Combi vs System vs Conventional Boiler: Which Suits Your Home?

When a boiler needs replacing, the first question is often whether to stick with the same type or change. There are three main kinds of gas boiler in British homes, and each suits a different kind of home. This guide explains how they work, who they suit and what is involved in switching from one to another.

The three types at a glance

Combi boilers

A combination boiler heats water directly from the mains the moment you open a hot tap, and heats the radiators the rest of the time. There is no cylinder and no tank in the loft.

Suits: flats and smaller houses with one bathroom, homes short on cupboard space, and households that rarely run two hot taps at once.

Watch out for: a combi can only heat so much water at a time. Run a shower and a kitchen tap together and both slow down. It also depends on your mains supply: weak flow means weak hot water, so the flow rate is worth measuring before choosing one.

System boilers

A system boiler heats the radiators directly and heats a hot water cylinder, usually an unvented cylinder that runs at mains pressure. The pump and expansion vessel are built into the boiler, so there are no tanks in the loft.

Suits: homes with two or more bathrooms, families where showers run at the same time, and larger houses where a combi would struggle to keep up.

Watch out for: you need space for the cylinder, and stored hot water can run out after heavy use until the cylinder reheats. An unvented cylinder needs its own annual service.

Conventional boilers

A conventional (or regular, heat-only) boiler works with a hot water cylinder and two tanks in the loft: a cold water tank that feeds the cylinder and a small expansion tank for the heating. Many older homes in South London were built with this layout.

Suits: homes that already have the tanks and pipework, properties with low mains pressure, and older systems where changing the layout would mean a lot of new pipework.

Watch out for: hot water pressure depends on how high the loft tank sits, so upstairs showers can be weak without a pump. The tanks take up loft space and need checking.

Changing from one type to another

A like-for-like swap is the simplest job: the new boiler goes where the old one was, using the same pipework and flue route where it suits. Changing type is a bigger job, and worth thinking about carefully.

• Conventional to combi: the loft tanks and cylinder come out, freeing space, but the pipework changes and the old system has to cope with mains pressure. Older radiators and joints are checked first.
• Conventional to system: the loft tanks go and an unvented cylinder replaces the old one, giving mains-pressure hot water while keeping stored hot water for busy mornings.
• Combi to system: worth considering when a household outgrows a combi, but it needs space for a cylinder and new pipework to reach it.

Whatever the type, a new boiler in England has to meet current building regulations for efficiency and controls, and the installation is registered with Gas Safe and the local authority through the installer.

Questions to answer before choosing

1. How many bathrooms are used at the same time? One points towards a combi; two or more towards a system boiler.
2. What is your mains pressure and flow? This decides whether a combi will give you a good shower.
3. Do you have space for a cylinder, and do you want the loft back? Space often settles it.
4. Where can the flue go? The boiler's position depends on where the flue can safely exit.
5. How old is the rest of the system? Sludged radiators and old pipework can affect a new boiler, which is why many installations include a system clean.

Getting advice in South London

InstaGasWorks installs combi and system boilers across South West London, Croydon and Sutton, starting with a free survey that checks your water pressure, space, flue position and existing system before you get a written quote. The boiler installation page has the current prices and what each installation includes.

If your current boiler is still working but you are weighing up when to replace it, a boiler service is a good point to ask for an honest view of its condition.