Most of us take our boilers for granted, until it is time to service, repair or replace them. If you’ve decided it’s time to shop around for a new boiler, you may suddenly find yourself caught out by all the different types of boiler that suddenly appear to be available. If you live in an average sized residential property, an engineer or heating supplier may well have recommended that you choose a combi boiler. But what exactly is a gas combi boiler and how does it work? Read on to find out all you need to know.

What is a combi boiler?

Gas combi boilers - the basics

Why should I choose a gas combi boiler?

Gas condensing combi boilers are are by far the most popular choice of boiler in the UK. They are usually recommended because for the average household, they are the most efficient way to provide hot water and heating. Comprising or only one single unit, they are very space efficient and with new technology it is now possible to save even more space with an ultra compact model. Combi boilers are also very simple to install, operate and maintain.

Why should I choose a gas combi boiler?

In a gas combi boiler, the word ‘combi’ is short for ‘combination’. It is called a combination boiler because it is a single unit which serves two functions - heating and hot water. One of the key attractions of a combi boiler is that no separate water heater is required. Many older homes had an immersion heater located upstairs in an airing cupboard, supplied by a header tank in the attic. When hot water was required, it was necessary to heat up the immersion heater in advance. As well as being much less efficient than a combi boiler, this system required a lot of space and was impractical.

Understanding how combi boilers work

Why are gas combi boilers more efficient?

The main reason that combi boilers are more efficient is that water is only heated when you need it. They are designed to maximise the amount of energy that is directly used to heat the water, and there is no need to heat up and store more water than is required. This helps to reduce your carbon footprint and also cut down on your energy bills.

How does a combi boiler work?

A combi boiler works by taking water directly from the mains. When you turn on a hot water tap, the gas burner is ignited and a powerful system called a heat exchanger transfers the energy to the water. This system allows hot water to be generated almost instantly.

To help deliver hot water even more quickly, modern boilers also have a built in pre-heat system. This is a small reservoir of water that is always kept at a certain temperature and means that you will instantly have hot water when you turn on a tap. Most boilers will turn on the burner around once an hour to keep this reservoir hot, which is why you may notice your boiler turning on even when nobody is running a tap. It is possible to increase your boiler’s efficiency even more by turning off this function, however in most domestic environments this is not recommended.

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What is condensing technology?

Building regulations in the UK now require all gas combi boilers to incorporate condensing technology. These rules were introduced to help homeowners benefit from dramatically increased efficiency. With older boilers, as much as 50% of heat energy was lost as the combustion gasses escaped into the flue, taking their heat with them. In a condensing boiler, on the other hand, the heat contained within the combustion gasses is captured within the system and used to preheat cold water as it enters the system.

Condensing technology helps to protect the environment as it reduces your home’s carbon footprint by reducing CO2 emissions. It can also save you a lot of money on your fuel bills by making better use of your energy.

Choosing the right combi boiler

What size combi boiler do I need?

There are several factors to consider when deciding which size gas combi boiler to buy. The most important factors are generally how much demand there is likely to be for hot water and how many radiators you will be heating.

As a rule, a 20-35kW combi boiler will generally be sufficient for an average 3-4 bedroom house, whilst those with ensuite bathrooms may find that a 30-35kW boiler is more suitable. Much bigger houses will typically need a larger 35-42kW boiler, although it may be worth considering a system boiler as an alternative.

What else should I consider when choosing a combi boiler?

When choosing a gas combi boiler, it is also worth considering what options are available to control your heating. Many of the latest boilers have computerised controls and new generation thermostats that can help you maximise efficiency and save money on your heating bills.

Also think about whether you’d like to have access to smart functionality, which allows you to control your heating remotely, usually from an app on your smartphone.

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