Combi boilers
A combination boiler heats water straight from the mains as you use it. There is no hot water cylinder and nothing in the loft, so it saves the most space, and the hot water does not run out.
The catch is flow. A combi can only heat water so fast, and the mains has to supply it. One shower at a time is usually fine; a shower and the kitchen tap at once, less so. Combis suit flats and smaller homes with one bathroom and a decent mains supply.
System boilers
A system boiler heats a hot water cylinder, usually an unvented one fed from the mains, so you get good pressure at several taps at the same time. There are no tanks in the loft, but the cylinder needs a cupboard.
They suit homes with two or more bathrooms, as long as the mains can keep the cylinder supplied. Unvented cylinders must be installed by someone qualified to fit them, and should be serviced regularly.
Regular boilers
A regular, or heat-only, boiler works with a cold water tank and a small feed and expansion tank in the loft, and a vented cylinder in the airing cupboard. The hot water is gravity-fed, so the pressure depends on how high the tank sits, and showers often need a pump.
They are the usual choice where mains pressure is low, where the existing system is old and open-vented, or where keeping the current layout means the least disruption.
Why a combi is not always the upgrade
Replacing a regular boiler and its tanks with a combi is a popular upgrade, and often a good one. It is not always right:
- A busy household with two bathrooms may find a combi cannot keep up in the morning
- Low mains flow means weak hot water at the tap, whatever the boiler
- Converting an old open-vented system to a sealed one puts it under more pressure. Tired radiators, valves and joints can start to leak, so they need checking first
- You lose the cylinder, and the immersion heater that gives you hot water when the boiler is out of action
- If a heat pump is likely in the next few years, you will need a cylinder again
What decides it
A survey that measures your water supply and looks at the existing system usually settles it. If a combi will not suit your home, we will tell you. The questions that matter:
- How many showers and baths run at the same time at the busiest point of the day
- What your mains flow and pressure actually are. We measure them rather than guess
- Whether you have room for a cylinder, or would rather have the cupboard back
- Whether you want the loft tanks gone
- Whether a heat pump or solar panels are on the cards later, since both work with a cylinder