On this page
- What does the kW rating on a combi actually mean?
- How much hot water does each size deliver?
- How many bathrooms can a combi handle?
- Why does your mains water supply matter?
- Is a bigger combi boiler always better?
- Which Navien combi sizes do we fit?
- Getting the right combi size in Stoke-on-Trent
- Frequently asked questions
Quick answer
For a combi, the kW rating mainly decides how much hot water it can deliver at once. As a rough guide, 24 to 28kW suits one bathroom, 30 to 35kW suits a bathroom plus an en suite used one at a time, and 35kW or more suits bigger demand, provided your mains water supply can keep up.
Key takeaways
- A combi's headline kW figure is mainly about hot water flow, not heating.
- Each extra kW adds roughly 0.4 litres a minute of hot water at a 35C rise.
- Your incoming mains flow can cap performance, whatever the boiler size.
- Two showers at once often points to a system boiler and cylinder instead.
Combi boilers are sold by their kW rating, and it is tempting to assume a bigger number means a better boiler. For most homes that is not how it works. Getting the size right is about how you use hot water and what your mains supply can deliver, and that is worth understanding before you get quotes this summer.
For boiler sizing more generally, including system and regular boilers, see our existing boiler size guide. This post focuses on combis only.
What does the kW rating on a combi actually mean?
On a combi boiler, the headline kW figure is normally its maximum hot water output. It tells you how quickly the boiler can heat mains water as it flows through. The central heating output is often lower than the headline figure, and most homes need far less heat for radiators than a combi can supply for hot water.
So when you compare a 28kW and a 35kW combi, you are mainly comparing how much hot water each can produce at once, not how warm they will keep the house.
How much hot water does each size deliver?
Manufacturers quote hot water flow in litres per minute at a 35C temperature rise, meaning mains water heated from around 10C to around 45C. The physics is simple: each kW heats roughly 0.4 litres a minute through that rise. So a larger combi delivers a stronger flow of hot water at the same temperature.
| Combi size | Approximate flow at 35C rise | Typically suits |
|---|---|---|
| 24kW | Around 10 litres per minute | Flat or small house, one shower, modest demand |
| 28kW | Around 11 to 12 litres per minute | Two or three bedrooms, one bathroom |
| 35kW | Around 14 litres per minute | Bathroom plus en suite used one at a time |
| 40kW or more | Around 16 to 17 litres per minute | Larger homes with a strong mains supply |
Always check the figure for the exact model, because real-world output depends on design. In winter the incoming water is colder, so the same boiler has to work harder and the flow at your chosen temperature drops a little.
How many bathrooms can a combi handle?
A combi heats water on demand, so it is best with one outlet at a time. One bathroom is ideal. A bathroom plus an en suite works well with a larger combi if showers are rarely used together. If two showers regularly run at the same time, a combi will struggle whatever its size.
That is when a system boiler with a hot water cylinder usually makes more sense, because it stores a tank of hot water ready to share between outlets. It is worth being honest about how your household uses hot water on a busy weekday morning.
Why does your mains water supply matter?
A combi can only heat the water that arrives at it. If your incoming mains delivers a weak flow, a 40kW boiler will not give you a better shower than a 28kW one, because it has less water to work with. Before recommending a size, a good installer measures your mains flow rate and pressure.
This comes up in older streets around Stoke-on-Trent, where some homes still have a narrow or ageing supply pipe from the road. A quick check at the kitchen tap tells the engineer whether the supply can support a bigger boiler or whether that money is better spent elsewhere.
If you are in the Potteries, remember the local water is hard, and it builds limescale inside combi hot water components over time. Ask your installer about scale protection on the cold feed into the boiler, and keep up with annual servicing so any build-up is spotted early.
Is a bigger combi boiler always better?
No. Oversizing brings very little benefit if your supply or usage does not need it. A boiler much larger than your heating needs can also cycle on and off more often in mild weather, which is less efficient and adds wear. It also costs more to buy. The Energy Saving Trust has independent advice on choosing and running an efficient boiler.
The right size is the smallest one that comfortably meets your peak hot water demand on the mains supply you actually have. Common sizing mistakes include:
- Choosing by number of bedrooms alone, without counting showers and baths.
- Upsizing to fix a weak shower when the real problem is mains flow.
- Keeping a combi in a home that now has two busy bathrooms.
- Ignoring the heating output and controls, which affect running costs.
Which Navien combi sizes do we fit?
We supply and fit the Navien NCB300 combi in 28kW, 37kW and 41kW outputs. Each package comes with a 5 year warranty, a magnetic filter and a wireless room thermostat. The 28kW suits most one-bathroom homes, while the larger outputs are for homes with higher hot water demand and a mains supply to match.
New boilers in England also need to meet Boiler Plus rules on controls, which we cover in Boiler Plus rules for heating controls. You can see our current options on the new boiler page.
Getting the right combi size in Stoke-on-Trent
The easiest way to get the size right is to let an engineer see the property and test the supply. We offer free remote video surveys by WhatsApp, FaceTime or Zoom, and you can start with a free boiler quote. We fit boilers across the area, and once your new boiler is in we can look after it, with boiler service in Crewe and boiler service in Nantwich as well as across Stoke-on-Trent.
Call 07896 677083 to talk through your household's hot water use before you decide.
Frequently asked questions
Can I replace a 24kW combi with a 35kW one?
Often, yes, but only if your mains supply can deliver enough water and your gas supply pipe is large enough for the bigger boiler. The installer should check both. If either is limited, upgrading the boiler size alone may not improve your showers.
Does a bigger combi use more gas?
Only when it is working harder. A boiler burns gas in proportion to the heat it produces, so a larger combi running a shower at the same flow and temperature uses about the same gas. The bigger cost differences come from purchase price and how well it is set up for your heating.
Can a combi boiler run two showers at once?
A large combi on a strong mains supply may manage two outlets, but the flow and temperature at each will drop. For households that regularly use two showers together, a system boiler with a hot water cylinder is usually the more comfortable choice.
How do I measure my mains water flow rate?
A simple method is to time how long it takes to fill a container of known size from the kitchen cold tap fully open, then work out litres per minute. An installer will use a proper flow meter and also check pressure, which gives a more reliable result.
Sources and further reading
Abdul runs Gasmaster Plumbing & Heating and has worked on boilers, heating and gas appliances across Stoke-on-Trent since 2005. He is Gas Safe registered (reg. 220932), and these articles come from his day to day work in homes and businesses across Staffordshire, Cheshire and Derbyshire.
