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How Does a Combi Boiler Work? [Boilers Explained]


Lee Devlin

Published on: 23/06/2023

What is a combi boiler?

Before we delve into how a combi boiler works, it’s important to establish what it is exactly. A combi boiler, otherwise known as a combination boiler, is a highly efficient and compact boiler that acts as both a water heater and a central heating unit. With a combi boiler, hot water is supplied directly from the mains and is heated on demand using a gas burner and heat exchanger. This makes it highly energy efficient and cost-effective, as you only ever pay for the water you use.

Combi boilers are also a great choice for smaller households due to their space-saving features and quick reaction times.

Modern combi boilers are incredibly reliable and can be used to heat water whenever needed, making them the most popular type of boiler in the UK.

How does a combi boiler work?

A combi boiler is a single unit that combines a water heater and a central heating boiler system. When you turn on the hot water through a tap, this will signal the boiler to start heating water inside the system and the hot water supply will begin quickly.

So how does a combi boiler work? The heat exchanger inside the combi boiler transfers the majority of the heat from the burnt gas inside the boiler to the cold water, making it hot, and then delivers the hot water to the taps as required.

The combi boiler is powered by either mains gas, oil, or LPG, which enters a sealed combustion chamber inside the boiler and an electric ignition sets it alight. The primary heat exchanger within the boiler is then heated up to heat incoming water up to 60 degrees, flowing through all the radiators and heating up your home. While the primary heat exchanger heats and recycles the water that goes through the pipes into the radiators, the secondary plate heat exchanger heats up the hot water that goes into your taps.

The control valves in the combi boiler will operate in different directions, either allowing water to flow through the central heating system or diverting it to the hot water tap as required. This eliminates the need for a storage tank that is required in the case of a conventional boiler.

What are the advantages of having a combi boiler?

1. Greater Energy Efficiency and Lower Costs

In comparison to conventional boilers, a combi boiler installed can provide several advantages in terms of energy efficiency and saving you money.

Combi boilers are incredibly efficient, boasting an efficiency rating of over 90%. The energy-efficient heat exchanger in combi boilers is designed to help maximise energy efficiency and reduce energy costs. It works by using the waste heat from the boiler to pre-heat the water before it enters the boiler, using the waste energy to heat the water more efficiently. This reduces the amount of energy required to heat the water, resulting in lower bills and fewer carbon emissions.

Additionally, combi boilers provide on-demand hot water, so you will never have to wait for hot water to be heated up, saving you both time and money.

2. Easier Control and Regulation

Having a combi boiler makes controlling and regulating the boiler easier due to its easy-to-use controls. A central heating pump or hot water storage cylinder is not required in a combi boiler, which means the need for complex plumbing is eliminated. This means the installation time is also reduced.

Combi boilers also come with temperature and time controls, allowing you to program the system to turn on when needed, giving you more control over your heating costs.

Furthermore, the lack of a water storage tank and hot water storage cylinder means that there will be no long wait times for hot water, allowing you to have hot water on demand.

3. Compact Design and Installation

The compact design and ease of installation of a combi boiler provide numerous benefits to homeowners. Firstly, its size makes it ideal for smaller homes where there may not be room for bulky tanks and cylinders.

In addition, it is easier to install than traditional boiler systems, often taking less than six hours to complete. A combi boiler does not require the installation of any additional tanks or cylinders, which can take up valuable space. Furthermore, its quick and easy installation saves on labour costs and simplifies the process of replacing an existing boiler.

4. Variety of Models and Sizes

The models and sizes of combi boilers vary, but they generally come in a range of sizes from 24-34 kW, with a variety of output speeds. They also come in a range of sizes to suit different-sized households and can be wall-mounted or floor-standing.

5. Reduced Carbon Footprint

Having a combi boiler can help reduce the carbon footprint by being more energy efficient. Combi boilers are proven to be 90%+ efficient which means less heat is wasted, resulting in fewer unnecessary carbon emissions.

Furthermore, combi boilers don’t burn fuel to produce central heating and hot water, reducing the use of fossil fuels which are a leading cause of climate change. By using an electric combi boiler with solar PV, running costs can also be reduced.

6. Heating, Hot Water, and Central Ventilation in One Unit

A combi boiler combines a water heater and central heating into one single unit, meaning no additional hot water tank is required. This makes it a more compact solution than other units and is ideal for commercial premises with limited space.

Secondly, they provide excellent water pressure with no need to install a pump, and instant hot water supply with no long wait times.

7. Safety and Security Benefits

Combi boilers are also eligible for UK government schemes, allowing you to benefit from additional financial support. It also comes with built-in safety features, including pressure and temperature control, to ensure the safe operation of the system.

10. Warranty and Maintenance Costs Reduced

Regular servicing is important to ensure the boiler is working efficiently. Many combi boilers come with an annual boiler service included in their warranties, so you don’t have to worry about extra costs for maintenance.

Heat pump technology is also becoming increasingly accessible and affordable, and is an efficient way to keep your home warm in the long run.

What are the things to consider when buying a combi boiler?

1. Quality of Fuel

When considering the purchase of a combi boiler, it is important to consider the quality of the fuel that will be used to power it. Different fuel sources, such as gas, oil, LPG, and electricity, can all be used to power a combi boiler. While each fuel source has its pros and cons, it is important to ensure that whichever fuel is chosen is of high quality, as this can have a major impact on both the efficiency of the boiler and its running costs.

High-quality fuels burn more efficiently and produce fewer emissions, helping to reduce energy costs and make the home greener. Quality fuels also help to ensure that the boiler is running smoothly and efficiently, ensuring that it lasts longer and requires fewer boiler repairs.

2. Energy Efficiency Rating

The energy efficiency rating of a combi boiler is an important factor to consider when making the decision to buy one. A combi boiler that has a high energy efficiency rating means that for every £1 spent on heating your home, only 7p is spent on wasted energy. This can result in considerable savings compared to conventional boilers which have an efficiency rating of around 70%.

Combi boilers with an A-rated efficiency can save you up to 40% on your energy bill, making them a very cost-effective solution. Furthermore, less energy will be wasted which will result in fewer unnecessary carbon emissions, contributing to a greener environment.

3. Size and Capacity

When choosing a combi boiler based on size and capacity, several considerations should be taken into account.

Firstly, the size of the home must be taken into account. For a smaller property with one bathroom and up to ten radiators, a 24-27kw combi boiler would be suitable. If a house has four bedrooms, one or two bathrooms (including ensuites) and up to fifteen radiators, a 28-34kw combi boiler should be used. For larger homes with two bathrooms and up to twenty radiators, a 35kw or 42kw combi boiler would be recommended.

It is also important to consider the hot water demand in the household, as well as the existing system, as this will affect the boiler size and type. If multiple people are demanding hot water simultaneously, a higher output boiler is recommended, whilst a conventional boiler would be better suited for larger households with varying hot water needs. The number of bathrooms in the house is also a factor to consider, as multiple bathrooms will require a larger boiler to maintain the water pressure.

In general, combi boilers are compact and efficient, providing hot water on demand and lasting up to 15 years. Conventional boilers, however, are better suited for larger households, as they can provide hot water to multiple bathrooms simultaneously.

Any More Boiler Questions?

If you have any other boiler-related issues, get in touch! Homecure Plumbers are certified gas engineers and we’ve repaired and installed on all the most popular boiler brands. Let us know how we can help!

Lee Devlin
Publication date: 23/06/2023

Lee Devlin is the managing director of Homecure Plumbers, London's favourite local plumbing service. Since 2009, Lee's been leading a large team of gas safe plumbing and heating engineers as well as growing a successful business built on quality customer service and workmanship, in a competitive industry within the Capital. He's been in the game for a long time, so he regularly shares his knowledge and experience with the public. His expert tips have been featured in lots of well known publications such as: The Sun, Metro, Insider, Homes And Gardens, Country Living and more!

Read More About Lee Devlin.

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