Choosing and planning
Combi vs System Boiler: Which One Does Your Home Need?
Choosing between a combi and a system boiler comes down to how your home uses hot water. A combi heats it from the mains as you run a tap, and a system boiler stores it in a cylinder, so your bathrooms, household, cupboard space and heat pump plans decide which fits. Here is how combi, system and regular boilers compare, with sizes and the cost of switching.
Combi, system and regular boilers compared
A combi boiler heats water from the mains on demand, with no tank or cylinder. A system boiler heats water and stores it in a hot water cylinder. A regular boiler, also called a conventional boiler, has a cylinder plus a cold water feed tank, usually in the loft, and is common in homes built before the 1990s.
Here is how the three compare, on Checkatrade’s comparison of boiler types.
| Combi | System | Regular | |
|---|---|---|---|
| Hot water store | None | Cylinder | Cylinder plus loft feed tank |
| Best for | 1 bathroom, smaller households | 2 or more bathrooms, larger households | Replacing an existing regular boiler |
| Installation | Quickest, most compact | Straightforward, no loft tank | More involved, needs loft space |
| Hot water to several taps at once | Limited | Good | Good |
| Heat pump ready | No, needs a cylinder | Yes | Yes |
Combi boiler pros and cons
A combi is compact and usually fits in a kitchen cupboard. It gives instant hot water, there is no tank to freeze, and Checkatrade says every new combi installation must meet an efficiency rating of 92% or more.
Installation is quick and simple. A like-for-like combi swap in the same place costs £2,500 to £4,000 supplied and fitted, and takes 4 to 6 hours, on Checkatrade’s figures.
A combi can struggle with high demand, and it depends on your incoming mains pressure. Older pipework may need modifying before one goes in.
System boiler pros and cons
A system boiler has no cold water feed tank, gives strong, consistent pressure and handles several bathrooms at once, and it is compatible with solar thermal. The trade-off is the cylinder, which takes up cupboard space, needs insulation and limits your hot water to what it holds.
Most heat pumps rely on a hot water cylinder, so a system boiler makes a later switch to a heat pump easier.
Do not pick a combi if a heat pump is in your plans
A combi has no hot water cylinder, so switching to a heat pump later means paying extra to add one, on top of the heat pump itself.
Regular boiler vs combi
A regular boiler needs loft space for its feed tank, and fitting one new is more involved than fitting a combi or system boiler. If you already have one, replacing it like for like is often more straightforward than converting, and you keep the cylinder most heat pumps rely on.
On Checkatrade’s figures, a new regular boiler costs around £3,000 supplied and fitted, and switching a regular or system boiler to a new combi costs an average of £4,000.
Keeping a regular boiler, or switching it to a combi
| New regular boiler, supplied and fitted | Around £3,000 |
|---|---|
| Switch to a new combi, supplied and fitted | £4,000 on average |
Extra to switch Around £1,000
Both rows are on Checkatrade's figures. Converting to a combi takes 1.5 to 2 days on site, against 4 to 6 hours for a like-for-like installation.
What size boiler do you need?
A boiler’s output is measured in kW, and Checkatrade’s sizing table sets the output a home needs by its radiators, bedrooms and bathrooms.
| Radiators | Bedrooms | Bathrooms | Boiler output |
|---|---|---|---|
| Up to 5 | 1 | 1 | 9 to 18 kW |
| 5 to 10 | 2 | 2 | 24 to 27 kW |
| 10 to 20 | 3 | 3 | 28 to 34 kW |
| 20 or more | 4 or more | 4 or more | 35 to 43 kW |
Getting a new boiler fitted
Fitting a gas boiler of any type is a job for a Gas Safe registered engineer. The Planning Portal says a new boiler needs building regulations approval, because of safety and energy efficiency, and a registered installer notifies building control directly or through its competent person scheme. The installer should give you a commissioning certificate, such as a Benchmark certificate, once the boiler is in.
Ask each installer you shortlist which type suits your home, and for a written quote that names the boiler, its output in kW and, for a system boiler, whether the cylinder is included.
Asking an installer which boiler you need
Do this
We have a regular boiler with a tank in the loft, 2 bathrooms, 4 of us showering most mornings and an airing cupboard upstairs. Would a system boiler or a combi suit us, is our mains pressure enough for a combi, and what output in kW do we need?
Not this
What boiler should we get?
Among the Nottingham heating engineers compared on accreditations and guarantees, Warm For Life in Beeston is a Worcester Bosch Accredited Installer and Service Partner, and Fahrenheit Plumbing & Heating, also in Beeston, is a Worcester Bosch Accredited Installer and a Baxi accredited installer. MB Plumbing & Heating in Arnold is a Vaillant Advance installer, and C-T Boiler Installations in Carlton is a Worcester Bosch accredited installer with G3 qualified engineers.
Which boiler do you need?
Checkatrade’s starting point is a combi for a home with 1 bathroom and 1 to 2 people, and a system boiler for 2 or more bathrooms or a busy family home. Cupboard and loft space, mains pressure and heat pump plans settle the rest.
Before you choose a combi, system or regular boiler
- Count your bathrooms, since 1 points to a combi and 2 or more to a system boiler
- Count the people who use hot water at the same time
- Find a cupboard for a hot water cylinder if you want a system boiler
- Check there is loft space for the feed tank if you keep a regular boiler
- Ask the engineer whether your incoming mains pressure suits a combi
- Ask whether older pipework needs modifying for a combi
- Decide whether a heat pump is in your plans
- A combi suits a home with 1 bathroom and 1 to 2 people, a kitchen cupboard for the boiler and no plans for a heat pump.
- A system boiler suits 2 or more bathrooms or a busy family home, with a cupboard for the cylinder and plans for a heat pump or solar thermal.
- A regular boiler suits a home that already has one and the loft space for its feed tank, where replacing it like for like is often more straightforward.
Common questions
Is a combi or system boiler better?
A combi is better for a home with 1 bathroom and 1 to 2 people, and a system boiler for 2 or more bathrooms or a busy family home, as a starting point. A combi is compact and gives instant hot water, while a system boiler gives strong, consistent pressure and handles several bathrooms at once.
What size boiler do I need?
A home with 5 to 10 radiators, 2 bedrooms and 2 bathrooms needs a boiler of 24 to 27 kW, and one with 10 to 20 radiators, 3 bedrooms and 3 bathrooms needs 28 to 34 kW, on Checkatrade's sizing table. A home with up to 5 radiators, 1 bedroom and 1 bathroom needs 9 to 18 kW, and one with 20 or more radiators, 4 or more bedrooms and 4 or more bathrooms needs 35 to 43 kW.
What is the difference between a regular boiler and a combi?
A regular boiler stores hot water in a cylinder and has a cold water feed tank, usually in the loft, while a combi heats water from the mains on demand with no tank or cylinder. Switching a regular boiler to a combi costs an average of £4,000 and takes 1.5 to 2 days on Checkatrade's figures, against around £3,000 for a new regular boiler.
Does a system boiler need a tank in the loft?
No, a system boiler has no cold water feed tank, so there is no tank in the loft. It stores hot water in a cylinder instead, which takes up cupboard space and needs insulation.
Can a combi boiler run more than one shower at once?
Not always. A combi can struggle with high demand, and its hot water to several taps at once is limited, while a system boiler handles several bathrooms at once and gives strong, consistent pressure.
Can you switch from a combi to a heat pump later?
Yes, but adding a hot water cylinder for the heat pump costs extra, because most heat pumps rely on one and a combi has none. A system or regular boiler already has a cylinder, which makes a later switch to a heat pump easier.
Where the figures come from
- Checkatrade, combi boiler vs system boiler read 2026-09-16
- Checkatrade, new boiler cost guide read 2026-09-16
- Planning Portal, boilers and heating building regulations read 2026-09-16
Next
The Best Plumbers in Nottingham
Ten firms, ranked on trade memberships, written guarantees and years in the trade.