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Combi or system boiler: which suits your home?
How the three main types of gas boiler work, and the things about your house that decide between them.
When a boiler needs replacing, the first question is usually whether to go for a combi. For a lot of homes a combi is the right choice. But not for every home. The answer depends on how many bathrooms you have, how many people want hot water at once, and what your mains water supply can deliver.
The three main types of boiler
Combi boiler
A combination boiler heats your radiators and heats hot water directly from the mains when you turn on a tap. There is no hot water cylinder and no tanks in the loft.
- Good for: flats and smaller houses, homes with one bathroom, and anyone who wants the airing cupboard back.
- Watch out for: a combi can only heat so much water at once. Run the shower and the kitchen tap together and both feel it. How well a combi performs also depends on the pressure and flow of your incoming mains water.
System boiler
A system boiler heats the radiators and a hot water cylinder, usually in an airing cupboard. The main heating parts, like the pump and expansion vessel, are built into the boiler, so there are no tanks in the loft.
- Good for: homes with two or more bathrooms, or where several people need hot water at the same time.
- Watch out for: you need room for a cylinder, and hot water has to be heated in advance. If the cylinder runs out, you wait for it to reheat.
Conventional (heat only) boiler
A conventional or regular boiler works with a hot water cylinder and usually a cold water tank and a small feed and expansion tank in the loft. It is common in older houses.
- Good for: homes that already have this setup and work well with it, and some properties with low mains water pressure, because the hot water is fed from the loft tank by gravity.
- Watch out for: it takes the most space, and tanks in the loft need insulating in winter. Gravity-fed showers can be weak without a pump.
The questions that decide it
How many bathrooms, and how many people?
This is the big one. A couple in a two-bed house will usually be very happy with a combi. A family of five with a bathroom and an en suite, where two showers often run at once in the morning, will often get a better result from a system boiler and a cylinder.
What is your mains water like?
A combi can only deliver what comes in through the mains. If the flow at the kitchen tap is poor, a combi shower will be poor too. This is worth measuring before deciding, not after the boiler has gone in.
Do you need the space?
Removing a cylinder and loft tanks frees up a cupboard and some loft space, which is one of the main reasons people convert. If space is not a concern, it is less of a deciding factor.
Do you want a backup for hot water?
A cylinder can have an electric immersion heater, so if the boiler breaks down you can still get hot water. With a combi, no boiler means no hot water until it is fixed.
Thinking about solar or a heat pump later?
Heat pumps and some solar setups work with a hot water cylinder. If that is on the horizon, ripping the cylinder out now may not be the best plan. It is worth mentioning when you get a quote.
What a conversion involves
Switching from a system or conventional boiler to a combi is more than swapping one box for another. The cylinder and loft tanks usually come out, pipework is rearranged, and the gas pipe may need upgrading because a combi needs more gas when the hot tap is running. It typically takes longer than a like-for-like swap. There is more detail in how long a boiler replacement takes.
Whichever type you choose, the system should be cleaned and a filter fitted before the new boiler goes on. Sludge from the old system is one of the main reasons new boilers fail early.
Getting the right boiler in Ashford and Folkestone
I fit combi, system and conventional boilers, including system to combi conversions, across Ashford, Folkestone and the surrounding area. You get a straight answer on which type suits the house, with the running cost difference explained, rather than whichever is easiest to fit. See boiler installation for how it works, or boiler servicing and repairs if you would rather keep the current boiler going for now.
Common questions
Is a combi boiler better than a system boiler?
Not always. A combi suits smaller homes with one bathroom. A system boiler with a cylinder usually suits homes where two or more people want hot water at once.
Will a combi work with low water pressure?
A combi relies on your mains supply for hot water, so poor mains flow means poor hot water flow. It is worth checking the flow before choosing.
Can I keep my hot water cylinder with a new boiler?
Yes. A system or conventional boiler works with a cylinder, and a cylinder can have an immersion heater as a backup if the boiler breaks down.