Combi Boiler vs System Boiler: Which is Best for Your Home?
Combi boilers offer an energy efficient option for smaller properties with one bathroom.
But when there’s a need to run multiple showers at once, upgrading to a system boiler is usually necessary.
Deciding between a combi vs system boiler requires careful consideration. As a homeowner, you’ll need to factor in your property type and overall usage needs.
To help you make an informed purchase decision, the WhatPrice team has put together the following guide on system vs combi boilers to guide you.
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Introduction
Plumbers often describe boilers as the beating heart of your home. That’s because boilers are responsible for powering your central heating system and providing all of your taps with hot water.
Only when a boiler breaks down or becomes less efficient over time does the magnitude of its importance become clear. From cold showers to high energy bills, when it’s time to replace your boiler the signs will be difficult to miss.
Before you rush into a decision, it’s worth understanding the different types of boilers so that you can choose a boiler type and size (kW) that will best meet your needs.
In this post, we’ll be focusing our comparisons on combi and system boilers which are two of the most popular types of boilers in the UK.
What you need to know before choosing a boiler
🔍 What is your existing boiler type?: To determine whether you want to opt for a like-for-like replacement or switch to a different type of boiler (i.e. combi to a system boiler).
🏠 What is your household demand?: The number of bedrooms, bathrooms and radiators will impact whether you need a modest combi or a more powerful system boiler. Likewise, the size of the boiler needed for your specific property.
👨🔧 How do you want to get hot water?: System boilers store hot water in a cylinder, whereas combi boilers heat water on demand.
🚿 Do you have good water pressure?: To be able to choose a combi or system boiler you will need good water pressure.
💷 What’s your budget?: New combi boilers have fewer components and so tend to be cheaper to install than new system boilers.
Understanding boiler types
Different boiler types exist because no two properties are the same in terms of their size, plumbing systems and overall usage requirements.
While other boiler types exist, we’re about to outline what sets combi vs system boilers apart.
Since combi and system boilers are the most popular boiler types in the UK, it is most likely you will be considering one of the two types for your next boiler.
What is a combi boiler?
A combi boiler gets its name from its design which combines all the components of a traditional central heating system into one compact unit. Combi boilers can provide both central heating and hot water without the need for a hot water cylinder or tank in the loft.
How combi boilers work is they take water from the mains and heat it up as it is needed on demand. This means there is no waiting for hot water as is the case with other types of boilers. Instead, the boiler starts to make hot water as soon as the demand is made, such as when you call for central heating or turn on a hot tap.
Once the boiler fires up, it starts burning gas in a combustion chamber. The hot gas then rises through a series of pipes into a component called the heat exchanger, which in turn heats up the surrounding cold water.
Combi boiler advantages
- Combi boilers only heat water as it’s needed rather than having to heat a full tank of water
- No need for a separate cylinder or hot water tank
- Great for homes with modest hot water demands
- New combi boilers have excellent energy efficiency compared with other boiler types
- Compact design means they can usually fit in a small cupboard
- Easier to install and maintain
Combi boiler considerations
If your property has more than one bathroom, including if you want to use multiple showers at once then a combi boiler might not be powerful enough for your needs.
While a new combi boiler can sometimes improve low water pressure in terms of the hot water supply (due to replacing old parts and flushing the system), if your local area in general has low water pressure then a combi boiler isn’t suitable. For the same reason, combi boilers aren’t compatible with power showers.
What is a system boiler?
A system boiler stores hot water for future use in a hot water cylinder. So long as there is enough water in the cylinder, it’s possible to run multiple hot taps at once. This makes system boilers ideal for larger homes where the demand for hot water is higher.
Unlike a conventional boiler, system boilers don’t have a separate feed tank in the loft. The lack of a tank works to free up space in your home, although you will still need to store the hot water cylinder in an airing cupboard.
The key to choosing the right system boiler is understanding your usage needs. During periods of high demand, it is possible to run out of hot water, although backup options such as an immersion heater can be added to avoid this scenario.
System boiler advantages
- Better equipped to handle high hot water demands
- You don’t need a tank in the loft
- Hot water is provided efficiently without losing pressure
- Best boiler type for power showers
- Compatible with solar systems
System boiler considerations
Unless you have a very high demand for hot water (i.e. there’s more than one bathroom in your property) a system boiler might be excessive for your needs. Likewise, should you exceed the tank capacity you may have to wait a while for the hot water to replenish. Opting for a system boiler will require a hot water cylinder if you don’t already have one installed. Also, the industry average efficiency for system boilers is slightly less than that of combi boilers.
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Combi vs system boilers: A direct comparison
| Feature | Combi boiler | System boiler |
|---|---|---|
| Best for | Small to medium homes with 1 bathroom | Medium to large homes with more than 2+ bathrooms |
| How is hot water provided? | On demand | Stored in a hot water cylinder |
| Simultaneous hot water use? | Yes, if the mains pressure is compatible | Yes |
| Space requirements | Minimal. Can fit in a kitchen cupboard | Requires a hot water cylinder |
| Loft tank required? | No | No |
| Water pressure requirements | Needs good water pressure | Needs good water pressure |
| Efficiency | Excellent efficiency | Good efficiency for larger homes |
| Installation complexity | Typically very straightforward | Moderate |
| Average installation costs (including price of boiler) | £2,802 | £2,900 |
| Maintenance requirements (excluding yearly servicing) | Minimal unless the boiler is heading towards the end of its life | More complex due to having more components including a separate hot water cylinder |
| Average boiler lifespan | 10-15 years | 10-15 years |
The key differences between combi and system boilers relate to the type of demand they are designed to handle. While combi boilers are ideal for one bathroom homes, powering multiple devices at once is better suited to a system boiler.
Another deciding factor in the combi vs system debate is available space, since combi boilers are far more compact than system boilers which require separate hot water cylinders.
Unless your hot water demands can justify all that extra power, combi boilers are widely considered cheaper and more energy efficient to run. Modern combi boilers can also be found with very long warranties spanning 10-12 years which can offer peace of mind should your boiler fail before it’s due to be replaced.
Which boiler is right for you?
As a homeowner or landlord, the choice of combi vs system boilers depends on your property size and overall user demands.
Best choice for small homes & flats
Combi boilers are geared towards small homes with modest requirements. Also, where there is a lack of space to store a hot water cylinder.
Best choice for large homes & families
For homes with more than one bathroom, a combi boiler may not be able to meet demand. In this case, a system boiler would likely be a better fit.
Considerations for future-proofing your home
If you are considering extending your property, think carefully about whether your current boiler type and size would be able to meet the demand. In particular, adding extra bathrooms and radiators will require more from your boiler. Combi boilers are the most likely to struggle to keep up as they aren’t designed to supply larger homes.
Another key consideration for future-proofing the boiler is the fuel type. It’s now possible to find both combi and system boilers that are hydrogen-ready. With hydrogen predicted to replace gas in the coming years, purchasing a hydrogen-ready boiler can avoid the need to buy a new boiler too soon.
Alternatives to combi and system boilers
Should a combi or system boiler not quite meet your needs, the good news is that other options also exist. This can be in terms of the boiler type itself, or even switching from a boiler altogether to a greener energy alternative.
Conventional boilers (also known as a regular boiler)
The closest boiler type to a system boiler is a conventional boiler. However, unlike a system boiler which only needs a hot water cylinder, a regular boiler will also require a feed tank in the loft.
Opting for a conventional boiler is recommended if your area has low water pressure meaning a system or combi boiler wouldn’t be compatible. Typically, this is in older properties which have traditional plumbing systems.
Renewable options
Want to get off gas completely? If your budget is unlimited or if you qualify for a grant, a heat pump is a fantastic option. Otherwise, the other fuel options to consider are solar or biomass. Whichever fuel you choose, future-proofing your home with a greener energy solution is a smart idea, not least due to the sharp increase in gas prices in recent years.
Expert recommendations & best boiler brands
Once you’ve chosen which side of the system vs combi boilers debate is the best fit for your property, all that’s left is to select a boiler make and model.
We’re here to help as we’ve compiled the top-rated combi and system boilers based on user reviews and overall brand reputation.
Top combi boiler models & reviews
| Boiler make & model | Why is it top rated? |
|---|---|
| Alpha E-Tec NX Combi Boiler | One of the cheapest combi boilers with a long warranty |
| Ideal Logic+ | Fits into the smallest of spaces |
| Vaillant EcoTec Plus | One of the best combi boilers for high hot water demands |
| Viessmann Vitodens 100-W | Exceptional build quality to promote boiler longevity |
| Worcester Bosch Greenstar 4000 | All round winner for performance and reliability |
Read more: The top 10 combi boilers
Top system boiler models & reviews
| Boiler make & model | Why is it top rated? |
|---|---|
| Baxi 800 System 2 | Excellent build quality including brass hydraulics and stainless steel heat exchanger |
| Ideal Vogue Max System | Long warranty period and has a user-friendly interface |
| Vaillant ecoTEC Plus System | Endless choice over boiler sizes |
| Viessmann Vitodens 100-W System | Best overall energy efficiency |
| Worcester Bosch Greenstar 8000 Life | Has intelligent features to reduce the need for boiler repairs |
Read more: What is a system boiler and how much do they cost?
Costs & installation guide
| Boiler type | Typical boiler price range | Typical price range of boiler and installation included |
|---|---|---|
| Combi boiler | £1,000 – £1,990 | £2,000 – £4,000 |
| System boiler | £1,000 – £1,650 | £2,500 – £4,500 |
On the whole, combi boilers tend to be cheaper to purchase and install. If opting for a like-for-like boiler swap, according to prices submitted to WhatPrice the average costs will be around £2,802. Though, the cost can be up to £4,000 if converting from another boiler type or if you want to relocate your combi boiler.
System boilers involve more components and so although the initial purchase price is similar to a combi boiler, the installation will usually be more expensive.
Our user submitted prices have found the cost of a like-for-like system boiler swap to be £2,967. However, should your cylinder also need replacing or if you will be converting to a system boiler from another boiler type, the cost of installation can be up to £4,500.
Boiler prices: what to expect
As we’ve just touched upon, opting for a like-for boiler swap (i.e. the same type of boiler installed in the same location as your existing boiler) will always be cheaper. Simply put, these straightforward swaps involve less parts and labour.
Relocating or converting your boiler will always be more expensive. So consider whether the cost is worth it, or whether upgrading your existing boiler would fix any issues you are currently experiencing.
Brand names also impact the cost of a new boiler. While top names like Vaillant and Worcester Bosch may cost more initially, their superior build quality and longer warranty periods can actually save you money in the long run.
Installation process: what you need to know
Installing a boiler can look different depending on the type of boiler, its fuel source and whether it’s a straightforward swap or a complete conversion.
However, the general steps include:
- Turning off the gas, water and electricity
- Removing the old boiler
- Draining down the system
- Installing the new boiler
- Connecting the pipework, flue and electrical components
- Filling and testing the system
- Configuring the boiler systems
When it’s a like-for-like swap, your engineer may only need 1 day to complete the installation.
But for extremely complex boiler installations (i.e. conversions from another boiler type) it can take 4-7 days to install a boiler.
Before you go ahead, your engineer will explain the timeframes involved based on the scope of the job so that you’ll always know what to expect.
How to save money on a new boiler
- Understand which boiler type and size you actually need to avoid overpaying for a boiler that is far greater than your requirements.
- Take your time to collect quotes and think about your decision before going ahead.
- Ensure your home is energy efficient to keep your energy bills as low as possible once your new boiler is installed.
- Run your boiler off solar energy where possible.
See if you qualify for any energy grants to reduce the cost of replacing your boiler. - Once installed, ask your plumber about the best settings so that your boiler is set to work efficiently but isn’t working harder than necessary.
Conclusion & next steps
Final verdict: Which boiler should you choose?
Purchase a new combi boiler if you have a flat, or your house only has one bathroom.
For larger homes with multiple showers, a system boiler would best meet your needs, provided you can accommodate the hot water cylinder.
With this information in mind, you can start to get quotes for your new boiler installation. You will need to count the number of bedrooms, bathrooms and radiators your property has to determine the boiler size (kW) you need.
An engineer will then visit your property to scope up the job. If any new pipework or accessories are needed, this will be added to your quote.
💡Handy tip: Use our boiler installation calculator to get free boiler quotes
Looking for a new boiler?
Get free quotes and compare prices with our simple enquiry form today
Combi vs system boilers FAQs
Still have questions about whether you should invest in a new combi or system boiler?
We’ve answered some of the most popular queries below. You may also find our other boiler content helpful including to learn more about the best boiler brands and typical prices of new boilers.
Is a combi boiler cheaper to run than a system boiler?
When undertaking the combi boiler vs system boiler comparison, running costs are naturally one of the biggest concerns as a homeowner.
If your hot water demands are high (i.e. you have multiple bathrooms), then a system boiler will be cheaper to run. That’s because a combi boiler is only designed for more modest hot water demands, rather than running two showers at once for example.
Otherwise, combi boilers are considered cheaper to run and overall have slightly better energy efficiency than system boilers. Since combi boilers don’t need to keep a hot water cylinder hot, they are more cost-effective.
By choosing the right boiler type for your property and specific usage you can choose the most efficient system for your home. In any case, upgrading your boiler will always reduce running costs especially if your existing boiler is 10+ years old and no longer meets the current standards for boiler energy efficiency.
To maximise cost savings, you should also look to improve energy efficiency throughout your property as a whole. Simply put, the more efficient your home is, the less the boiler will need to work to keep up.
Can I switch from a system boiler to a combi boiler?
Yes! While most homeowners make the switch from a combi to a system to gain more performance, it is also possible to convert a system boiler to a combi boiler.
Reasons for switching from a system to a combi boiler can include to get access to hot water on demand, to free up space in your home or if your hot water demands are lower (i.e. you only have one bathroom).
That said, since converting a boiler can be expensive, you should also seek professional advice before you go ahead. For instance, it is sometimes the case that a hot water cylinder used by system boilers becomes less efficient over time which can impact the boiler’s performance. However, some simple modifications or even a replacement could remedy the issue, saving you the need to convert the entire system.
How long does a new boiler installation take?
A new boiler installation can take anywhere from 1-7 days.
As you might expect, the exact timeframe is dependent on the scope of the job.
The quickest boiler installations are when you just need a like-for-like swap. These straightforward boiler replacements usually take around a day to complete. Since there are no major pipework changes, the job mostly involves removing the old boiler, installing the new boiler, testing everything and calibrating the system.
Where things get more complicated is when you are looking to relocate the boiler or convert your existing boiler to a different type. Depending on how much new pipework needs to be installed, this is where the job can take several days to complete.
On the whole, combi boilers are considered the easiest boilers to install compared with system boilers. Although, both combi and system boilers are still less complex to install and maintain than conventional boilers.
What size boiler do I need for my home?
Choosing a boiler size (kW) is based on the number of bedrooms, bathrooms and radiators your property has.
We’ve included a basic guide in a chart below. Crucially, you’ll notice that system boilers (and also regular boilers) often require lower power capabilities compared with a combi boiler.
The reason why combi boilers need a bigger size is due to the different way they heat water, since they don’t use a hot water cylinder or loft tank. Instead, combi boilers must heat the water required to supply your hot water supply and central heating from the boiler unit itself.
What size combi boiler do you need?:
| Bathrooms | Radiators | Bedrooms | Recommended combi boiler size |
|---|---|---|---|
| 1 | 0-10 | 1-2 | 20-25kW |
| 2-3 | 10+ | 3-4 | 25-30kW |
| 3+ | 20+ | 4+ | 35-43kW |
What size system boiler do you need?
| Bathrooms | Radiators | Bedrooms | Recommended system boiler size |
|---|---|---|---|
| 1 | 0-10 | 1-2 | 9-11kW |
| 2-3 | 10+ | 3-4 | 11-15kW |
| 3+ | 20+ | 4+ | 15-30kW |
Remember, opting for a boiler that’s not powerful enough will mean you may struggle to get adequate heating and hot water. On the flip side, choosing a boiler that is too great for your needs can actually waste energy and ultimately money on increased energy bills.
What are the best energy efficient boilers?
Combi boilers are widely regarded as the most energy efficient boiler type, with an industry average of 95% efficiency. Depending on the specific boiler model, it is possible to find combi boilers which are up to 98% efficient.
Since 2018, all newer gas boilers must have an efficiency of at least 94%.
Regardless of the boiler type, opting for a new boiler will always increase efficiency. If your existing boiler is particularly old (i.e. it was installed more than 10 years ago), our research suggests you can improve efficiency by as much as 20%.