Combi vs System vs Conventional Boiler
Which Type Do You Need?
Choosing the wrong boiler type can cost you thousands in unnecessary pipework and installation. Here is how to pick the right one in five minutes.
Updated April 2026
At-a-Glance Comparison
| Feature | Combi | System | Conventional |
|---|---|---|---|
| How it works | Heats water on demand from mains | Heats water, stores in hot water cylinder | Heats water, needs cylinder and cold water tank in loft |
| Install cost | £1,800 - £3,500 | £2,000 - £4,500 | £1,700 - £4,000 |
| Best for | 1-3 bed, 1 bathroom | 3-5 bed, 2+ bathrooms | Large homes, low mains pressure |
| Not ideal for | Homes with 2+ bathrooms | Small homes without cylinder space | Small homes, no loft space |
| Hot water delivery | Instant, unlimited but one tap at a time | Stored, multiple taps simultaneously | Stored, multiple taps, gravity fed |
| Needs cylinder? | No | Yes | Yes |
| Needs loft tank? | No | No | Yes |
| Space needed | Smallest footprint | Boiler + cylinder | Boiler + cylinder + loft tank |
| Typical lifespan | 10-15 years | 12-15 years | 15-20 years |
Combi Boilers
A combi (combination) boiler heats water directly from the mains when you turn on a tap. There is no hot water cylinder or cold water tank. It is the most popular boiler type in the UK, fitted in roughly 70% of homes.
Advantages
- Compact. Fits in a kitchen cupboard
- No cylinder or tank needed. Frees up space
- Cheapest to install of all three types
- Unlimited hot water (one tap at a time)
- Lower heat loss since water is not stored
Disadvantages
- Struggles with two or more taps running at once
- Water pressure depends on mains supply
- Not ideal for homes with 2+ bathrooms
- Cannot use an immersion heater as a backup
- Not compatible with solar thermal hot water
Installed cost: £1,800 to £3,500 depending on brand and output. Labour typically £800 to £1,500 for a like-for-like swap.
For a detailed breakdown of combi boiler pricing, sizing, and brand recommendations, see our dedicated guide at CombiBoilerCost.com.
System Boilers
A system boiler heats water and stores it in a separate hot water cylinder, usually in an airing cupboard. It does not need a cold water tank in the loft, making it simpler than a conventional boiler but more capable than a combi for larger households.
Advantages
- Delivers hot water to multiple taps at once
- Strong, consistent water pressure from mains
- Compatible with solar thermal panels
- No cold water tank needed in the loft
- Better suited to large homes than combi
Disadvantages
- Hot water can run out if the cylinder empties
- Requires space for a hot water cylinder
- More expensive to install than a combi
- Heat loss from the cylinder (even with insulation)
- Takes longer to heat a full cylinder vs instant combi
Installed cost: £2,000 to £4,500 including the cylinder. Labour typically £1,000 to £2,200 for a straightforward installation.
Conventional (Regular) Boilers
A conventional boiler (also called a regular or heat-only boiler) needs both a hot water cylinder and a cold water storage tank, usually in the loft. It is the oldest type of boiler system and is still found in many pre-1980s homes.
Advantages
- Works with low mains water pressure
- Can supply very high hot water demand
- Longest lifespan of any boiler type (15-20 years)
- Compatible with solar thermal and immersion backup
- Cheapest like-for-like replacement in older homes
Disadvantages
- Needs the most space (boiler + cylinder + loft tank)
- Lower water pressure than mains-fed systems
- Slowest to heat a full tank of water
- Higher heat losses from stored water
- Risk of frozen loft tank in winter
Installed cost: £1,700 to £4,000. Labour typically £800 to £2,000. A like-for-like replacement in an older home is the cheapest scenario.
Which Type Should You Choose?
Choose Combi If...
- Your home has 1-3 bedrooms
- You have only 1 bathroom
- You want the cheapest installation
- You have limited space (no airing cupboard)
- Your mains water pressure is good
- You live alone or with 1-2 other people
Choose System If...
- Your home has 3-5 bedrooms
- You have 2 or more bathrooms
- Multiple people shower in the morning
- You have space for a hot water cylinder
- You want solar thermal compatibility
- Strong mains pressure is available
Choose Conventional If...
- Your home already has a conventional system
- Your mains water pressure is low
- You have a very large home with high demand
- You have loft space for the tank
- A like-for-like replacement is cheapest
- You want the longest boiler lifespan
Conversion Costs
Switching between boiler types costs more than a like-for-like replacement because of the additional pipework, cylinder removal, or cylinder installation required.
| Conversion | Typical Cost | What is Involved |
|---|---|---|
| Conventional to combi | £2,500 - £4,000 | Remove cylinder and loft tank, reroute pipework, upgrade mains feed |
| Conventional to system | £2,200 - £3,500 | Remove loft tank, keep cylinder, upgrade controls |
| Combi to system | £3,000 - £5,000 | Install new cylinder, additional pipework, potentially larger boiler |
| System to combi | £2,000 - £3,500 | Remove cylinder, reroute hot water pipes, cap off cylinder connections |
Boiler Lifespan by Type
Combi
10-15 years
Higher component complexity means shorter lifespan. Annual servicing is essential.
System
12-15 years
Fewer moving parts than a combi. Cylinder may outlast the boiler itself.
Conventional
15-20 years
Simplest design with fewest failure points. Many last well beyond 15 years with annual servicing.
Signs Your Boiler Needs Replacing
- Your boiler is over 12-15 years old
- It breaks down more than once a year
- Repair parts are becoming hard to source
- Your energy bills are rising despite no usage change
- The boiler makes banging, whistling, or gurgling noises
- Radiators take a long time to heat up
- The boiler flame is yellow instead of blue
- There is a visible leak or corrosion around the unit
Frequently Asked Questions
Can I switch from a conventional to a combi?
Yes, but it costs more than a like-for-like replacement. Expect to pay £2,500 to £4,000 for the conversion, which includes removing the cylinder and loft tank, rerouting pipework, and upgrading the mains water supply. It is worth it if you want to free up space and your home has 1-3 bedrooms with 1 bathroom.
Which boiler type is cheapest to run?
All three types run on the same fuel (gas) at the same price per kWh. The difference in running cost comes from efficiency. New combi, system, and conventional boilers all achieve 90-94% efficiency. The main running cost difference is heat loss from the cylinder in system and conventional setups, which adds roughly £30 to £60 per year.
Does a system boiler need a hot water tank?
A system boiler needs a hot water cylinder (usually in an airing cupboard) but does not need a cold water tank in the loft. This makes it simpler than a conventional boiler while still handling multiple bathroom demands.
How do I know what boiler I currently have?
If you have no hot water cylinder and water heats instantly, you have a combi. If you have a cylinder but no tank in the loft, you have a system boiler. If you have both a cylinder and a tank in the loft, you have a conventional boiler.