Replacing your boiler with a combi boiler?
If you’re thinking about switching to a combi boiler then you’re in good company. Around 80% of homes in the UK have combi boilers and it’s easy to see why. They save space, heat water instantly, and can be more energy-efficient than older systems.
But is a combi boiler right for you? And what does the switch involve? In this guide, we’ll walk you through their benefits, the cost of replacing a conventional or system boiler with a combi boiler, and what to expect during installation. So you can decide if it’s the right choice for your home.
What is a combi boiler and how does it work?
A combi boiler (short for combination boiler) provides both your heating and hot water from one compact unit. This means there’s no need for a separate water tank or cylinder. Instead, it heats water directly from the mains. So you get hot water on demand, without having to wait for a tank to warm up.
Combi boilers vs other boiler types
Not sure what type of boiler you currently have or what might suit your home best? Let’s take a quick look at the three main boiler types in the UK: combi, conventional, and system.
- A combi boiler heats water directly from the mains, so you get hot water instantly. There's no need for a separate hot water cylinder or storage tank, which makes it ideal for smaller homes.
- A conventional boiler (sometimes called a regular or heat-only boiler) stores hot water in a cylinder and usually requires a cold-water tank in the loft. It’s often found in older properties.
- A system boiler is somewhere in between. It works with a hot water cylinder like a conventional boiler, but the key components are built into the boiler itself – so it doesn’t need a separate tank in the loft.
If you want to take a deep dive into the different types of boiler, check out our in-depth guide. Alternatively, here’s how the three types compare at a glance:
| Feature | Combi boiler | Conventional boiler | System boiler |
|---|---|---|---|
| Hot water supply | Heated instantly from mains | Stored in a tank, can run out | Stored in a cylinder, better for large homes |
| Space needed | Compact, no water tank needed | Requires a hot water cylinder and loft tank | Needs a hot water cylinder but no loft tank |
| Installation process | Simpler and faster | More complex, extra pipework needed | Easier than conventional but still needs a cylinder |
| Energy efficiency | Very efficient, no stored heat loss | Can lose heat from stored water | More efficient than conventional but less than combi |
| Running costs | Lower – heats water only when you need it | Higher – stores hot water even when it’s not in use | Moderate – stores hot water, but system is more efficient than conventional |
| Best for | Small to medium homes with 1–2 bathrooms | Larger homes with high hot water demand | Homes with 2+ bathrooms and high demand |
Why switch to a combi boiler?
If you're thinking about replacing your current boiler with a combi boiler, there are plenty of reasons to make the change. Not only are they compact and convenient, but they can also help you cut down on energy bills and free up space at home.
Here’s a closer look at the key benefits:
Energy efficiency
Combi boilers are one of the most efficient boiler types on the market.
- They only heat the water you use – no stored water, no wasted energy
- Combi boilers boast over 90% efficiency, helping reduce your carbon footprint and monthly costs
- New models could cut your energy bills by up to 22%.
Space-saving design
Unlike conventional and system boilers, combi boilers don’t need a hot water cylinder or storage tank.
- They’re a great choice for smaller homes, flats, or anyone looking to reclaim their airing cupboard or loft space
- With fewer parts involved, installation is often simpler too.
Cost savings
A combi boiler conversion can be a smart move for your wallet – not just at installation but for years to come.
- Combi boilers are typically cheaper to run because they don’t store hot water unnecessarily.
- They also have fewer components, which can mean lower maintenance and repair costs over time.
Instant hot water
Say goodbye to waiting around. One of the biggest perks of a combi boiler is that hot water is always available.
- With a combi boiler, you’ll get hot water as soon as you turn on the tap
- There’s no waiting for a tank to heat up, which means better efficiency and more convenience.
How to switch to a combi boiler
Upgrading to a combi boiler is a straightforward process when handled by a Gas Safe registered engineer. If you're switching from a conventional or system boiler, here’s what to expect:
1. Removing the old system
Your old boiler, hot water cylinder, and cold-water tanks (if you have them) will be safely disconnected and removed. The engineer will also take out any unused pipework or thermostats that are no longer needed.
2. Upgrading the pipework
Because a combi boiler works differently, your engineer may need to adjust or upgrade your existing pipework to ensure the system runs efficiently. This is particularly important if your current pipes are older or undersized.
3. Installing the new combi boiler
Your new combi boiler will be fitted in a suitable location, often in the same spot as your old boiler to keep things simple. If you’re moving the boiler to a new location, extra work may be required.
4. Testing and system flush
Once installed, the engineer will flush the system to remove any debris and make sure your heating and hot water are working properly. They’ll test the pressure, gas connections, and efficiency to ensure everything is running smoothly.
5. Final checks and handover
Your engineer will walk you through how to use your new boiler, including setting up smart controls if applicable. They’ll also register your warranty and provide any necessary paperwork, so you’re fully covered.
Is my home suitable for a combi boiler?
Combi boilers are a great choice for many homes, but they’re not a one-size-fits-all solution. Before making the switch, here are some key factors to consider to make sure your home is compatible.
Choosing the right combi boiler for your household size
The size of your home and the number of bathrooms you have will determine what boiler output (kW) you need. A boiler that’s too small won’t meet your hot water needs, while an oversized boiler could be inefficient and cost more to run.
Here’s a quick guide:
| Household Size | Bathrooms | Radiators | Recommended output |
|---|---|---|---|
| Small home (1-2 bedrooms) | 1 | 0-10 | 24-27kW |
| Medium home (3-4 bedrooms) | 2-3 | 10+ | 28-34kW |
| Large home (4+ bedrooms) | 3+ | 20+ | 35-42kW |
Understanding the water pressure requirements for a combi boiler
Water pressure plays a big role in how well a combi boiler works. Since a combi boiler heats water directly from the mains, it needs a good flow rate to deliver strong showers and fast-filling taps.
How to check your water pressure:
- Turn on your cold tap – if it’s slow, your mains pressure may be low
- Ask your water supplier for your mains water pressure reading.
What to do if your water pressure is too low:
- Install a mains booster pump to increase flow
- Fit an accumulator tank to store and release water at a higher pressure.
If you're unsure, speak to a heating expert to check your home’s suitability.
Is your existing pipework compatible with a combi boiler?
Your home’s pipework is key to making sure everything runs smoothly when you switch to a combi boiler. If your pipes are old or too small, they might need upgrading to handle the strong water flow and pressure from a combi boiler.
Here’s what to consider before installation.
Pipe size and condition:
- If your home has narrow or outdated pipework, it may not be able to deliver hot water effectively.
- Older pipes, particularly those made from lead or steel, may need replacing with modern copper or plastic alternatives to ensure the best performance.
Gas supply pipe upgrades:
- Combi boilers require a steady gas supply to heat water on demand
- Many older boilers run on a 15mm gas pipe, but most modern combi boilers need a 22mm pipe to function efficiently. If your gas supply pipe is too small, it may need to be upgraded.
Leaks and blockages:
- Leaking or corroded pipes can lead to pressure loss and inefficient heating
- Before installation, an engineer will check for blockages, rust, or weak points in your system to prevent future issues.
If you're unsure whether your pipework is suitable for a combi boiler, it's best to get a professional assessment.
Combi boiler compatibility checklist
Use this quick checklist to see if your home is suitable for a combi boiler.
- Household size – Combi boilers work best in small to medium homes with one or two bathrooms.
- Water pressure – Your home should have good mains water pressure. If not, a booster pump may be needed
- Existing pipework – Your pipes need to be modern and wide enough to handle a stronger flow rate
- Gas supply – Your home should have a 22mm gas pipe, but it can be upgraded if you don’t
- No need for large hot water storage – You don’t require a hot water cylinder or loft tank.
If you’ve ticked most of these boxes, a combi boiler could be a great choice for your home. Next, we’d recommend speaking to a boiler expert face-to-face. They’ll talk you through your options and find the right boiler for your home and budget.
Why choose British Gas to install a new combi boiler?
We install more boilers than anyone else in the UK. And with 200 years of experience under our belts, we understand the ins and outs of Britain’s homes too.
If spreading the cost of a new boiler makes things easier, you can choose up to 3 years’ interest-free credit, with £0 deposit and no hidden fees. And with a warranty on every new boiler, if anything ever needs fixing, we’ll get it sorted.
Got questions about replacing your boiler with a combi boiler? You're not alone. Here are some of the most common things homeowners ask when upgrading their heating system.
Frequently asked questions
How long does combi boiler installation take?
A typical combi boiler installation takes 1-2 days, depending on your home and whether you need to upgrade any existing pipework. We aim to get everything up and running as quickly as possible. Plus, we handle every step of the process for you – from safely removing your old system to testing your new boiler before handover.
What maintenance does a combi boiler need?
To keep your combi boiler running smoothly all year round, regular maintenance is essential. Here’s how to look after it:
- Check your boiler pressure – Low pressure can affect performance
- Bleed your radiators – Helps keep your heating working efficiently
- Book an annual service – A British Gas engineer will check for any issues, clean components, and ensure everything is working safely.
British Gas offers annual servicing and maintenance plans to help keep your boiler in top condition, reducing the risk of breakdowns.