Fed up with the hot water running out halfway through a shower? Or perhaps you’re looking to reclaim your loft space by ditching the hot water tank? Maybe your old conventional boiler is ineffiicent and you’re looking to save money on your energy bills.
If so, you are probably considering a change to a more efficient combi boiler. But given the upfront investment required, is it actually worth it?
For the majority of UK homes, a modern combi boiler is the preferred option. They are compact, don’t require a hot water cylinder or cold water storage tank, and are designed to supply hot water and heating from a single unit. Along with taking up less space, combis are typically more energy-efficient and can help reduce your energy bills.
To help you make an informed choice, in this blog, we’ll explain how a combi boiler system works, how it compares to a conventional boiler, and the key benefits and drawbacks to consider before making the switch.
For advice on a new boiler in the Winchester and Southampton areas, please do not hesitate to contact JPS Plumbing, Gas and Electrical.
What is a combi boiler?
A combi (short for combination boiler) is a type of gas boiler that provides both heating and hot water directly from the mains. They heat water on demand whenever you turn on a tap or switch on your heating.
This is in contrast to conventional boiler systems, which require both a cold water tank and a hot water cylinder to store water, typically stored in the loft or airing cupboard.
Because everything is contained in a single compact unit, a combi boiler system is a great space-saving solution. That’s one of the reasons it has become one of the most popular heating systems for the average UK home.
How do combi boilers work?
While the technology inside a combi boiler is quite sophisticated, the way it works in practice is fairly straightforward. Here’s a brief explanation of how it works:
- The combi boiler takes cold water directly from the mains supply. When you open a hot tap, a flow sensor inside the boiler detects the demand for hot water.
- The boiler’s gas burner fires up and heats water almost instantly as it passes through a powerful heat exchanger.
- The heated water is then sent straight to the tap or shower, while the boiler temporarily prioritises hot water over central heating.
- Once the tap is turned off, the boiler switches back to circulating heated water through the radiators to maintain the home’s heating.
All of this happens within seconds, which is why combi boilers can deliver hot water almost instantly. Not only is this convenient, but the on-demand process means there is no need for a separate hot water cylinder, so there is less energy wastage and more efficient heating.
What is the difference between a combi boiler and a conventional boiler system?
The biggest difference between the two systems is how they store and deliver hot water.
| Feature | Combi Boiler System | Conventional Boiler System |
|---|---|---|
| Water storage | No storage tank | Uses a hot water cylinder and a cold water storage tank |
| Space required | One compact unit which is very space-saving | Requires loft and/or airing-cupboard space for the tank and hot-water cylinder. |
| Hot water supply | Instant hot water on demand | Stored hot water in a water cylinder – when it’s gone, it has to be replenished before you can enjoy hot water again |
| Water pressure | Uses mains pressure | Gravity-fed, so lower pressure |
| Installation | Often simpler with fewer components | More complex pipework and tanks |
| Suitability | Ideal for one or two bathrooms | Better for homes with multiple bathrooms or very high hot water demands |
Benefits of a combi boiler system at home
For many homeowners, installing a new combi boiler offers a few key advantages, both day-to-day and long-term.
Cost savings
One of the main reasons people switch to a combi is to save money on energy costs. Because a combi system only heats up water when it’s needed, there’s no heat being lost from a storage tank or water cylinder sitting in the loft or airing cupboard. That means less wasted energy and better overall efficiency.
Combis are condensing boilers, meaning they capture and reuse heat from exhaust gases that traditional boilers would simply vent outside. By recovering this extra heat and transferring it back into the heating system, they can achieve efficiencies of 90% or more, which is why many modern combis are A-rated.
Modern combi boilers can also be integrated with smart heating controls that learn your heating habits, adjust temperatures automatically, and ensure the boiler only fires when heat or hot water is actually needed. This helps lower energy consumption, reduce energy bills, and help minimise your home’s carbon footprint.
Instant hot water
Another major advantage of a combi boiler is instant hot water the moment you turn on a tap. People with conventional boilers who have had to stand and wait a few minutes for hot water to flow, or who have experienced hot water running out mid-shower, can tell you what an inconvenience it is!
Space saving
Conventional gas boilers can take up a surprising amount of room. A hot water cylinder in the airing cupboard, a cold water tank in the loft, and loads of additional pipework can eat into valuable storage space.
You can have a combi boiler installed in a kitchen cupboard or utility room, with no need for extra components, which is great for smaller homes.
Good water pressure
Combi boilers draw water directly from the mains, meaning they can typically deliver stronger water pressure. Older conventional systems are gravity-fed because they rely on a cold water tank located in the loft. The height of the tank creates natural pressure, allowing water to flow down through the hot water cylinder and pipework under gravity. For those used to low-pressure showers from gravity-fed systems, upgrading to a combi can be a big improvement.
Considerations of switching to a combi boiler
Although combi boilers work well for many homes, they’re not always the perfect fit. It’s important to consider the following before replacing your conventional boiler with a combi.
Not ideal for large homes
Choosing the right boiler depends on your home’s size and hot water demands, so a combi boiler isn’t right for everyone.
A combi boiler heats up water instantly, but it can only supply so much at once. In properties with multiple bathrooms, where several people are likely to be showering or running hot taps at the same time, the system can be stretched. In that case, a system boiler or regular boiler with a hot water tank may still be the better option.
Conversion costs
There is obviously a cost to installing a brand-new boiler, with the unit itself ranging from several hundred to several thousand pounds, depending on the brand, output, and efficiency rating. If you’re replacing an old system, the installation process may involve removing the old water tank, cylinder, and some existing pipework, increasing the cost further.
However, while a new boiler installation requires an upfront investment, the long-term cost savings and efficiency improvements can make it worthwhile. For example, if you are upgrading from an old F-rated boiler to a new A-rated combi, you could save around £200 a year on your energy bills (source: Energy Saving Trust, July 2025).
An experienced heating engineer can quickly assess your existing setup and explain exactly what would be involved.
No backup supply of hot water
Traditional systems sometimes include an immersion heater in the hot water cylinder as a backup when something goes wrong. With a combi boiler, hot water is produced on demand, which means there’s no stored water available if the boiler stops working.
Fortunately, modern boilers are extremely reliable when properly serviced each year.
Thinking of changing your boiler to a combi? Enquire today
If your current boiler is ageing, inefficient, or just too bulky, upgrading to a modern combi boiler could make a big difference.
At JPS Plumbing, Gas and Electrical, our experienced engineers can help you choose the most efficient boiler based on your property’s size, your hot water demands, and your current heating system. We work with a number of leading boiler brands and can also provide annual servicing and boiler cover plans to protect your new investment.
Get in touch today to discuss your options or get an instant boiler quote online now.
Watch our founder, Jamie, talk about different types of boilers on our YouTube channel.
Frequently asked questions about switching to combi boilers
Are combi boilers more efficient than conventional boilers?
Modern combi boilers are designed to heat water only when it’s needed, so they are typically more efficient than conventional boilers.
How long does it take to replace a combi boiler?
Replacing an existing combi boiler with a new combi boiler can often be completed in a single day. However, if you are converting from a conventional boiler system to a combi system, it may take longer if your cold water tank, hot water cylinder, and additional pipework need to be removed. In most cases, the installation process takes no more than three days, depending on how complex the system is.