FAQs
What size boiler do I need for a 3-bed house?
A typical 3-bed home needs a 24 to 30kW combi boiler. Insulation, the number of radiators, bathrooms and mains water pressure can all change that, so the size should be confirmed at survey.
Is a bigger boiler better?
No. An oversized boiler switches on and off more often, which adds wear and can reduce efficiency. The best boiler is one sized to match your home's heat loss and hot water needs.
Can I replace my boiler with the same size?
Sometimes, but not always. Many older boilers were oversized, and homes change over time with insulation, extensions and extra bathrooms. A survey confirms the right size for your home today.
What does kW mean on a boiler?
kW stands for kilowatts and measures the boiler's heat output. A combi boiler usually lists one figure for central heating and a higher figure for hot water.
Is a combi boiler right for a house with two bathrooms?
It depends on whether both are used at the same time and on your mains water pressure. For homes with high hot water demand, a system boiler with a cylinder is often a better fit.
Who can size a boiler in Derby?
Etta Plumbing & Heating Services is a Gas Safe registered (166252) heating company based in Derby, trading since 1997. We assess boiler sizing as part of every free, no-obligation quote across Derby and Derbyshire.
The right size starts with the right survey
There is no single answer to what size boiler you need. A guide table gets you close, but your home's heat loss, hot water use and water pressure decide the right fit.
A free survey from a local Derby engineer takes the guesswork out. You get a boiler that heats your home properly, delivers the hot water you need and does not waste energy doing it.
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