What is a Combi Boiler And How Do They Work?

What is a Combi boiler?

Stuck for time? Bookmark this detailed guide covering what combi boilers are and how they work for later, and read this summary instead. 

And if you’re simply looking for a quote on a brand new combi boiler, jump over to our 2-minute boiler quote tool here. 

So, what is a combi boiler and how does it work?

They heat hot water on demand, meaning there’s no need for cold water storage tanks or hot water cylinders, unlike conventional and system boilers. 

This means they’re suited to average properties in the UK, such as a 3-bed semi detached with 1 shower or bathtub. They’re not suited to larger properties with 3+ showers or bathtubs, or those with low mains pressure.

They work by using a burner to warm up the heat exchanger, which transfers heat to water that’s filtered around your property. Combi boilers also work by prioritising hot water over heating; if you feel radiators cooling whilst running taps, this is why.

Typically, a new combi boiler will be much more efficient than your current unit (cash savings on energy bills), compact (they use very little space) and are the cheapest type of boiler to install.

Size wise, the most popular combi boiler sizes are 25kw, 30kw and 35kw. 

25kw-30kw combi boilers are typically fitted in 2-3 bedroomed properties with 1 shower. Depending on the exact layout of your heating system, a 35kw can power a reasonably sized central heating system, as well as up to 2 showers (this needs to be checked with your boiler engineer or using our boiler tool here).

In Summary: What is a Combi Boiler?

A combi boiler heats hot water on demand, rather than using cold water storage tanks or a hot water cylinder. They’re becoming increasingly common as people upgrade from old system and regular boilers to a more efficient combi boiler setup. 

However, as a combination boiler has no storage function, they are generally designed for average sized properties (2-3 bedrooms) rather than large ones. Typically, larger properties will be better suited to system boilers which use a hot water cylinder, or a regular boiler.

How Does a Combi Boiler Work?

Now you know what this type of boiler is, how exactly does a combi boiler work? 

There are dozens of crucial components within a combi boiler, the burner and heat exchanger being two of the most important. These are incredibly expensive parts to replace, which is why in our other articles we always recommend buying a boiler with a 10-year or longer warranty. 

Whenever a hot tap, shower or your central heating is turned on, your boiler will fire up the burner, with the heat being absorbed by the heat exchanger. Using a series of pipes within the boiler and around your property, hot water is then delivered to taps, showers and radiators.

An important function of a combi boiler is it will always prioritise hot water over heating. 

The component in your boiler that determines where to send hot water / heat, is the diverter valve; it flicks between hot water being sent to taps and diverting water to radiators. This is one of the reasons you might find that radiators start to cool down if you have a long shower or bath. 

There are more details on diverter valves and specifically, diagnosing a faulty boiler diverter valve here. 

How Combis Differ To System & Regular Boilers

Although combi boilers are the most popular type of boiler in the UK, there are two other main types: 

• Conventional boilers
• System boilers

Put simply, conventional boilers use a cold water storage tank to feed the system rather than mains water. This is then heated and stored in a hot water cylinder. A system boiler works in a similar way, but doesn’t use a cold water storage tank. 
Generally speaking, both of these types of boiler are used in bigger properties where heating water on demand simply wouldn’t produce enough hot water for the amount of taps, radiators and showers.

Top Benefits of a Combi Boiler

Save Money With Efficiency

With energy bills sky high, any savings are welcomed with open arms. And according to the Energy Saving Trust, changing from an old G-rated boiler to a new A-rated combi boiler could save up to £310 per year in gas and electricity bills. This assumes a semi detached house with a new thermostat, programmer and thermostatic rad valves. 

Older boilers that are D-G rated are usually operating at 70-80% efficiency. On the flip side, a new A-rated combi boiler from a leading brand such as Alpha, Worcester Bosch, Ideal, Vaillant or Viessmann would usually operate at 90%+ efficiency. 

Use Less Space

As we mentioned above, system and conventional boilers require the use of water storage tanks and cylinders. These take up huge amounts of space in lofts and airing cupboards. 

A combi boiler works without the need for a cold water storage tank (it takes water from the mains) or a hot water cylinder (a combi heats water on demand). Ultimately, this means installing a heating system using a combi boiler rather than a conventional / system boiler, is going to save a lot of space!

Cheap To Install

Combi boilers are also the easiest units to install. Extra pipework as well as physically buying storage tanks / cylinders for system and conventional boilers, means a full heating setup can cost thousands more when compared directly with a combi. 

If you are able to meet hot water and heating demand using a combi heating system, this is going to be the most cost effective system to install. You can see the exact cost of installing a combi boiler using our clickable boiler tool here.

When is a Combi Boiler Not Suitable?

Low Mains Water Pressure

Combi boilers work by taking water from the mains. But for this to happen, you’ll need to ensure that the current mains pressure is reasonable at the very least. A Gas Safe engineer will be able to check that your mains water pressure is sufficient to supply a combi boiler.

Large Properties With Multiple Showers

Almost any property with less than 15 radiators and just one bathtub / shower will be able to use a combi boiler setup. Combis come in a range of different sizes, and a Gas Safe boiler engineer will be able to spec a boiler that’s most suited to your property. 

However, if you have an exceptionally long heating run (pipework), 15+ radiators or multiple baths and separate showers, it’s like your property would benefit from a conventional or system central heating setup.

What Size Combi Boiler Do You Need?

The size combi you need isn’t a one-size-fits all approach. For example, one 3-bedroom semi detached property might have 1 shower and 10 radiators, whereas another may have 15-20 radiators and 2-3 showers. 

Generally, larger combi boilers are able to run up to 2 showers, but they’d need to be in the 35kw-40kw range. Whereas a 3-bedroom property using just 1 shower would be able to use a smaller 30kw unit.

Our clickable boiler quotation tool here takes out all the guesswork. Tell us a few details about your property and you’ll have a fixed price on screen within 2-minutes for the most suitable boiler for your property.

Combi Boiler Installation Process & Prices

Ready to get your combi boiler installed? Typically an installation will take 1-2 days and includes: 

• Removal of your old boiler and any outdated pipework
• Upgrading pipework to comply with current Gas Safe standards
• Installation of your new boiler, plus any smart controls you’ve chosen to add-on
• Chemical flush to make your existing pipework and radiators operate more efficiently. 

Prices vary a great deal depending on the size of your property and complexity of an installation. But for those with an average 3-bedroom semi detached house with average hot water and heating demand, expect installation costs to sit in the £2,500-£3,000 range. 

A more detailed guide to combi boiler installation costs can be found here, or jump straight over to our boiler tool and get a fixed price on screen within 2-minutes.