What Is a Combi Boiler and Is It Right For Your Home?

A combi boiler caters to your heating and hot water demands all without the need for a tank or cylinder. Boasting a compact design and an excellent efficiency, it’s easy to see why combi boilers remain the most popular type of boiler in the UK. 

Are you looking to buy a combi boiler for the first time? Or has it been a while since you upgraded your existing boiler? 

Learn from the experts here at WhatPrice about what a combi boiler is and whether it’s the right choice for your home. Plus, what to look out for when buying a combi boiler to make a sound purchase decision. 

How does a combi boiler work?

A combi boiler (also known as a combination boiler) works by taking water directly from the mains and heating it on demand.  

Unlike other types of boilers, a combi boiler doesn’t need any separate hot water tanks or cylinders to work. Instead, the boiler unit is responsible for heating the water needed for both your hot water supply and your central heating. 

The benefits of this system are that hot water can be provided as required, with hot water taps often becoming very warm after just a couple of seconds. However, this method of supply is designed to only heat one hot water source at once, making combi boilers best for one bathroom homes (or two at a push). 

The key components of a combi boiler include:

• Burner
• Condensate pipe
• Control PCB (Printed Circuit Board)
• Diverter Valve
• Expansion vessel
• Fan
• Flow sensor
• Flue
• Heat exchanger
• Pressure relief valve
• Pump

If that all sounds like a lot of stuff, don’t worry! In reality, your combi boiler will comprise a central unit (i.e. the boiler itself) which contains most of the components. For gas combi boilers, you’ll also need a flue which leads outside. 

Your central heating system which is powered by the boiler will be connected by a series of pipework. This enables your combi boiler to take cold water from the mains, heat it and distribute it throughout your hot water sources and radiators as required. 

If you are upgrading an existing boiler and are keeping the boiler in the same place, your existing pipework will likely stay as it is. But, if you are moving the boiler or changing to a combi boiler from another type (i.e. a system or regular boiler), some pipework changes will be needed. 

Advantages of a combi boiler

80% of UK households have a combi boiler installed. They remain the most popular type of boiler due to being cost-effective to install and run. Their popularity also means it’s usually very easy to source spare parts should your boiler ever need repair. 

So long as you maintain your new combi boiler, it will offer excellent efficiency.

The main advantages of a combi boiler include:

✅Gives you on demand hot water (no waiting for a tank to heat up!)

✅Space saving boiler with no need for a hot water cylinder or tank

✅Excellent energy efficiency to lower your energy bills

✅Easy to install, especially if it’s a like-for-like swap

✅Maintenance made hassle-free due to modern combi boilers having remote fault monitoring and diagnostic tools 

Disadvantages and limitations to consider

So long as how a combi boiler works suits your property, there aren’t too many  

combi boiler disadvantages to worry about.

But, you should know:

❌Combi boilers aren’t designed for large homes (i.e. 2+ bathrooms)

❌Simultaneous hot water supply is limited  

❌Your property must have good mains water pressure

❌Requires regular servicing for safety and longevity 

Combi boiler vs. other boiler types

Boiler type	How does it work?	Who is it best for?	Considerations
Combi boiler	A central unit which takes water from the mains to provide heating and hot water.	Households with small to medium demand (i.e. preferably just one bathroom) who want heating and hot water on demand. Those who want a no water tank boiler. Combi boilers are also the best compact boiler for small homes.	Combi boilers struggle to heat multiple hot water sources at once. Requires good mains pressure to work.
System boiler	The boiler uses a hot water cylinder to provide hot water and heat radiators.	Larger households where there is a need to run 2 showers at once. Also, where there is enough space to store the cylinder in a loft or cupboard.	The hot water is limited to the capacity of the cylinder. System boilers are slightly more expensive.
Regular (conventional) boiler	Regular boilers need two tanks including a cold water tank and a hot water cylinder.	Large traditional homes with the existing tanks and pipework. Also, low water pressure areas.	More complex to install and the setup also requires the most space. Some heat loss is experienced from the way the hot water is stored.

If you have an old boiler, it won’t be very efficient. Sometimes, upgrading to a newer model is all that’s needed. However, going through the boiler buying process is also the perfect time to check whether you have the right boiler type installed for your home. 

Psst: Want to know more about the differences between combi and system boilers? Check out our free combi vs system boiler guide! 

Choosing the right combi boiler

Boilers are important investments. Not only are boilers responsible for heating your home and providing hot water, but they’ll also directly impact your energy bills. 

Plus, if you’re in your forever home, the boiler you’re considering buying now could be with you for the next 10-15 years. 

So here’s what you need to know to make the best purchase for your property and overall usage needs. 

How to size a combi boiler correctly

When you’re sizing up a boiler for your home, thankfully, no measuring tape is needed! 

But you will need to count the number of bedrooms, bathrooms and radiators your property has. 

From there, your boiler installation team will be able to advise on the best boiler type, brands and sizes to suit your property. The size will be given in kW, with the average size range running between 15kW and 43kW.

Example scenarios:

25kW-27kW boiler: Small 1 bed property with a low demand for hot water. 1 bathroom and 6-10 radiators. 

28kW-34kW boiler: Medium 3 bedroom home. 1 bathroom and 10-15 radiators

35kW-42kW boiler: Large 5 bedroom home. 2 bathrooms and 15-20 radiators 

The most common size range for combi boilers is between 25kW and 35kW. However, other types of boilers (i.e. system or regular boilers) are sized differently. That’s why when you get quotes for a combi boiler, you’ll need to enter the boiler type so that the correct size range can be suggested. 

Fuel type (gas, electric, LPG or hydrogen-ready)

The most popular option for combi boilers is one which runs on natural gas, also known as a gas combi boiler. If you already have a gas boiler, upgrading your existing boiler is straightforward, at least where the fuel supply is concerned. But if your home isn’t on the mains, you’ll either need to pay to get connected or consider another fuel source for your boiler. 

Outside of gas, the other fuel options for combi boilers include:

Electric: It’s really easy to install an electric combi boiler. There’s also no flue required because the boiler doesn’t produce emissions. But, electricity has higher running costs than gas, so this is something to consider if you don’t have solar panels. 

LPG (Liquefied Petroleum Gas): For off-grid homes which aren’t connected to the gas mains, LPG is one of the top fuel options for a combi boiler. It’s cleaner than oil, but requires an on-site storage tank. The running costs are higher than gas, although upgrading your boiler can improve efficiency and reduce costs as a result. 

Hydrogen-ready: If you still want to buy a gas boiler but want to future-proof it for the eventual move away from gas, hydrogen-ready is the best option currently available. It runs on a blend of up to 20% hydrogen and the boiler prices are similar to standard gas boilers. However, the use of hydrogen isn’t yet widespread, so you’ll need to check what’s available in your area.

Top-rated combi boiler brands

Based on our research, the top selling and most popular combi boilers include those made by: 

• Alpha
• Baxi
• Ideal
• Vaillant
• Viessmann
• Worcester Bosch

If you’d like to discover the top models from each of these brands, we’ve written a guide on buying the best combi boilers. Here, we cover all of the boiler features and user feedback, plus we cover the typical prices for each model. 

Smart features and efficiency ratings to look for 

Combi boiler feature	What does it do?	What to aim for
ErP label	Tells you the energy efficiency rating of the boiler	A+ or A++
Condensing	A condensing boiler reuses heat from exhaust flue gases that would otherwise be wasted which improves efficiency.	All new boilers must be condensing to meet current standards.
Seasonal efficiency	How well the boiler performs not just at full power, but in everyday conditions.	Look for high part-load efficiency, or models advertised as having good modulation (see below). Boilers getting ~94-98% in lab or ErP tests are among the best.
Noise levels (dB)	Can impact the enjoyment of the space if the boiler is too loud.	The lowest dB possible! Boilers which produce around 45dB are considered quiet. But anything over 55dB would be bothersome.
Smart thermostat and programmable controls	Gives you more control over your boiler and helps best manage its running costs.	Smart tech and app pairing. Being able to set different schedules, i.e. ‘Away mode’ helps reduce wastage.
Weather compensation	Measures outside temperature and adjusts boiler output accordingly. If it’s milder outside, the boiler works less hard which saves fuel.	Check whether the boiler or control system includes this. Some smart thermostats or controllers have this built in.
Fuel-blending or hydrogen-ready compatibility	With the UK moving toward decarbonisation, some boilers are being built to run on a future gas-hydrogen blend. This can help future-proof your heating.	If you plan to keep your boiler for more than 10 years and don’t want to switch to a heat pump, check for different fuel compatibility.
Remote monitoring & alerts	Provides early warnings to avoid breakdown or inefficient operation. Also lets you track usage to target savings.	The boiler can give you reports status, plus tell you about faults or performance data via an app. Some controllers can warn of leaks or low pressure.

Installation & running costs

Boilers remain one of the most popular ways to heat your property, including providing a source of hot water. Out of all the boiler types, combi boilers are seen as the most efficient. But as with everything in your home, there is the initial purchase cost, as well as ongoing running and maintenance fees to consider.  

Typical installation costs in 2025

WhatPrice users are fantastic at submitting their prices, including telling us how much they paid for their new combi boiler. 

Our latest figures suggest the average cost for a new combi boiler with installation is £2,776.

The amount you’ll pay for a new combi boiler depends on:

• The boiler brand
• The boiler size (kW)
• The quality of the boiler
• Whether it’s a simple swap or involves converting to a new boiler type
• If you’ll be installing the boiler in the same location in your property
• If gas supply upgrades are needed
• Whether smart controls are included
• System cleaning and powerflushing
• The warranty length

When you collect quotes for a new combi boiler, all of the above will be explained to you, so that you’ll have a better idea of what to expect and the costs involved.

Buyer’s beware: Lots of websites now compete for boiler installations, including tempting you with ‘cheap’ quotes or promising ‘next day’ installations. Unfortunately, when a boiler quote seems too cheap or too good to be true, it probably is. It can result in your boiler installation being outsourced or important steps being skipped. Always compare quotes from trusted local installers near you and never be tempted by pushy sales tactics. 

Expected lifespan & warranty

A decent combi boiler should last 10 years, with premium models capable of lasting 15 years or longer. Beside average lifespans, what will matter most is the safety and efficiency of your boiler. When it’s deemed beyond repair, it will be time for a new boiler regardless. 

Something which can offer a degree of protection is choosing a boiler which comes with a longer warranty period. Short boiler warranties are considered 1-5 years. However, many boiler manufacturers offer up to 10 years of warranty, or the option to extend your warranty for a small fee. 

Make sure you also have coverage for boiler breakdowns and repairs (especially anything not covered by a guarantee or warranty). Our research found the average call out fee for a boiler repair is £450, whereas we found monthly boiler coverage starting at £3.45 per month with Hometree. This included unlimited call outs. 

Average annual running costs

The running costs of a combi boiler are subject to change due to fluctuating energy prices. However, at the time of writing, you can expect to pay an average of £840 per year, if your boiler uses 12,000 kWh of energy.

If your existing combi boiler is older than 10 years, you can make savings by upgrading to a new and more efficient model. Further savings can be experienced if you upgrade the energy efficiency of your home, plus regularly compare your energy tariffs to check you have the best deal. 

You may also like: How Much Do Replacement Windows & Doors Cost?

Grants or government schemes available

If you’re on a low income, have a disability or are over 60, it’s always worth ensuring you receive all of the benefits you are entitled to. You can check for free by visiting the government Benefits Calculator. 

In terms of government help for the cost of a new boiler, there are two main options currently available:

ECO4: The ECO4 grant will cover up to 100% of the cost of a new boiler. To qualify, you’ll need to receive certain means-tested benefits (i.e. Universal Credit or Pension Credit). Your boiler must be over 18 years old, or be non-condensing with a low energy rating. The boiler can be gas, oil, electric or LPG. ECO4 runs until March 2026. 

BUS: BUS stands for the Boiler Upgrade Scheme. BUS funding will give you up to £7,500 to switch from a boiler to a heat pump. Or, you can get £5,000 towards the cost of a biomass boiler. Unlike the ECO4 scheme, BUS funding isn’t means-tested. Instead, the scheme is aimed at making our home energy greener.

Maintenance & servicing

If you want to see the best return on your boiler investment, then keeping on top of any maintenance is key. 

Unlike other areas of your home, only qualified professionals (i.e. a Gas Safe engineer) can perform gas combi boiler repairs or servicing. However, it remains your responsibility as the homeowner to schedule any servicing and also report any faults promptly. 

So long as your boiler still has warranty coverage, any faults should be repaired for free or for a minimal charge. But, you’ll also need to pay for an annual service as part of a plan or one-off fee. 

How often should a combi boiler be serviced?

Combi boilers need servicing every 12 months. This isn’t just good practice to ensure safety and efficiency, it’s often a stipulation of your boiler warranty. Therefore, skipping your annual boiler service can invalidate your boiler warranty. This could mean the free repairs or even full replacement that you would have been eligible for if your boiler fails while still under warranty would no longer stand. 

Common problems and how to prevent them

Boiler problem	What are the signs?	How to prevent
No heating or hot water	Boiler works intermittently or not at all	Run the heating occasionally in summer to keep valves moving. Keep thermostats clean and test them regularly. Schedule annual servicing to catch early signs of valve or pump wear.
Low pressure	When your radiators aren’t heating properly. The boiler may shut off or give you an error code.	Check the pressure gauge monthly (should be 1-1.5 bar when cold), top up using the filling loop if it drops. If the pressure drops often, get your system checked for leaks.
Frozen condensate pipe	The boiler won’t fire up, or you get error codes specific to this issue.	Insulate the external pipe. The pipe should also be short and with a downward slope to drain water. During freezing weather, run the heating regularly to keep the flow warm.
Leaks or drips coming from the boiler	You’ve noticed water pooling under the boiler or dripping from pipes.	Depending on where the leak is, it can happen due to corroded internal seals or pipes. It may also be down to a faulty pressure relief valve. If the boiler is new, it could be because the installer did a poor job with the pipework. Investigation is needed to fix the leaking element.
A noisy boiler	The boiler is making banging or whistling noises. These noises have suddenly become noticeable as your boiler used to be much quieter.	Limescale buildup on the heat exchanger is a common culprit. Fit a scale reducer or water softener if you live in a hard-water area. Have your system flushed every 5 to 7 years. Use inhibitor chemicals to reduce limescale and corrosion (this should be standard for all new boiler installations).
Radiators don’t feel warm all over	Cold patches at the top or bottom.	This can happen when air is trapped in the system, or there is a sludge buildup in radiators or pipes. Preventing bleeding radiators once or twice a year. For sludge, get a powerflush of the system and ask your plumber to use inhibitor fluid.
Pilot light keeps going out	In older boilers, the flame will not stay lit.	There is a fault with the boiler which could relate to the thermocouple or a gas supply issue. Regular boiler servicing is imperative, but ultimately, these older style boilers are well overdue replacement.

When to replace your combi boiler

Most combi boilers last for 10-15 years before needing to be replaced. If you installed your current boiler, then you should have the paperwork to find out the age of the boiler. But, if your boiler was installed by a previous homeowner, you might need to do some research to find this information. 

One thing you’ll notice if your boiler is on the older side is poor efficiency. For instance, when you struggle to get hot water or your radiators always feel lukewarm despite adjusting the settings. Your energy bills might be far higher than expected, even with the rising costs of energy factored in. 

Warning signs that you need to replace your boiler without delay include if your boiler fails its annual inspection, or has any other safety issues that are deemed beyond repair. 

Beyond obvious problems, sometimes the boiler isn’t the right type for your property, or could be sized incorrectly for your needs. If in doubt about whether you should replace your boiler, always consult a Gas Safe engineer for an expert opinion. 

How to get accurate new boiler quotes

As a homeowner, boilers aren’t something you buy that often. So when it’s time to replace yours, it can be difficult to know where to look to find accurate quotes, let alone quotes from trusted sources. 

Our top tips for getting combi boiler quotes include: 

✅Get multiple quotes: Always get a minimum of 2-3 quotes. Sure, this will take more time than going with the first quote you receive. But it’s important to ensure that you’re being fairly quoted. Plus, you can take the opportunity to vet the installer to find the best company for the job.

✅Use an online quotation system: Use our free boiler installation cost calculator to find combi boiler quotes from local installers near you. Simply answer a couple of questions to get your price estimate!

✅Understand what will impact the cost: Get clued up on how the size of your home, plus the number of bedrooms and bathrooms will affect the cost of a combi boiler. The greater the demand, the larger size boiler you’ll need. 

✅Choose local installers: When there are tried and trusted boiler installers who live locally, this can offer peace of mind. Contrary to what you may think, local installers don’t necessarily charge more to install boilers than national companies. Going for a local company can also prevent being overcharged for delivery.

Combi boiler FAQs

Combi boilers might be the most common type of boiler in the UK, but that doesn’t mean everyone instantly knows everything about them! We’ve answered some of the most popular questions below to explain all.