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How to Replace a Conventional Boiler With a Combi [Cost Guide 2023]

Looking to change a conventional boiler to a combi, but not sure if it’s the right move?

No problem! This guide will cover everything you need to know before making the switch, and even show you how to quickly compare quotes for your combi conversion.

Changing from conventional boiler to combi

Is Changing From a Conventional to a Combi Boiler Worth It?

Yes — in most cases, changing to a combi boiler is a worthy investment. That’s because combi boilers take up less space, heat water on demand, require less maintenance, and are generally more efficient.

However, there are circumstances in which your replacement boiler should be a newer conventional boiler model rather than a combi boiler. For example, if your household uses lots of hot water at once — which is true of many larger families — a combi boiler may not keep up with the demand. This means that during peak usage times, like morning and evening, the water pressure in your taps may be greatly reduced to a trickle.

So, if you have robust demand for hot water, you should consider a regular or system boiler instead of a new combi boiler.

If you want to see what your optimal replacement options are — whether conventional or combi boilers — we suggest you use Heatable’s calculator. You’ll have to answer a few quick questions about your existing boiler and property, and you’ll get fixed quotes for a range of gas boiler options on your screen immediately.

We recommend that you do this calculation three times — this will get you separate quotes for combi boilers, system boilers, and conventional boilers. This way, you’ll see how much each boiler type costs to install.

Heatable boiler installations

How Much Will Changing From a Conventional to a Combi Boiler Cost?

Typically, gas combi boiler installation costs for a small-medium property hover around £1,800-2,200. This figure includes a full combi boiler replacement, where you get a new boiler to replace an old one of the same type.

However, a combi boiler conversion (changing from a conventional boiler to combi) is a lot more work than simply switching a broken appliance with a new one. Here are a few extra costs you can anticipate in a combi boiler conversion:

Cost to Remove Hot Water Tank

If you’re changing from a conventional to a combi boiler, one of the biggest benefits you get is all the extra space freed up as you remove the water storage tanks. Of course, their removal isn’t free; typically, the average cost of this procedure is £100-£150.

Removing the Water Feeder Tank

And that’s not all — regular boilers also come with a water feeder tank, which be should be removed as well. This typically costs between £200 and £300.

If you’re changing from a back boiler to combi, you can expect to pay even more. That’s because back boilers are installed behind a fireplace and have a special decommissioning procedure that keeps them safe during removal. You can read more about back boiler replacement in our guide here.

There are a lot of other factors that will affect the installation price. But overall, assuming your combi boiler conversion is a simple one, you should expect to pay around £2,700-£3,000.

You can use a cost calculator and get a fixed price on-screen for your combi conversion, which will account for all of these factors.

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What Happens When You Change to a Combi Boiler

Combi boiler conversions vary when it comes to the workload. And naturally, the increased workload means an increase in price. Here are a few essential steps that a Gas Safe heating engineer will have to perform during a combination boiler conversion:

  • Moving a boiler to a new location
  • A new core hole needs to be drilled for a flue in a new location
  • The gas run needs to be upgraded to current standards
  • Heating controls need to be upgraded from the originals
  • The installation of a magnetic system filter (advised), and a limescale filter (advised for heating systems in hard water areas)
  • Adding TRVs to the central heating system
  • Type of boiler warranty (i.e. a low budget appliance with a 2-year warranty, over a leading brand with 5-10 years warranty).

Other things that increase the price of changing from a conventional boiler to a combi, rather than a straight combi swap, include:

  • Removing the old hot water cylinder
  • Removing the cold water feed tank (usually in the loft)

As you can see, it’s not a one size fits all approach.

Reasons For Changing to a Combi

The average household is suited to a combi boiler. Unless you have a high demand for heating and hot water (dozens of radiators, and several showers running at once), there’s a good chance a combi boiler is right for you.

Even if your family has a high demand for hot water and heating, there might be a storage combi boiler to suit.

To double-check, you can use this boiler size calculator, which will offer up the best solutions based on clickable multiple-choice questions it asks about your property.

So, what are the benefits of upgrading to a combi boiler, over your existing conventional (or system boiler) setup?


Unlike a conventional boiler, a combi heats hot water on demand, rather than having a constant supply of hot water in a cylinder; that makes them more efficient.

The Energy Saving Trust estimates that extremely old boilers in large properties could be adding £840 per year to energy bills compared with a new condensing boiler. And that doesn’t even factor in efficiency savings from your combi upgrade.

This efficiency upgrade is one reason many people need a new boiler in the first place (a boiler breakdown is the other).

Space Saving

If you’ve ever been house hunting, the likelihood is, you’ve come across more than one redundant airing cupboard in your time. Most likely, the pipework has been capped, the cylinder that it used to house has been removed, and the property’s heating system has been upgraded to a combi.

You’ll save tonnes of space when changing to a combi. Not only can you remove the cylinder from your cupboard and the cold feed tank from the loft, but there are also plenty of manufacturers offering compact boilers, which will sit in a standard sized kitchen cupboard.

For larger properties, that bonus might mean freeing up kitchen space and improving aesthetics by removing a wall-hung boiler. For flats and apartments, where space is prime, it’s extra storage that you wouldn’t have had otherwise.

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The Best Combi Boilers to Install

There are tonnes of incredible models and reputable manufacturers from which to choose.

Typically, you can use the criteria below to narrow down your options:

  • kW output – is the boiler powerful enough for your property?
  • Physical dimensions – will you need to fit it into a small space, such as a cupboard?
  • Warranty – does the included warranty show the manufacturer’s confidence in their product?

Regarding warranties — we’d not suggest sacrificing warranty to save £100 or so on a new boiler.

The installation could cost upwards of £2,700, but the boiler is only to make up around 40% of that cost.

If you want to go with products from brands that have confidence in their OWN products’ longevity, we’d suggest choosing one that has a warranty to match. Typically, that means going for a 5-year warranty as a minimum. However, there are boilers with 10 year warranties, so if your budget can handle it, it’s going to be money well spent.

If you want to see which boilers will suit your property best, you can fill in this quick, anonymous form from Heatable and get the model options — along with fixed installation quotes — on your screen in minutes.

Which Installer Should You Choose For Your Combi Conversion?

There are literally thousands of installers across the UK. Some are small and local, some are national. And from what’s available, you’ll see a huge differential in prices quoted for the same job.

Our suggestion? Start researching and get some prices.

For smaller local installers, you can jump over to this network, and request up to 3 quotes from installers that work specifically in your area.

National companies tend to be much more expensive.

Online Boiler Installers

However, we find Heatable is always worth a look. They use an automated online quote system, removing costs associated with other national firms (phone reps, on-site salespeople, and so on) and that means they tend to be on par with local installers in terms of prices, rather than national installation companies.

Worcester Bosch Greenstar 4000

You can check the prices here.

FAQs About Replacing a Regular Boiler With a Combi

Below, we will answer some of the most common questions about combi boiler conversions.

Can I change from a conventional boiler to a combi boiler?

Yes, you can. Just keep in mind that such a conversion would entail substantial changes to your central heating infrastructure, including:

  • Rerouting gas, water, and central heating pipework
  • Removing the hot water cylinder and water storage tanks
  • Installing the new boiler in a new location
  • Peripheral work required to support the tasks listed above

Needless to say, you’ll pay more for a conversion than a straight swap for the same boiler type.

However, getting a combi boiler can help you reclaim precious space at home and save on your energy bills if your demand for heating and hot water is modest.

Is it worth changing to a combi boiler?

It depends on your property size and hot water consumption. If you live in a small-to-medium-sized home and don’t use too much hot water at the same time, a combi boiler is a great option. It’ll help you cut on energy bills and save space, since combis don’t require water tanks or cylinders to operate.

However, if you have a busy household that frequently runs multiple taps or showers at once, a combi boiler may not be able to keep up with your hot water needs. Instead, you should consider getting a new regular or system boiler.

To summarise, if you’ve got a conventional boiler and you need a new one, your options are:

  • Replace conventional boiler with combi (cost will reflect added labour)
  • Replace with a system boiler (costs less, but more than a straight swap)
  • Straight-swap for a new conventional boiler

How many years does a combi boiler last?

You can expect modern combi boilers to last around 15 years, given that you service them annually and employ preventative measures to avert sludge and limescale build-up.

That said, even the best boiler brands have their budget models, and these typically have a short lifespan. Want a good indication of how long a particular model will last? Look at the warranty.

Boiler warranties essentially tell you how long the manufacturer expects their boiler to function without flaws. So, whilst a 10-year warranty instills confidence, its measly 2-year counterpart — and the poor boiler it protects — should be avoided like the plague.

What’s Next?

Hopefully, that’s everything you need to know before changing from a conventional boiler to a combi.

Got questions? Drop us a line through our contact form and we’ll get back to you ASAP.

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