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What size combi boiler do I need?
It’s mostly about how much hot water you want at once, not how many radiators you have. A combi’s size is its hot water output, and your water supply sets the limit on what any combi can give you.
- 2.4kW, roughly, for each extra litre a minute of hot water
- 1.0bar: what Scottish Water aims for at your stopcock
- 6 to 9%efficiency a typically oversized boiler can lose
Greenstar 4000
It’s one of the first questions people ask when they’re replacing a boiler, and most online calculators answer it by counting bedrooms or radiators. For a combi, that’s the wrong place to start.
A combi heats your hot water as you use it, straight from the mains. That’s the hard job. Heating the radiators in a typical Fife house needs far less power than heating a running shower. So the size of a combi is really about hot water, and about how much water your mains can give it.
What the kW number means
It’s the hot water output, not the heating.
When a combi is sold as 25kW, 30kW or 40kW, that’s its output when it’s heating water for your taps. Its heating output is usually lower. Worcester’s Greenstar 4000 25kW, for example, heats the radiators at up to 24kW, and can turn itself down as low as 3kW.
More kW, more hot water
A bigger combi can heat more litres of water a minute. As a rough guide, each extra litre a minute of hot water at a typical 35°C temperature rise needs about 2.4kW more.
- One shower: a modest flow does the job
- A shower and a tap at once: you need more
- Two showers at once: a big combi, or a cylinder
Sized at the survey
Worcester Bosch combis at a glance
| Sizes | Hot water flow | Worcester says it suits | |
|---|---|---|---|
| Greenstar 1000 | 24kW, 30kW | 8.5 and 10.8 litres a minute | Smaller homes |
| Greenstar 4000 | 25kW, 30kW | 9 litres a minute (25kW) | Small to medium homes with one bathroom |
| Greenstar 8000+ | 32kW to 50kW | 12 to 16 litres a minute | The biggest and busiest family homes |
Flow rates from Worcester Bosch product pages at a 40°C temperature rise, checked 2 November 2026. At the 35°C rise some brochures use, the same boiler shows about 14% more. What you actually get depends on your water supply.
Your water supply sets the limit
A bigger combi can’t make more water arrive.
A combi can only heat the water your mains delivers. If your supply is weak, a bigger boiler won’t make the shower stronger. It’ll just be capable of more than it ever gets.
Scottish Water aims to provide at least 1.0 bar at your stopcock, but it can’t guarantee it, and some homes get less. The bigger combis need more: Worcester’s guidance for its earlier Greenstar 8000 45kW asks for 2.2 bar while water is flowing to give full flow.
How we check it
At the survey we measure how much water your mains delivers, in litres a minute, and the pressure while it’s flowing. That, plus how many bathrooms you use at once, decides the size. It’s also how we know when a combi isn’t the right answer at all.
Pressure and flow
Why bigger isn’t better
An oversized boiler wastes gas.
It’s tempting to go up a size to be safe. But a boiler that’s too big for the heating switches on and off far more often, called short cycling, and never settles into its most efficient running. A UCL modelling study found typical oversizing can cost 6 to 9% in efficiency.
Scotland’s guidance for heating installers says heating should be based on a heat loss calculation for the house and shouldn’t be significantly oversized.
What matters as much as the headline size is how low the boiler can turn itself down. A combi that can run at 3kW on a mild day cycles on and off far less than one that can’t go below 6 or 7.
Greenstar 4000
Our approach
We size on your heat loss and your water supply, not the number of bedrooms. If a smaller boiler does the job, we’ll say so.
Which one sounds like you?
- Flat or one bathroomA mid-size combi
One shower at a time, done well.
- En suite, rarely used at onceA larger combi
If your mains can feed it.
- Two showers at once, every morningBig combi or a cylinder
Our combi or system guide.
- Weak mains supplyA cylinder may suit better
We check at the survey.
Common questions
Is a 30kW combi enough for a three bedroom house?
For the heating, almost always, as most three bedroom homes need far less than that. The real question is hot water: how many showers or baths run at once, and how strong your mains is. We check both at the survey.
Will a bigger combi give me a better shower?
Only up to what your mains supplies. If the water coming in is limited, a bigger boiler can’t heat water that isn’t there. We measure your flow before recommending a size.
Can a combi run two showers at once?
Most combis share their hot water, so both showers drop. A larger combi on a strong mains does better. Worcester markets its 8000+ for busy family homes, but for two bathrooms used together, a cylinder is often the better answer.
Does a bigger boiler cost more to run?
It can. A boiler too big for the house cycles on and off more and runs less efficiently. One modelling study found typical oversizing can cost 6 to 9% in efficiency.
Can I keep my electric shower with a combi?
Yes. An electric shower heats its own water from the cold main, so it works whatever the boiler is doing. When both run at once, they share the incoming water.
How do you work out the size?
At a free survey we look at the house and its heat loss, measure your mains flow and pressure, and ask how you use hot water. Then we recommend the size that fits, and confirm the price.
How much does a new combi cost?
It depends on the model and what your home needs. Our online quote gives you a guide price in a couple of minutes, and the survey confirms it.
Related pages
Sources: Worcester Bosch, combi boilers; Worcester Bosch, Greenstar 4000 specification; Worcester Bosch, Greenstar 8000+; Scottish Water, your water pressure explained; CIBSE Journal, oversizing and boiler efficiency; Scottish Government, Domestic Building Services Compliance Guide 2022. Checked 2 November 2026.
Not sure what you need? Give us a shout.
Book online in a couple of minutes or give the office a call, Mon to Fri, 9am to 5pm. We’ll tell you what we think, and if the answer is to leave things as they are, we’ll say that too.
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