Combi, or combination, boilers are the most common type of boiler found in UK homes. With their ability to provide central heating and instant hot water on demand, combis are an ideal boiler choice for their efficiency, convenience and space-saving design.
This guide explains everything you need to know about combi boilers including how they work, key benefits, disadvantages to consider, and costs to install. Read on to learn whether a combination boiler is the right heating system option for your home.
What is a combi boiler and how does it work?
A combi boiler can heat up water for the hot taps and also for the central heating radiators in one system. This saves space as you don’t need a storage tank and also saves time because it heats up water on demand and only as much as you need. A powerful boiler can heat up larger amounts of water quickly.
How Combis Work
Combi boilers provide both central heating and domestic hot water in one system. Here’s an overview of how they operate:
- When you turn on the heating, the gas burner ignites and heats water in the main heat exchanger loop connected to your radiators.
- A heating pump circulates this hot water around your radiators.
- For hot water, the diverter valve sends cold mains water through a plate heat exchanger.
- The main heat exchanger loop on the other side heats the water.
- Hot water flows to taps while the heating loop returns to the main heat exchanger.
| Function | Component Used |
|---|---|
| Burn Fuel | Gas burner |
| Transfer Heat | Main heat exchanger |
| Circulate Water | Heating pump |
| Switch Flow | Diverter valve |
| Generate Hot Water | Plate heat exchanger |
In a combi, the same heating loop provides central heating and also produces hot water as needed in one compact system.
Benefits of Combi Boilers
Hot Water On Demand
One of the biggest benefits of a combi boiler is its ability to provide hot water instantly, without needing a storage tank. When you turn on a hot tap, the combi boiler takes cold water directly from the mains supply and passes it through a heat exchanger to heat it up.
This makes combis extremely convenient and efficient. There’s no waiting time for a storage tank to heat up or worry about running out of hot water. The boiler provides continuous hot water to any number of taps as its needed.
Some combis also have an internal plate heat exchanger that keeps a small amount of hot water ready in the boiler so it’s instantly available when you turn on the tap. This gives even faster hot water delivery.
| Type of Boiler | Hot Water Delivery |
|---|---|
| Combi | Instant continuous supply |
| Regular | Limited by storage tank capacity |
| System | Faster than regular but still limited |
So if quick unlimited hot water is important in your home, a combi boiler is by far the best choice. Families with high demand from multiple bathrooms/showers especially benefit from the instant hot water supply of a combi.
The only time hot water flow may be restricted is if multiple taps are opened simultaneously, which can cause a pressure drop. But for general use one tap at a time, a combi boiler provides all the hot water you need, when you need it.
Efficient Operation
Combi boilers utilize advanced condensing technology to maximize efficiency. This means they can recover latent heat from the hot flue gases that would normally be wasted in non-condensing boilers.
The gas burner flame heats water in the heat exchanger. The resulting flue gases pass through an extra secondary heat exchanger that condenses the steam and uses this extracted heat to pre-warm the cold water entering the boiler.
| Boiler Type | Typical Efficiency |
|---|---|
| Condensing Combi | 90-95% |
| Non-Condensing | 60-80% |
This condensing process enables the boiler to achieve up to 95% efficiency, compared to just 60-80% in a non-condensing boiler. This equates to significant savings on your gas usage and carbon emissions.
Modern building regulations require all new gas boiler installations to be high efficiency condensing types. So if your existing boiler is over 10 years old, upgrading to a new combi boiler will drastically improve your heating system’s efficiency.
Compact Size
One of the biggest advantages of a combi boiler is its compact and space saving design.
Since it does not require an external hot water cylinder tank like a regular or system boiler, all the heating components are integrated into a single, wall-mounted unit.
| Boiler Type | Components |
|---|---|
| Combi | Single wall-mounted unit |
| Regular | Boiler unit + hot water tank + cold water tank |
| System | Boiler unit + hot water tank |
This compact all-in-one configuration means combi boilers are perfect for smaller homes where space is limited. They can easily fit inside a standard kitchen cupboard.
There is also no need for a cold water tank in the loft, freeing up space for storage or conversion. And eliminating any separate tanks or cylinders means less pipework complexity.
Overall, the combi design minimizes installation time, cost, and space requirements – making them the ideal boiler option for apartments or flats where room is tight. A combi ensures no wasted space for heating system components.
Cost Savings
Choosing a combi boiler over a conventional regular boiler and tank system can lead to significant cost savings.
Since combis integrate all heating components in one unit, you only need to purchase and install a single appliance. This is generally cheaper than buying a standalone boiler, hot water cylinder, cold water tank, and all associated pipes and valves.
There is also less time required for installation as there is no complex pipework linking tanks and cylinders. This reduces labor costs.
Overall, installing a new combi boiler can cost £500-£1000 less than a comparable regular boiler system.
In the long run, you will also save on energy bills. Combis are more energy efficient, providing hot water on demand rather than constantly heating a tank. And there is no heat loss from hot water pipes traveling to external cylinders.
With lower purchase costs, faster installation, and better efficiency, a combi boiler offers excellent value for money compared to outdated regular boiler configurations. The savings quickly add up.
Disadvantages of Combi Boilers
Water Pressure
Combi boilers provide hot water directly from the mains supply rather than a storage tank. This means the water pressure at your taps depends on the incoming mains pressure.
Higher mains pressure enables combis to deliver more hot water more quickly to taps and showers. However, opening too many taps at once can exceed the boiler’s flow rate capacity and cause a pressure drop.
This may result in a short-term reduction in water flow if multiple taps are opened simultaneously, especially in larger homes. Running a bath and shower together may be difficult depending on the combi’s power.
Upgrading to a more powerful combi boiler with a higher maximum flow rate can help ensure sufficient hot water delivery for larger homes with high demand.
Installing a small pressure booster pump can also help maintain water pressure if opening multiple taps causes issues.
Overall, for average households, a standard combi boiler should provide adequate hot water for typical daily usage across 1 or 2 bathrooms. But larger 3+ bathroom homes may need to opt for a system boiler with a hot water cylinder to avoid pressure limitations.
- Limited hot water supply: Combi boilers are not suitable for larger homes or properties with high hot water demand, as they may struggle to supply enough hot water to multiple taps or showers at once.
- Incompatible with some showers: Some types of showers, such as power showers, may not be compatible with combi boilers due to the pressure drop issue mentioned above.
- No backup hot water supply: If your combi boiler breaks down, you will not have a backup hot water supply, which can be inconvenient.
- Limited choice of models: Combi boilers are not suitable for all types of properties, so there may be a limited choice of models available to choose from.
Typical Boiler Installation Costs
Here are some approximate costs for installing common boiler types:
| Boiler Type | Price Range | Additional Information |
|---|---|---|
| Combi | £2000-£3000 | Condensing combi boilers provide the best value for money. Budget options start from £1000. |
| System | £2500-£4000 | More expensive than combis due to extra hot water cylinder and associated components. |
| Regular | £3000-£5000 | Most expensive option as requires full system of boiler unit, hot water cylinder, cold water storage tank, and pipework. |
| Oil | £3000-£5000 | Oil fuelled boilers have comparable costs to a regular gas boiler. |
| Electric | £2000-£3500 | Don’t require gas or oil fuel lines. Good option if electricity is your primary power source. |
Combi boilers are generally the most cost effective boiler choice thanks to their compact all-in-one design.
System or regular boilers may be preferable for larger homes needing more hot water capacity, but will cost more to purchase and install.
Always get multiple quotes to find the best local deals. Installation costs can vary across different regions and engineers.
Is a condensing boiler the same as a combi boiler?
Rather than being a type of boiler, like a combi, system, or conventional, the term condensing refers to the way it functions – at high efficiency. So all modern gas boilers (installed since 2005) are condensing, including the combi.
Why do I have a combi boiler and a water tank?
In the UK, it is common to have a combi boiler and a water tank in your home. The reason for this is that while combi boilers are highly energy efficient and space saving, they may not be suitable for larger homes or properties with high hot water demand. A water tank, on the other hand, can provide a larger supply of hot water, making it ideal for homes with multiple bathrooms or large families.
By combining a combi boiler with a water tank, homeowners can enjoy the benefits of both systems, with the combi boiler providing energy-efficient heating and hot water on demand, and the water tank providing a larger supply of hot water when needed. Choose the right size of water tank for your home to ensure that you have enough hot water without wasting energy or money on heating water that isn’t being used.
If you’re unsure whether a combi boiler and water tank system is right for your home, it’s best to seek professional advice from a qualified heating engineer.
What sizes do combi boilers come in?
Combi boiler sizes vary depending on the number of radiators they need to serve and what the hot water demand is like for a household. The domestic boiler sizes go from 24kW to 40kW for the majority of manufacturers, but some combi boilers can go up to 50kW.
To check what sizes are available right now, click here to go to Boxt Limited, a leading name in the newly installed boiler industry.
*The information in this article should be used for general guidance only and not as financial advice. Full details are on the link in the footer to our disclaimer page. Always discuss your requirements with a competent and suitably qualified professional before undertaking any work.