There are three main types of gas boiler in UK homes. Which one suits you depends on how many bathrooms you have, how much hot water you use at once, and what is already in the house. Here is how they differ.
Combi boiler
A combi (short for combination) heats water directly from the mains when you turn on a tap. There is no hot water cylinder and no cold water tank in the loft.
Good for: small to medium homes with one or two bathrooms, and anyone short of space. They are compact and quick to install.
Watch out for: two showers or baths running at once will share the flow, so the output drops. Combis also need decent mains pressure and flow to work well.
System boiler
A system boiler heats a hot water cylinder, which stores the water ready to use. It does not need a cold water tank in the loft, because the major parts of the heating system are built in.
Good for: larger homes with two or more bathrooms, where several people use hot water at the same time. It works well with unvented cylinders at mains pressure.
Watch out for: you need space for the cylinder, and once the stored hot water is used up you wait for it to reheat.
Regular (conventional) boiler
A regular boiler, also called a heat-only boiler, works with a cold water tank in the loft and a hot water cylinder. It is the traditional arrangement and still common in older houses.
Good for: homes with an existing tank-fed system, older properties where mains pressure is weak, and big households that use a lot of hot water.
Watch out for: it takes the most space, and the loft tank can freeze if it is poorly insulated.
Questions that decide it
- How many bathrooms do you have, and how often are they used together? One bathroom usually points to a combi. Several points to a system or regular boiler with a cylinder.
- How strong is your mains water? Combis and unvented cylinders both rely on it.
- Where would it go? Loft tank and airing cupboard space may be available, or you may want to free it up.
- What is there now? Changing from a regular boiler to a combi is possible, but it can mean altering the pipework, so we quote that properly after a survey.
Size matters as much as type
A boiler that is too big wastes fuel, and one that is too small leaves you cold. The right output depends on your heat loss, hot water demand and the type of system. Be wary of anyone who names a kilowatt rating before they have seen the house.
Rules that apply to new installations
In England, a new gas combi boiler must be installed with time and temperature controls and at least one extra energy-saving measure, such as weather compensation or a smart control with automation. This is often called Boiler Plus. Scotland and Wales have their own rules, which differ.
If you are weighing the options, our guide to combi boiler pros and cons goes into more detail. We install and replace gas and oil boilers, and we will recommend the type that fits how you live. See our boiler installation page, or ask for a free quote.
