Combi vs System Boiler: Which Is Right for Your Home?

The short answer

If one person showers at a time and you have a single bathroom, a combi is usually the neater choice. If two or more people regularly want hot water at the same moment, a system boiler with a cylinder usually copes better. Almost everything else in this guide is detail behind that one question: how much hot water does your home need at once?

I have been in the trade for 25 years and have run C Gill Plumbing & Heating for the last 12, and this is one of the questions I am asked most. There is no type that is better in every house, only one that fits yours.

How a combi boiler works

A combi heats water as it passes through, straight from the cold mains. Turn on a hot tap and the boiler fires up; turn it off and it goes back to heating the radiators. There is no cylinder and no tank in the loft.

The strengths are space and simplicity. You gain the airing cupboard, you never run out of hot water because nothing is stored, and there are fewer parts to look after. The limit is flow. A combi can only heat so much water per minute, and that is shared between every open tap. Run the kitchen tap while someone is in the shower and both feel it. It also depends on decent mains pressure and flow coming into the house.

How a system boiler works

A system boiler heats the radiators and heats a cylinder of stored water, usually in an airing cupboard. The pump and expansion vessel are built into the boiler, so there is normally no need for a cold tank in the loft. When you open a tap, the hot water comes from the cylinder.

The strength is that several taps and showers can run together without the flow collapsing, which suits family homes with more than one bathroom. The limits are space for the cylinder and the fact that stored hot water runs out if the cylinder is too small for the household. Once it is used, it takes time to reheat, so sizing the cylinder properly matters.

Switching from one to the other

Going from a system boiler to a combi means removing the cylinder, capping or removing the pipework that fed it, and connecting your hot water directly to the new boiler. Before recommending it, I check your mains pressure and flow, because a combi with poor incoming water will give you weak showers. You also need to be comfortable with one strong outlet at a time.

Going the other way, from a combi to a system boiler, needs space for a cylinder and new pipework to and from it. It is less common but can make sense when a household grows or a second bathroom goes in. Either change is more work than a like-for-like swap, so it shows up in the price. Our guide to new boiler costs explains how that and other choices affect the quote.

Questions to ask yourself

Before we visit, it helps to think about these points. Your answers usually make the choice fairly clear:

• How many bathrooms and showers do you have?
• Do two people often need hot water at the same time?
• Is the cold tap in your kitchen strong, or does it splutter?
• Would you use the airing cupboard space for something else?
• Do you fill a large bath regularly?

Get advice from a Gas Safe installer

I would always rather look at a house than guess. On a free, no-obligation survey I check the water pressure, the bathrooms, the space and the existing pipework, then explain which option suits you and why. If you would like to read more first, see our pages on combi boiler installation, system boiler installation and boiler installation in general.

We are Gas Safe registered, family-run and based in Briton Ferry, covering roughly 30 miles around SA11 2JG. Ring me on 07429 632804, send a WhatsApp or use the form on our contact page.

Side-by-side comparison