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2023 Combi Boiler Installation Guide – Size, Fitting Cost & Best Boilers

Are you looking to upgrade your old, unreliable, or inefficient boiler with a new one but don’t know where to start? This guide to combi boiler installation is going to help. We’ll cover all the information you need to make a sound buying decision, and show you how to compare combi boiler installation costs right here on your screen

What Is A Combi Boiler?

Short for ‘combination boiler’, combis supply hot water for both your taps and radiators. They provide hot water on demand rather than heating water and storing it. It’s worth noting that combis prioritise domestic water over heating, so if yours is struggling to keep up with both functions (for instance, if you’re running two showers while central heating is also on), it will focus on the showers.

Combi boilers’ primary advantage is that they save tonnes of space. Heating water on demand removes the need for large water storage tanks and hot water cylinders – a massive benefit for those of us living in smaller homes.

Typically, combi boiler installation is cheaper than the replacement of system or regular boilers. And that’s because combis don’t use storage tanks, which means there’s less infrastructure and fewer moving parts to replace.

If you don’t have water storage tanks or a hot water cylinder on your property, you probably already have a combi boiler. In this case, all you need is a straight boiler swap. Lucky you — at least from a bank balance perspective.

If you have an existing system or regular boiler, you CAN upgrade to a combi boiler and rid your property of all those storage tanks and cylinders. That said, beware that removing all this infrastructure will add to the boiler installation costs.

Combi Boiler Sizing

Before getting a combi boiler installed, you’ll need to determine what size boiler you need. To clarify — when we say size, we mean power output in kilowatts, not physical dimensions.

And this is where things can get slightly confusing. You can’t base boiler sizing solely on the number of bedrooms or bathrooms your property has. You’ll also need to consider the square footage of your property and the number of radiators and towel rails. What’s more, you’ll have to assess the overall demand for hot water; for instance, does your family ever run two showers at once?

The latter is essential. As we’ve mentioned, combi boilers heat water on demand. That’s why supplying two showers simultaneously while also circulating hot water through the radiators will strain a boiler if it lacks the right capacity.

Combi Boiler Size Chart

To give you a rough idea, we’ve created this simple boiler size chart below. Alternatively, you can use this 90-second boiler size calculator to get a more precise boiler size recommendation.

Boiler Size Radiators Bathrooms Bedrooms
24kw-27kw – Check price Up to 10 1 1-3 bedrooms
28kw-34kw – Check price Up to 15 2 2-4 bedrooms
35kw-42kw – Check price Up to 20 3 3-5 bedrooms

Compact Boilers Explained

Apart from the power output, you should also factor in the physical dimensions of your new boiler. If you’re like most UK residents, space is likely scarce — and precious.

Luckily, most boiler manufacturers offer what’s known as a “compact” boiler. These are tiny combis that fit neatly into a kitchen cupboard — out of sight, out of mind.

As an example, the Worcester 32 CDi Compact is 690 x 390 x 280, whereas the non-compact Worcester 8000 Style Combi measures 780 x 440 x 365. As you can see, the size difference is significant enough that the 8000 Style Combi will need to be installed in a garage, airing cupboard, or get hung on a wall. Its compact counterpart, on the other part, will easily squeeze into a kitchen cabinet.

And don’t fret — despite their negligible footprint, compact boilers still typically accommodate a range of power output options.

Bear in mind, not all manufacturers label their compact boilers as such. The Ideal Logic Plus is a good example — by reading the fine print, we can surmise that it fits into a cupboard; however, this information is not immediately obvious. That’s why so it’s always worth checking dimensions before making a purchase.

Combi Boiler Installation Costs

Like sizes, new combi boiler costs can vary, even for very similar properties. For example, replacing a boiler in one 3-bed semi might cost £1,900, but over £2,000 in another.

A lot of it comes down to the type of boiler you need. A more powerful boiler costs more money. Likewise, a boiler from a leading brand with a lengthy, 10-year warranty (we’ll get into that more below) will command a premium over a less desirable brand with a 2-year warranty.

For average homes with 2-3 bedrooms, you’ll be looking at the £1,800-£2,100 mark if you want a boiler from a top brand like Worcester with a 10-years warranty.

The cost of moving a boiler is another critical factor to keep in mind. If you are looking to install your new boiler in a place within your home, expect to shell out an additional £300 or more to cover the costs of relocating critical plumbing infrastructure.

We input details for a small house into this fixed price form, and here is the price we got:

Worcester Bosch Greenstar i from Heatable

Combi Boiler Installation Cost Comparison

Below are some estimates of boiler fitting costs for combis, based on the amount of bedrooms, bathrooms, and radiators your property has.

Boiler Size Radiators Bathrooms Bedrooms Installation Cost
24kw-27kw Up to 10 1 1-3 bedrooms £1,770-£2,000 – Check price
28kw-34kw Up to 15 2 2-4 bedrooms £2,000-£2,400 – Check price
35kw-42kw Up to 20 3 3-5 bedrooms £2,400-£3,000 – Check price

Which New Combi Boiler Brand Should You Choose?

You’ll get a different answer depending on which installer you ask. But the best boiler brands are all similar in price and quality, so it all comes down to warranty and installation cost being offered.

One installer might favour a brand like Baxi, whereas another might specialize in fitting Vaillant or Potterton boilers. Likewise, an installer that fits 100 Vaillant boilers a year will offer you a better deal on Vaillant than Worcester, simply because they buy Vaillant boilers in bulk.

That said, Worcester Bosch is a well-known, reputable brand that most installers tend to fit — they’ve won the Which? Best Buy award 9 years running.

As an example, Heatable are one of the biggest installers in the UK. With their combi boiler installation numbers running into the thousands every year, they can offer ridiculously low prices. But they also offer 10-year warranties on most Worcester combi boilers, which is a huge step up from what boiler fitters typically provide. You can check prices here.

If you’re unsure whether your installer will fit a reputable brand, jump over to our page covering the best combi boiler brands.

Installing Combi Boilers — Warranties and Other Things to Consider

You might be happy with any one brand on the list above, but the decisions don’t stop there. Here are a few questions you may find yourself asking:

  • Do you want a compact boiler to fit in a kitchen cupboard, or prefer to save some cash and go with a standard sized boiler?
  • Will you go for the cheapest combi boiler installation, with the lowest warranty? Or, are you prepared to pay a little extra to get that additional peace of mind?
  • How much is a combi boiler going to cost if you have to move it to a different location on the property?

Once your boiler is spec’d to make sure it’s powerful enough for your home, the next most vital feature is warranty.

You’ll see some boilers with painfully short, 1-year warranties. Avoid them like the plague. Though they may tempt you with immediate savings of around  £300, these pathetic warranties leave you on the hook for all repairs and replacement after the 1-year period.

That’s why we’d recommend paying that little bit extra. Ideally, go for a warranty with a minimum duration of 5-years. But also note a 10-year warranty will seldom cost too much more than a 5-year one. And, the trivial price difference may well be worth it if it guarantees your peace of mind for 5 extra years.

To get a fixed price on a boiler with a 10-year warranty, you can use this clickable online form. It takes around 90 seconds and will give you fixed prices on-screen.

Or, check out our thorough guide to the best combi boilers here.

Cash Savings With Your New Combi Boiler

If your boiler is still “kind of” working, there’s a good chance you’re on the fence about repairing versus replacing it.

One thing worth considering are the cash savings you’ll get with a new boiler, as opposed to the constant outlay on repairs with your old one.

The Energy Saving Trust worked out that if you’re replacing an old G-rate boiler, you could save anywhere up to £840 per year with a new A-rated replacement. That works out to £70 per month in savings. Since boiler finance deals are available from most boiler fitters in the £25-30 per month range, depending on the rating of your boiler and repair costs, a new boiler is usually a no brainer.

new boiler energy savings


What’s Next?

There are plenty of decisions to make before booking in your boiler replacement. Hopefully, our guide to combi boiler installation has covered everything you need to know to decide.

Still got questions about getting your new boiler fitted? Drop us a line via our contact page, and we’ll get back to you as soon as we can.

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