What Is a Combi Boiler?
A combi boiler, also known as a combination boiler, is a heating system that provides hot water as well as central heating. It got its name because it was originally intended to combine all of the components of a traditional central heating system into a single compact unit.
How does a Combi Boiler Work?
A combi boiler works by extracting latent heat from the flue gases and using condensing technology to efficiently heat the system water. When hot water or heating is required, the combi boiler’s gas burner is ignited. The heat from the flame is then transferred to the primary circuit via the main heat exchanger and directed to the radiators or your hot water taps via a plate heat exchanger.
Many modern combi boilers also include a pre-heat system, which allows the boiler to store a small amount of water that can be quickly drained. This water is kept at the temperature you specify throughout the day and is ready to be delivered to your hot water tap when needed.
What size combi boiler do I need?
When choosing a combi boiler, it’s crucial to select the right size for your home. The output rating, which is an indicator of the boiler’s power, must match your hot water demand to avoid any inefficiencies. If the output rating is too low, the boiler won’t be able to meet your hot water needs. On the other hand, if it’s too high, it will result in higher energy costs.
Combi boilers have two output ratings, one for central heating and another for hot water. To determine the central heating output rating you need, refer to the following table based on the number of radiators in your home:
- Up to 10 radiators: 24-27 kW
- 10-15 radiators: 28-34 kW
- 15-20 radiators: 35-42 kW
It is also critical that the combi boiler can handle the necessary pressure from the main supply of water. A combi boiler heats both central heating water and domestic hot water. If the mains water pressure is too high, the boiler may be unable to cope, resulting in problems such as low water pressure, leaks, or even boiler damage. If the mains pressure is too low, the combi boiler may struggle to deliver the hot water you require, resulting in lukewarm showers or a complete lack of hot water.
To ensure that your combi boiler runs efficiently and effectively, select a model that can handle the flow of water from the mains at the required pressure.
Benefits of Combination Boilers
Combi boilers are a popular choice for many homeowners due to their numerous advantages, which include:
Compact Size
One of the advantages of a combi boiler is its small size. Because there are no external storage tanks, the entire system, including heating and hot water, is controlled by the same appliance.
Efficiency
The efficiency of combi boilers is another advantage. Some modern combi boilers include a flue gas heat recovery system, which collects the gas produced by the combustion of gas or oil and uses it to heat water from the mains. This allows the system to use less fuel and achieve an efficiency of more than 90%, lowering a household’s heating bills.
Money Saving
When compared to a conventional system, which typically requires tanks, cylinders, pumps, and so on, purchasing a single combi boiler unit can be less expensive than purchasing multiple system components.
Hot water on demand
Combi boilers are also a good option because they can quickly heat water when needed. Unlike system boilers and regular boilers, which heat and store predetermined amounts of water in advance, combi boilers heat water as needed. As a result, a combi boiler can provide a constant supply of hot water.
Disadvantages of Combi Boilers
While combi boilers have numerous benefits, there are a few factors to consider before purchasing one:
Unsuitable for larger properties
The amount of water that a combi boiler can provide at any given time is limited by the size of the boiler. Only one tap or outlet can be used effectively at a time on average. While a combi boiler will serve many homes perfectly, larger homes with multiple bathrooms may require hot water at a rate that a combi boiler cannot meet. Because of its external hot water tank and mains pressure hot water supply options, a system boiler may be more appropriate in these cases.
Good mains pressure is required.
Another disadvantage of combi boilers is that because they draw water directly from the mains, water pressure can quickly drop if multiple taps and showers are turned on at the same time. Larger homes are often better suited to regular or system boilers that draw water from a hot water cylinder.
Recommended System boilers based on budget
Main Eco Compact System Boiler
The Main Eco Compact System boiler is designed to fit in a kitchen cupboard and is suitable for homes with more than one bathroom in order to satisfy hot water needs. Prices start from approximately £700 and are a great entry level boiler
Features
- Available in 25kW or30kW
- 5-years parts and labour warranty
- Boiler Plus compliant
- Light 29.5kg weight
- Compact cupboard fit
- Low NOx emissions – rated class 5
- Top central flue
- Optional stand off frame
- Rigid filling link
- Certified to work with 20% hydrogen
- Dimensions H 700 x W 390 X D 285
Baxi 600 Combi Boiler
This compact, powerful boiler can be operated using smart controls through an app on your phone or tablet.. Prices start from approximately £900 and are a great mid level boiler
Features
- 7 year parts and labour warranty*
- Built in the UK using top quality brass components
- Fits inside a standard kitchen cupboard
- EasyFill makes it easy to top up the central heating system pressure without the need to call an engineer
- Dimensions H 700 x W 390 X D 285
Vaillant ecoFIT pure combi boiler
The cupboard-size combi boiler is ideal for any size home. Prices start from approximately £1200 and are a great mid to high level boiler
Features
- Available in 25, 30 or 35kW heating outputs
- Compact design to fit in a kitchen cupboard
- Hand-built in Derbyshire manufacturing plant
- Quiet Mark approved, meaning it’s one of the quietest products in its category
- Guarantee up to 10 years available when fitted by a Vaillant Advance installer**
- High performance build, smart controls and efficiency
- A flow rate of up to 14.1 litres per min for fast instant hot water
- ErP A rated for both hot water and heating efficiency
- Dimensions in mm: H 700 x W 390 x D 295
Frequently Asked Questions
How Long Will a Combi Boiler Last?
Combi boilers typically have a lifespan of 10 to 15 years, depending on the model and how frequently they are serviced. Regular
Is a combi boiler electric or gas?
Combi boilers can be powered by gas or electricity, with gas models being the most common. Electric combi boilers are less popular because they have higher running costs than gas models.
What is the purpose of my combi boiler and water tank?
It could be due to the age of your home or the size of your household that you have both a combi boiler and a water tank. Some older homes may have had a tank and boiler combination installed, whereas larger households may require a hot water storage tank to meet their hot water demands.
What is a system boiler?
A system boiler is a type of central heating boiler that is similar to a standard boiler but has additional features such as a pressurised hot water cylinder. A system boiler, as opposed to a combi boiler, requires a separate hot water tank.
How can I tell what kind of boiler I have?
Examine its components and the location of its hot water tank, if any, to determine the type of boiler you have. You most likely have a conventional or system boiler if you have a hot water tank in a separate cupboard or loft space. You may have a combi boiler if you do not have a separate hot water tank.
What are the most common issues with combi boilers?
Some of the most common issues with combi boilers are as follows:
- Water pressure is low.
- Kettling noise
- Leaks
- Diverter valve malfunction
- Thermostat failure
- Heat exchanger is blocked up
- Pump failure
Is it best to keep a combi boiler on at all times?
It is not necessary to leave a combi boiler on constantly. In fact, it is more energy efficient to only turn on the boiler when hot water is required.
How often should combi boilers be serviced?
It is recommended that you service your combi boiler once a year to ensure that it is operating safely and efficiently. This can also help to keep potential problems from becoming more serious
Can a combi boiler run on hydrogen?
Some combi boilers are built to run on hydrogen. This technology, however, is still in its early stages of development and is not widely available.
Are combi boilers being phased out?
No, combi boilers will not be phased out. Because of their space-saving design and efficiency, they continue to be a popular choice for many households. The 2025 Boiler Ban however will prohibit the installation of combi boilers for new developments and new build properties
Are combi boilers electric or gas?
Combi boilers typically refer to gas powered boilers. But there are electric boilers available on the market that can provide both hot water and heating.
Can combi boilers run underfloor heating?
Yes, combi boilers can run underfloor heating, but this is dependent on the model and whether it is designed to do so.
Can combi boilers leak carbon monoxide?
Yes, combi boilers, like all gas appliances, have the potential to leak carbon monoxide if not properly installed, maintained, and vented.
Can combi boilers freeze?
Yes, combi boilers can freeze in extremely cold temperatures, especially if they are in unheated areas or are not properly insulated.
Can combi boilers explode?
While any boiler can explode, combi boilers are generally safe when installed and maintained properly.
Can I change a back boiler to a combi boiler?
It is possible to replace a back boiler with a combi boiler, but it can be complicated process that typically takes 2-3 days to complete
How can I tell if my boiler is a combi boiler?
The simplest way to determine whether your boiler is a combi boiler is to look for a separate hot water cylinder. If it doesn’t, it’s most likely a combination boiler.
Does a combi boiler use electricity?
Yes, electric power is used by combi boilers to power their electronic controls and ignition system.
What causes combi boilers to lose pressure?
Pressure can be lost in a combi boiler for a number of reasons, including a leak in the system, a faulty pressure relief valve, or air in the radiators.
What is the procedure for resetting a combi boiler?
Resetting a combi boiler varies depending on the model, but it usually entails turning off the power and pressing a reset button or adjusting a control.
What is the proper way to drain a combi boiler?
To drain a combi boiler, turn off the power, turn off the water supply, and connect a hose to the drain valve to release the water. Depending on the model, the exact procedure may differ.