If you’re looking to install or upgrade your home boiler, you generally have three options: a system boiler; a conventional boiler; and a combination boiler (usually called a combi boiler). We’re going to have a quick look at system and conventional boilers, but we’ll focus on the UK’s most popular type – the combi boiler. Finally, we’ll look at which is probably best for you.

  • Comparison with other boilers

First, we’ll take a brief look at conventional and system boilers, so we can show how combi boilers are different. All types have their pros and cons, so you need to decide which one to choose based on your expected usage patterns.

Conventional boiler

A conventional boiler (sometimes called a regular, traditional or open vent boiler) is, as the name suggests, the type of boiler that was standard in homes before the advent of the combi boiler. It consists of a cold water storage tank (usually in the loft), the main boiler and the hot water storage tank. Cold water is fed to the boiler, where it is heated up and sent to the central heating system and to the hot water tank until it’s needed.

Should you draw a bath, the hot water will need to be replaced, so you’ll have to wait for it to re-heat before you can have more hot water. However, if several rooms all demand a small amount of hot water at the same time (a shower or a basin, for example), it will cope because the water is already heated.

System boiler

Just like a conventional boiler, the system boiler feeds the central heating and the hot water storage tank, but there’s no need for a cold tank as the boiler is fed direct from the mains. They’re also at higher pressure than conventional boilers.

System boilers therefore work in the same way as conventional boilers: once the hot water in the tank is used up, you’ll need to wait for it to fill up with hot water before it can be used again.

How are combi boilers different?

That brings us to combi boilers. The most obvious difference between the combi and the boilers described above is that there is no need for a hot water storage tank.

A combi boiler feeds your central heating system in much the same way as the other two, but the way it delivers hot water is very different. Whenever you turn on a hot tap, the combi boiler detects the demand and instantly starts heating the water direct from the mains. Within a few seconds, you’ll have piping hot water flowing out. Because it’s under pressure already from the mains, the water flows naturally, and there’s often no need for a pump.

What are the advantages of combi boilers?

  • The main advantage of combi boilers is that you only boil the water you need. If you need to fill a sink, you only boil a litre or two. Unless you have a reasonably large family or you use a lot of hot water, it’s inefficient to have water boiled all the time just in case you need it. In that respect, combi boilers should cost less to run – and they’re greener, too.
  • Most people who have conventional or system boilers save money by having the heating on a timer. So for example they will set it to switch off at 11 p.m. and start heating at 6 a.m. But with a combi boiler there’s no need to do that as it’s available 24/7. If you get up early or come home late, you can still run a bath or have a shower. 
  • Another benefit of combi boilers is that they don’t take up much space. Most people have them in the kitchen, utility room or under the stairs, and the boiler you see is all there is – no cold or hot water tanks are required. That’s a huge plus if you live in a flat or don’t have a loft.
  • If you’re installing a boiler from scratch, the combi boiler will probably work out a bit cheaper as there’s less equipment and pipework to fit.

What are the limitations?

  • The biggest drawback of combi boilers is that they have relatively limited flow when it comes to boiling water. Generally speaking, either one hot tap or the central heating can be used at a given moment. When you turn on a tap, the central heating will momentarily switch off. That’s not usually noticeable, even if you run a bath for 10 minutes, as the radiators stay warm. But if you turn two hot taps on simultaneously, there will be a noticeable drop in pressure and/or temperature.
  • Combi boilers don’t tend to work well if you have low mains water pressure. 
  • If you have a large family and high hot water use (for example two bathrooms or showers), you could find that a large tank will serve you better, as you could have multiple hot taps and showers in use at once. If you heat it up smartly, it doesn’t necessarily have to cost more to run.
  • Many hot water tanks with conventional and system boilers can be fitted with an immersion heater, which means that if your boiler breaks down, you can still heat water. If the combi boiler breaks down, you’ll have to use the kettle.
  • Since most older homes were designed to accommodate a hot water tank and probably a cold water tank too, the space issue is less important for many people. Also, it’s quite useful to have a boiler in an airing cupboard as it helps keep clothes and towels dry. With a combi-boiler, this isn’t an option.

Choosing the right boiler

As a general guide, a combi boiler will probably be the best option if you’re in a relatively small household, and don’t have multiple demands on your hot water simultaneously.

As household sizes get larger, and the number of bathrooms grows, the case for a conventional or system boiler gets stronger.

If you need help deciding which is the best boiler for your household, use our new boiler selector tool, and of course, you’re always welcome to talk to our experts when it comes to picking a boiler that’ll fulfil all your needs.

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