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Combi vs Conventional Boilers: Pros and Cons + Costs

Nick Geary
By: Nick Geary
Updated: 4th December 2023

combi-boiler-vs-regular-boiler

Are you looking for a new boiler but can’t decide between a combi vs conventional boiler? What are the main differences between a regular boiler vs combi boiler? Comparing these boilers will help you find out which one of these is more suitable for your home.

Most homes turn to a combi boiler, but if you have two or more bathrooms, then your home could be better suited with a conventional boiler, also known as a regular boiler.

This is because regular boilers store hot water in a cylinder in order to meet the higher water demand. But if you don’t already have a regular boiler, it’s not recommended to install a new one. Instead, go for a system boiler - a modern and compatible popular boiler option for newer homes.

There’s plenty to think about, so let’s compare combi and conventional boilers.

If you’re seeking professional advice from Gas Safe registered engineers before you compare a combi and conventional boiler, then look no further. Our network of trusted engineers can help not only with advice but also with providing free quotes for your new boiler.

You can get 3 free boiler quotes from heating engineers in your area just by answering a few questions about your current heating situation.

From there, we will connect you with price quotes from local professional installers that are tailored to your home and specific needs. This way, you can comfortably compare your options and choose the one that gives you the best deal. To get started, click the link below.

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Difference between a combi boiler and a conventional boiler

combi boiler

A combi boiler delivers heating and hot water on demand. They pull their cold water directly from the mains.

One of their most significant benefits is that they don’t need to be fitted alongside any tanks or cylinders, so they don't take up much space. Everything they need to heat your home and hot water is within the boiler itself, making it an ideal solution for smaller homes.

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 work by heating and sending hot water to a tap on demand, they struggle to meet high demand.

What is a conventional boiler?

conventional 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. The cold water is delivered down to the boiler when it needs to be heated. Then the domestic hot water is stored in a cylinder where it stays 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.

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 with the existing pipework.

Don't forget about system boilers

system boiler
Comparing a conventional boiler vs combi doesn’t tell the whole story. System boilers are also worth mentioning as they serve as a modern and compatible option in the place of conventional boilers, which tend to be found more often in older homes.

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. However, system boilers do take up more space than combi boilers due to their water cylinder, which is something to account for.

If you want a full comparison between combi and system boilers, visit our complete combi boiler vs system boiler guide.

Already interested in a boiler system for your home? Look no further than Boiler Guide. By taking part in our 30-second survey, we can provide you with up to 3 free quotes from Gas Safe registered professional installers in your area. Save yourself hours of tiresome research, and compare quotes perfectly tailored to your specific needs. Click the button below to begin!

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Pros and cons of combi boilers and conventional boilers

Now that we know the difference between combi boilers and conventional boilers, let’s consider what they can each offer your home and what you need to keep in mind. In this section, we will have a look at the key pros and cons of combi boilers and conventional boilers.

Combi boiler pros and cons

combi boiler pros cons

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

  • You'll be saving space
  • Hot water will be sent to the taps as and when you need it
  • Installation is straightforward and relatively quick (2 days max)
  • A combi-for-combi boiler swap is the cheapest boiler replacement

As convenient 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:

  • A combi boiler can’t meet the hot water demands of homes with multiple bathrooms
  • Combi boilers are not suitable for homes with weak mains water pressure

Conventional boiler pros and cons

conventional boiler pros and cons

You should only consider a conventional boiler if you already have one installed. This is because the pipework won’t need to be replaced making the installation far more straightforward. Some benefits of installing a conventional boiler include:

  • Plenty of hot water capacity for homes with 2 or more bathrooms
  • The latest models can reach up to 98% fuel efficiency

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

  • Hot water is not in demand and requires re-filling when used up
  • Extra space for the boiler, cylinder and tanks in the loft will be needed
  • Heated water in the storage cylinder may cool slightly,  so you need to make sure that it’s well insulated
  • The installation can be a lengthy and costly process

Combi vs conventional boiler 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 pretty straightforward and won't take too long (at a stretch it can be completed in a day but 2 days is more realistic as the installer should perform a powerflush). Plus, you’ll be saving between £600 and £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 your cheapest option. This means replacing a cheap combi boiler with another combi, for example.

Boiler Replacement Estimated Cost
Replace an old combi boiler with a new one, in the same location £1,700
Replace a system boiler with a new system boiler £1,950
Replace a system boiler with a new combi boiler, including hot water cylinder removal £2,500
Replace a regular boiler with a new combi boiler, including hot water storage and cold water tank removal £3,500

Which is cheaper to run, combi or conventional boiler?

Generally, a combi boiler is cheaper to run than a conventional boiler. This is because it has a higher efficiency rating than conventional boilers and because combi boilers heat up water on demand. As a result, combi boiler users can expect to save up to £630 on their annual energy bills if they switch to an A-rated combi boiler. 

After seeing the combi and conventional boiler costs, it may seem a bit daunting. But there’s a strategy that you can use to find the best and most competitive deal. The strategy is that you need to compare quotes from multiple Gas Safe registered heating engineers, not just one. This not only gives you the freedom to choose the best deal, but it also eliminates any risk of being overcharged.

The trick is researching to find trusted and certified boiler engineers, which is not always easy and fast. With the endless sea of boiler and installer options out there, you can find yourself burnt out from weeks of research and vetting.

Luckily here at Boiler Guide, we have cultivated a  network of vetted professional boiler engineers that can cut your research time and deliver tailored quote prices shaped for your needs.

You can get free quotes from up to 3 different boiler engineers in our network just by answering a quick form about your heating situation. We’ll take it from there. And don’t worry, it’s all free and there’s no obligation to accept any quote.

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Best combi boilers

There are many top boiler manufacturers that have made some of the best combi boilers. Here are the manufacturers of the 5 best combi boilers:

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 based on warranty, efficiency and manufacturer. Within these ranges are a few different models, which explains the list of output ratings.

Best combi boilers CH Outputs (kW)* DHW outputs (kW) Efficiency Warranty
Baxi 800 21.2, 26.4 25, 30, 36 93% 10 years
Ideal Vogue Max 18, 26, 32 26, 32, 40 93% 10 years
Vaillant ecoTEC Plus 20.4, 25.7, 30.1, 32.2 25, 32, 35, 38 94% 10 years
Viessmann Vitodens 200-W 25.4, 32 30, 35 94% Up to 10 years
Worcester Bosch Greenstar i 24 25, 30 93% Up to 10 years

*CH = Central Heating; DHW = Domestic Hot Water.

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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. Some of the ranges are different though.

Unlike combi boilers, conventional boilers don’t have a domestic hot water (DHW) output. This is because they aren’t heating hot water on demand when you turn the tap on. So you only have to think about the central heating (CH) output.

Best conventional boilers CH outputs (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

*CH = Central Heating; DHW = Domestic Hot Water.

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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 cylinders from your conventional boiler system. Plus, the combi boiler itself 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. Even better, you’ll no longer have to wait for the cylinder to fill with hot water before you can have a bath.

Installing a combi in the place of a conventional boiler takes a lot of work. In total, this can take as long as 4 days. So, your best bet is to book the boiler replacement during the summer to avoid being without heating during the winter.

Which boiler is right for you?

When it comes to a combi boiler versus conventional, the winner will depend on the size of your property and the 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.

Suitable combi boiler output rating

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

Then there’s the system boiler. A system boiler gives you the best of both worlds: hot water storage in a cylinder and no need for tanks in a loft.

Installing the right boiler for your home is essential. Otherwise, your heating bills will rise unnecessarily or your boiler could fail to meet your 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 conventional boiler output rating

Number of radiators Recommended regular size
Up to 10 9-18 kW
10-15 18-26 kW
15-20 27-40 kW

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

Get free boiler installation quotes

Have you been able to crown a winner in the battle between a combi boiler vs conventional? Then now is the time to get quotes. Even if you’re still on the fence about which boiler is best, then a Gas Safe registered engineer can help advise you to make sure you make the right decision. Either way, contacting a heating engineer is the next step to getting a boiler.

Getting in contact with multiple engineers can help give you advice on which boilers are better for your home, but also lets you compare boiler prices. Nevertheless, the process of sifting through endless lists of engineers and installers is not an easy task. To individually ring up each installer and manually compare prices can leave you both exhausted and still unaware of the best deals that may be out there for you.

We make getting in contact with multiple Gas Safe heating engineers near you easy and quick! You can get free quotes from heating engineers in your area just by answering a few questions about your current heating situation. Then we’ll take it from there and you’ll just need to wait for up to 3 different heating companies to be in touch to arrange the quotes.

Once you have the quotes, feel free to compare them. Comparing quotes not only eliminates the risk of overpaying, but it will put you in the best position when it comes to finding the most qualified boiler installer at the best price. What are you waiting for? Simply click the button below to get started!

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Frequently Asked Questions

Which is better, combi or condensing boiler?
By law, all combi boilers need to be a condensing boiler. This ensures that the next time you're looking for a combi boiler, its efficiency will be above 90%.
Are combi boilers better than conventional boilers?
Combi boilers can be better for your home if it has 1-2 bathrooms. If your home has 2 or more bathrooms, then a conventional boiler is better for your home.
What’s the difference between a combi and conventional boiler?
The main difference is that a combi boiler has all of its parts within one unit, while a conventional requires the boiler unit, a hot water cylinder, and a feed tank. This difference also has an impact on which homes are better suited for each boiler type.
Which is cheaper, combi or conventional boiler?
When it comes to combi and conventional boiler costs, a combi boiler is cheaper. But depending on what installation work you need done, it may still be cheaper to buy a replacement conventional boiler.

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Nick Geary

About the author

Nick Geary

Nick has a wide range of experience writing about conventional and renewable heating solutions, and is always increasing his knowledge by researching any new heating technologies on the horizon.

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