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Combi vs Conventional Boilers: Pros and Cons

Comparing a combi vs conventional boiler will no doubt help you find the best heating system for your home. These are very different heating systems so it's highly likely that one will be far more suitable than the other.

What is a combi boiler?

Combi Gas Boiler
A combi boiler delivers heating and hot water on demand. They're compact heating systems which don't need to be fitted alongside any tanks or cylinders. This is because everything they need to provide heating and hot water is within the boiler itself. Making them a space-saving heating system.

While the space-saving qualities of a combi can't be understated, they do have their limitations. To meet high demands for domestic hot water, conventional and system boilers are installed alongside a hot water cylinder. As combi boilers heat and send hot water to a top on demand, they struggle to meet high demand.

What is a conventional boiler?

Regular Gas Boiler
The conventional boiler goes by many names – regular, heat-only and open-vent, to name a few – and is the oldest boiler type. As well as the boiler, a conventional heating system also includes tanks in the loft and a hot water cylinder.

A tank in the loft is full of cold water, which is delivered down to the boiler or cylinder as and when it needs to be heated. Then the cylinder is where the domestic hot water is heated and stored until a hot water tap is turned on.

As you can tell, plenty of space is needed for a conventional boiler and all of its external parts. So you'll need the room available in your property. Plus, all of these parts are connected by a unique and complex network of pipes. For this reason, a conventional boiler is often only recommended for properties considering a direct conventional-to-conventional boiler replacement.

What is a system boiler?

Blaze with hot water cylinder
Comparing a combi vs conventional boiler doesn't tell the whole story as a system boiler could also be worth considering.

System boilers offer the best of both worlds in the sense that they're directly connected to the mains water supply and store water in a cylinder.

This allows them to meet high demands for hot water without the need for a tank in the loft. Plus, you'll get stronger water pressure than a conventional boiler.

Find out if a system boiler is best suited to your home in System Boiler vs Combi Boiler.

Combi vs conventional boiler

Now that we know the difference between combi and conventional boilers, let's consider what they can each offer your home. As well as what you need to keep in mind.

Combi boiler pros and cons

For homes with a single bathroom (possibly two depending on the output rating) a combi boiler is the best option for these reasons:

  • Compact heating system
  • Hot water delivered to taps on demand
  • A combi-for-combi boiler swap is the cheapest form of boiler replacement
  • Installation is straightforward and relatively quick

As great as combi boilers are, they're only suitable for small to medium sized homes. Larger properties with multiple bathrooms should instead turn to a conventional or system boiler. And here's why:

  • Can't meet the hot water demands of homes with multiple bathrooms
  • Not suitable for homes with weak mains water pressure

Conventional boiler pros and cons

A conventional boiler is the best option for properties which already have one installed. This is because the pipework won't need to be replaced making the installation far more straightforward. Benefits of installing a conventional boiler include:

  • High levels of efficiency
  • The ability to meet high demand for hot water in properties with multiple bathrooms

As well as the potential pros, there are some considerations to make before installing a conventional boiler:

  • No instant access to hot water
  • Space for the boiler, cylinder and tanks in the loft will be needed
  • Water in the cylinder can lose heat when in the cylinder so you need to make sure that it's well insulated
  • Installation can be lengthy and costly

Combi vs conventional boiler costs

Blink Boiler Replacement Costs
When it comes to costs, the combi boiler is the more affordable option. A combi boiler will cost somewhere between £500 and £2,000 while conventional boiler prices can stretch to £2,750. System boiler prices tend to go up to £2,500.

Installation of a combi boiler is relatively straightforward and you'll be saving between £600 to £1,000 as you won't need a hot water cylinder.

As well as the boiler itself, you'll also need to consider the installation costs. A like-for-like boiler replacement is likely to be your cheapest option. This means replacing a combi boiler with another combi, for example.

Boiler type Average cost of boiler excluding installation Average labour cost of boiler installation Approximate total cost of boiler fully installed
Combi £500 - £2,000 £500 - £1,000 £1,000 - £3,000
Conventional £500 - £2,750 £500 - £1,000 £1,000 - £3,750
System £500 - £2,500 £500 - £1,000 £1,000 - £3,500


Best combi boilers

The best combi boilers are manufactured by Baxi, Ideal, Vaillant, Viessmann and Worcester Bosch. All of these boiler manufacturers offer a range of combi boilers to choose between but the table below shows which ones are arguably the best.

Best Combi Boilers Domestic Hot Water Output Ratings (kW) Efficiency Warranty
Baxi 800 25, 30, 36 93% 10 years
Ideal Vogue Max 26, 32, 40 93% 10 years
Vaillant ecoTEC Plus 25, 32, 35, 38 94% 10 years
Viessmann Vitodens 200-W 30, 35 94% Up to 10 years
Worcester Bosch Greenstar i 25, 30 93% Up to 10 years

Compare the Best Combi Boilers.

Best conventional boilers

The best conventional boilers are manufactured by the same brands as the best combi boilers: Baxi, Ideal, Vaillant, Viessmann and Worcester Bosch.

Best Conventional Boilers Central Heating Output Ratings (kW) Efficiency Warranty
Baxi 800 16, 25, 30 93% 10 years
Ideal Logic Plus Heat 12, 15, 18, 24.2, 30.3 93% 7 years
Vaillant ecoTEC Plus 400 12.3, 15.1, 18.3, 24.6, 30.5, 35.1 94% 10 years
Viessmann Vitodens 100-W Compact 13, 16, 19, 26, 35 94% Up to 10 years
Worcester Bosch Greenstar 8000 Life 30, 35, 40, 45, 50 94% Up to 12 years


Why replace a conventional with a combi boiler?

A combi boiler is a single cost-effective heating system so you'll be able to free up space by removing the tanks and cylinder. Plus, the combi boiler itself is a compact unit that can be tucked away in a kitchen cupboard.

Replacing a conventional boiler with a combi is a great way to modernise your heating system. You'll free up space around your home and get hot water on demand.
One of the best reasons for installing a combi is that you'll no longer have to wait for the cylinder to fill with hot water before you can have a bath.

There are many benefits to replacing a conventional boiler with a combi, however, the installation requires a lot of work. In total, this can take as long as 3 days. So, you'll want to plan the boiler replacement in advance and avoid winter.

Which boiler is right for you?

When it comes to a combi vs conventional boiler, the winner will depend on the size of your property and demand for domestic hot water.

"A combi boiler is the best option for homes with 1-2 bathrooms. Meanwhile, conventional boilers can meet higher demands for hot water in properties with over 2 bathrooms thanks to the cylinder."

Then there's the system boiler. A system boiler gives you the best of both worlds: hot water storage in a cylinder, mains water pressure and no need for tanks in a loft.
Boiler Size
Installing the right boiler for your home is essential. Otherwise, your heating bills will rise unnecessarily or it could fail to meet demand. To avoid this, you'll also need to get the output rating spot on too.

Boiler output rating, measured in kilowatts, shows the power of the boiler. Combi boilers have a central heating (CH) output and a domestic hot water (DHW) output while conventional and system boilers only have a CH output rating.

Suitable combi boiler output rating

Number of Radiators Recommended Combi Boiler Size (CH output)
Up to 10 24-27 kW
10-15 28-34 kW
15-20 35-42 kW

Suitable conventional boiler output rating

Number of Radiators Recommended Regular Boiler Size
Up to 10 9-18 kW
10-15 18-26 kW
15-20 27-40 kW

Find out more in What Size Boiler Do I Need?.

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Blaze: Best Gas Boilers
Have you been able to crown a winner in the battle between a combi vs conventional boiler? Then now is the time to get quotes. Even if you haven't been able to decide then a Gas Safe registered engineer is best suited to your home.

By completing our simple online form, you can get free quotes from heating engineers in your area. Up to 3 different heating companies will be in touch with you as soon as possible to arrange the quotes.

Once you have the quotes, you'll be in a position to compare them. Comparing quotes will put you in the best position when it comes to finding the most qualified boiler installer at the best price.

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Boiler Guide Team

About the author

Boiler Guide Team

Boiler Guide has been home to a host of heating experts who have helped millions of UK homeowners searching for advice and guidance. If you’ve got heating questions, we’ll have the answers.

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