Combi or system boiler: which is right for your home?

When a boiler needs replacing, the first question is often whether to stay with the type you have or switch. The two most common choices are a combi boiler and a system boiler with a hot water cylinder. Neither is better in every home: it comes down to how you use hot water.

How a combi boiler works

A combination (combi) boiler heats your radiators and also heats water instantly, straight from the mains, whenever you turn on a hot tap. There's no hot water cylinder and no tanks in the loft.

• Good for: flats and smaller homes with one bathroom, where people rarely need hot water from two outlets at once.
• Pros: saves space, hot water on demand, no cylinder to heat or maintain.
• Watch out for: the flow is shared. Run two showers, or a shower and a kitchen tap, and both will suffer. Performance also depends on your mains pressure and flow.

How a system boiler works

A system boiler heats your radiators and heats a store of water in a cylinder, usually in an airing cupboard. Many modern installations use an unvented cylinder, which delivers that stored hot water at mains pressure to every tap and shower.

• Good for: homes with two or more bathrooms, or households where several people shower around the same time.
• Pros: strong flow to several outlets at once, and a cylinder keeps the door open to a heat pump or solar later.
• Watch out for: you need space for the cylinder, the stored water has to be heated in advance, and an unvented cylinder must be installed and serviced by a G3 qualified engineer.

Which should you choose?

• One bathroom, small household: a correctly sized combi is usually the simplest choice.
• Two or more bathrooms, or a busy household: a system boiler with an unvented cylinder normally works better.
• Adding an en-suite: check your boiler before planning the room. A combi that copes with one shower may struggle with two.
• Low mains pressure: neither will perform well. I'll test the incoming flow and pressure before recommending anything.

What I'll check before quoting

1. How many bathrooms and showers you have, and how many people use them.
2. Your mains water pressure and flow rate.
3. Where a cylinder could go, if you need one.
4. The condition of your existing pipework and radiators, and whether the system needs cleaning first.

I'm Gas Safe registered (635712) and G3 qualified, so I can install combi, system and unvented cylinder set-ups. See my boiler installation page or ask for a free quote.