There are three main types of gas boiler in UK homes: combi, system and conventional (also called regular or heat-only). A combi heats water on demand straight from the mains. A system boiler heats a hot water cylinder, and a conventional boiler does the same but also relies on water tanks in the loft.
The right type depends on how many bathrooms you have and how strong your mains water supply is. Space matters too, since a cylinder and tanks take up room. We’ll explain how each type works and compare them side by side. We also cover the other boiler types you may come across, and help you decide which suits your home.
How Do The Main Types Of Boiler Compare?
Combis save the most space, system boilers handle the most hot water demand, and conventional boilers suit older homes with low mains pressure.
| Feature | Combi boiler | System boiler | Conventional boiler |
| How hot water is made | Heated on demand from the mains | Stored in a cylinder | Stored in a cylinder |
| Hot water cylinder | No | Yes, usually unvented | Yes, vented |
| Loft tanks | No | No | Yes, cold water storage tank and feed and expansion tank |
| Hot water pressure | Mains pressure | Mains pressure (unvented cylinder) | Gravity fed, unless a pump is added |
| Several taps at once | Limited | Good | Good, but pressure can be low |
| Space needed | Least | Boiler plus cylinder cupboard | Boiler, cylinder and loft space |
| Typical home | Flats and homes with one or two bathrooms | Homes with two or more bathrooms | Older homes with existing tanks or low mains pressure |
| Hot water if the boiler breaks down | None | From an immersion heater, if fitted | From an immersion heater, if fitted |
What Is A Combi Boiler?
A combi (combination) boiler gives you central heating and hot water from one compact unit, with no cylinder or loft tanks.
When you turn on a hot tap, the boiler fires up and heats cold mains water as it passes through. Nothing is heated in advance.
Combis are the most popular type of boiler fitted in the UK. They suit flats and small to medium homes, and they free up the space an airing cupboard and loft tanks would otherwise take.
The limit is flow. A combi can only heat so many litres a minute, and it can only deliver what your mains supply provides. Run a shower and the kitchen tap together and both may drop off.
Larger combis help, but they can’t fix a weak mains supply. Our guide on how to size a combi boiler explains how to match output to your home.
Combi Boiler Pros
- Hot water on demand, so you never run out
- No cylinder or loft tanks, which saves space
- Mains pressure showers without a separate pump
- Usually the cheapest type to install and run for smaller homes
Combi Boiler Cons
- Struggles when several hot taps or showers run together
- Performance depends on your mains pressure and flow
- No backup hot water if the boiler breaks down
What Is A System Boiler?
A system boiler heats your radiators directly and heats water stored in a cylinder, usually in an airing cupboard.
Most of the parts a conventional set-up needs, such as the pump and expansion vessel, are built into the boiler itself. There are no tanks in the loft.
Most system boilers today are paired with an unvented cylinder fed straight from the mains. You get strong pressure at every tap, and several people can use hot water at once. That’s why we usually recommend a system boiler for homes with two or more bathrooms.
It also suits homes that may add renewables later. A cylinder can be fitted with extra coils for solar thermal panels, and the same cylinder space is handy if you move to a heat pump. See our pick of the best system boilers for current models.
System Boiler Pros
- Plenty of hot water for several bathrooms at once
- Strong pressure from an unvented cylinder
- No loft tanks, so no risk of them freezing
- Backup hot water from an immersion heater if the boiler fails
System Boiler Cons
- Needs space for a cylinder
- Stored hot water can run out and takes time to reheat
- Unvented cylinders need an annual safety check
- Usually costs more to install than a combi
What Is A Conventional Boiler?
A conventional boiler is the traditional British set-up, using a vented hot water cylinder plus two tanks in the loft.
It’s also known as a regular, heat-only or open-vent boiler. A cold water storage tank sits in the loft alongside a smaller feed and expansion tank. Water from the loft tank feeds the cylinder by gravity, and the boiler heats it.
Plenty of older homes still have this arrangement.
It works well where mains pressure is poor, because the hot water doesn’t depend on the mains. It also suits older radiators and pipework that might not cope with the higher pressure of a sealed system.
The main downside is pressure. Gravity-fed hot water often gives a weak shower, especially upstairs, unless a shower pump is added. The tanks also take up loft space and need protecting from frost.
Conventional Boiler Pros
- Supplies several taps at once from stored hot water
- Works well where mains pressure is low
- Usually a like-for-like swap in homes that already have tanks
- Gentler on older pipework and radiators
Conventional Boiler Cons
- Takes up the most space, including the loft
- Gravity-fed showers can be weak without a pump
- More parts to maintain, including tanks and valves
What Is A Condensing Boiler?
It isn’t a separate type: condensing describes how a boiler works, and combi, system and conventional boilers can all be condensing.
A condensing boiler recovers extra heat from its flue gases, which turns the water vapour in them into liquid. That liquid, called condensate, runs to a drain through a plastic pipe.
Since 2005, new gas boilers fitted in England and Wales have had to be condensing models, so any new gas boiler you buy today will be one. Modern condensing boilers are ErP A rated and typically around 92% to 94% efficient. Many older non-condensing boilers sit well below 80%.
What Other Types Of Boiler Are There?
You may also come across storage combis, back boilers, oil and LPG boilers, electric boilers and heat pumps.
What Is A Storage Combi Boiler?
It’s a combi with a small built-in hot water store. You get a faster, stronger flow than a standard combi without a full-size cylinder, so it suits homes that sit between a combi and a system boiler.
What Is A Back Boiler?
It’s a boiler that sits behind a gas fire in the fireplace. They were common in UK homes for decades but are rarely fitted now. Most get replaced with a combi or system boiler, and the fireplace is often reopened or blocked up.
Are There Oil And LPG Boilers?
Yes, and homes off the gas grid often use them. Both come as combi, system and regular boilers and work much like gas models. See our LPG and oil boilers page for the options we fit.
When Does An Electric Boiler Make Sense?
Mainly in some flats and small homes, because it needs no flue or gas supply. Running costs are usually higher than gas, as electricity costs more per unit.
Is A Heat Pump A Type Of Boiler?
No, but it’s the main low-carbon alternative to one. It works best with a hot water cylinder and well-insulated rooms, and grants are available in some parts of the UK to help with the cost.
Which Type Of Boiler Do You Need?
It depends mainly on your bathrooms and mains pressure, and keeping your current type is usually the cheapest option.
Start with how you use hot water and what’s already fitted. Changing type is possible, but it adds work and cost.
Choose a combi if
- You have one bathroom, or one bathroom and an en suite that are rarely used together
- Your mains water pressure and flow are good
- You want to free up cupboard and loft space
Choose a system boiler if
- You have two or more bathrooms that are often used at the same time
- You have space for a cylinder
- You may add solar thermal or a heat pump later
Choose a conventional boiler if
- You already have loft tanks and a vented cylinder that are in good order
- Your mains pressure is low
- Your radiators and pipework are older and may not handle higher pressure
Can You Switch From One Type Of Boiler To Another?
Yes, and the most common change is from a conventional boiler to a combi.
The loft tanks and cylinder come out, the pipework is altered, and you gain an airing cupboard and loft space.
In larger homes, swapping a conventional boiler for a system boiler with an unvented cylinder is also common. It removes the tanks but keeps stored hot water.
Before converting, an engineer should test your mains pressure and flow, check your radiators and pipework, and look at how you use hot water. A conversion takes longer than a like-for-like swap and costs more.
How Much Does Each Type Cost To Install In 2026?
A like-for-like swap typically costs around £2,000 to £3,800, while a conventional to combi conversion runs nearer £3,000 to £5,000.
These are guide ranges for a new gas boiler fitted by a registered installer, including standard parts and labour. They aren’t quotes. Prices vary with brand, output, flue position and how much pipework needs changing.
| Job | Typical installed price guide |
| Combi replacing a combi | £2,000 to £3,500 |
| System boiler replacing a system boiler (cylinder kept) | £2,300 to £3,800 |
| Conventional boiler replacing a conventional boiler | £2,200 to £3,600 |
| Conventional to combi conversion | £3,000 to £5,000 |
| New unvented cylinder (if needed) | Add £1,200 to £2,500 |
For more detail by brand and model, read our guide to how much a new boiler costs.
Get A Free Boiler Quote From Clever Energy Boilers
Not sure which type you need? Our engineers check your hot water use, mains pressure and existing system before recommending a combi, system or conventional boiler. We install for homeowners and landlords across the North West, Yorkshire and the Midlands.
Every one of our new boiler installations is carried out by experienced, professional and registered installers, and we’re Worcester Bosch accredited.
Long, market-leading warranties are available through Clever Energy, and finance can help spread the cost.
Get a fixed price in minutes with our free online boiler quote.
Frequently Asked Questions
What Are The Three Main Types Of Boiler?
Combi, system and conventional (regular) boilers. Combis heat water on demand, while system and conventional boilers store hot water in a cylinder, and conventional boilers also use tanks in the loft.
Which Type Of Boiler Is Most Efficient?
They’re all similar, because every new gas boiler is condensing. A combi can waste slightly less energy as it doesn’t keep a cylinder of water hot, but correct sizing and good controls matter more than boiler type.
Is A Combi Boiler Better Than A System Boiler?
For smaller homes with one bathroom and good mains pressure, yes. A system boiler is better for homes with two or more bathrooms where hot water is often used in several places at once.
What Is The Difference Between A System Boiler And A Conventional Boiler?
A system boiler needs no loft tanks, while a conventional boiler does. Both heat water stored in a cylinder, but the system boiler has the pump and expansion vessel built in.
How Do I Know What Type Of Boiler I Have?
No hot water cylinder means you almost certainly have a combi. A cylinder with no loft tanks usually means a system boiler, and a cylinder plus loft tanks means a conventional boiler.