Types of boilers in the UK: combi, system and regular compared

The three types at a glance

The difference between the three is mainly about how hot water is produced and stored. Heating works in a similar way in all of them: the boiler heats water that circulates through the radiators.

Combi boilers

A combi, short for combination boiler, heats the radiators and also heats mains water instantly when you open a hot tap. There is no cylinder and no tank, which frees a cupboard and the loft. It is the most common choice for flats and smaller houses.

The limit is flow. A combi can only heat so much water per minute, and it shares the incoming mains with every other tap in the home. One shower runs well. A shower plus a kitchen tap plus a second bathroom at the same time is where a combi struggles, especially if the mains flow is modest. There is also no stored hot water to fall back on if the boiler breaks down.

A combi is usually a good fit when there is one main bathroom, the household rarely uses hot water in two places at once, and the mains supply has enough flow and pressure.

System boilers

A system boiler heats the radiators and a separate hot water cylinder. Key parts such as the pump and expansion vessel are built into the boiler, and the heating circuit is sealed, so there are no loft tanks. The cylinder is usually unvented, fed directly from the mains, which gives good pressure at the taps.

Because hot water is stored, several outlets can run at once until the cylinder is used up. After that, it needs time to reheat. A system boiler suits family homes with two or more bathrooms, as long as there is space for a cylinder and the mains can support an unvented cylinder. Most cylinders also have an electric immersion heater as backup, and a cylinder can later be paired with a heat pump or solar thermal panels.

Regular boilers

A regular boiler, also called a conventional, open-vent or heat-only boiler, is the traditional set-up in many older London houses. It heats a vented hot water cylinder, fed by a cold water storage tank in the loft, with a smaller feed-and-expansion tank topping up the heating circuit. Pumps and controls are separate from the boiler.

Hot water pressure comes from the height of the tank above the taps, so upstairs showers can be weak without a pump. The upside is that a regular boiler works well where mains pressure is poor, and replacing like for like keeps disruption down in a house whose pipework was designed around it. Where an older system cannot take mains pressure, keeping a regular boiler can be the safer choice.

Condensing boilers are not a separate type

People often search for condensing boiler versus combi, but they are not alternatives. Condensing describes how the boiler recovers extra heat from its flue gases, which makes it more efficient. Combi, system and regular describe how hot water is supplied. A modern combi is a condensing combi.

Building regulations in England have required new gas boilers to be high-efficiency condensing models for many years, so any replacement gas boiler you are offered will be condensing. It will also have a condensate pipe that needs a suitable drain, which is worth knowing when planning where the boiler goes.

Other options: electric boilers and heat pumps

Electric boilers work much like gas combis or system boilers but heat water with an element. They need no gas supply or flue, which can suit some flats, but running costs depend on electricity prices and the property's supply capacity.

Heat pumps are a different kind of system rather than a type of boiler. They usually run with a hot water cylinder and work best with larger radiators or underfloor heating. The government's Boiler Upgrade Scheme offers a grant towards installing one in England and Wales. If you are replacing a gas boiler now but may move to a heat pump later, a system with a cylinder can make that change easier.

Choosing the right type for your home

Start with how you use hot water, not with the boiler. Count bathrooms and showers, think about the busiest time of day, and find out what your mains flow and pressure are. Then look at where a boiler, cylinder and tanks can go, and whether you want to keep loft space.

Changing type is a bigger job than replacing like for like. A conversion from regular to combi removes tanks and a cylinder, which saves space but may leave a busy household short of hot water. Our combi or system boiler guide goes through that decision in more detail.

• One bathroom, small household, good mains: a combi is usually simplest
• Two or more bathrooms, good mains: a system boiler with an unvented cylinder
• Low mains pressure or pipework that cannot take mains pressure: a regular boiler with a vented cylinder
• Considering a heat pump later: keep a cylinder in the plan

Get advice on your boiler

Loyal Plumbers installs, services and repairs combi, system and regular boilers across London and inside the M25. Gas work is carried out by Gas Safe registered engineers. Tell us what you have now, how many bathrooms you have and what problems you want to solve, and we will recommend the type that suits the home, not just the quickest fit.