What Is A Condensing Boiler?

All new gas boilers fitted in domestic properties in the UK are condensing boilers because of a change in the law in April 2005. These new regulations were put in place to help protect the environment, as condensing boilers are much more efficient than older models.

Modern condensing boilers heat our homes and provide us with hot water by burning fuels, just like non-condensing models. However, they’re far more efficient than older boilers as they can recover heat from waste gases that would otherwise be lost.

Ideal Heating has pulled together all the information you need on how condensing boilers work in one handy place. We’ll look at how they’re different to non-condensing boilers and answer your questions about how these efficient appliances work.

How Does a Condensing Boiler Work?

When it’s powered on, a condensing boiler burns fuel to create heat that is transferred to the radiators in your home. One of the features that makes modern condensing boilers so impressive is that they operate in a way that means they waste very little heat. 

Unlike non-condensing boilers, they capture some of the heat energy from waste gases that are produced as they burn fuel. That energy can be used to warm up the water returning to the boiler from your radiators as it cycles through the central heating system — this means less energy is required to restart the process of reheating the water. 

Condensing Boiler Diagram

The image below shows all the parts of a modern condenser boiler and helps us understand how they work. A heat exchanger (2) helps heat the water that provides your hot water, as well as helping cool the waste gases that are produced as fuel is burned. 

Those gases are released through the outdoor pipe (also known as a flue) (1). As the gases are cooled, the latent heat energy they contain is released through condensation — this heat energy is in turn used by your boiler to continue powering your central heating. This process is what gives condensing boilers their name.

What Are the Benefits of a Condensing Boiler?

A modern condensing boiler has some great benefits over older models, but the main advantage they offer is much greater efficiency. They use much less energy to keep us warm, which means they keep our utility bills lower and they are better for the environment. 

Additionally, condensing models are also much smaller than old boilers, so they take up less space in our homes. This means these types of boilers can be installed in more areas, including your kitchen.

What’s the Difference Between Condensing Boilers and Combi Boilers?

All new boilers installed in domestic properties after April 2005 are condensing boilers, whether they are combi boilers, system boilers or heat only boilers. Combi condensing boilers are efficient models that supply your home with both central heating and hot water, all in one complete package. 

Which type of boiler is right for your home depends on a number of factors, and we have a quick and simple Find My New Boiler tool on our website to help you find your Ideal boiler. 

How Do I Know if I Have a Condensing Boiler?

One of the easiest ways to check if you have a condensing boiler is to find out when it was installed — as stated above, all domestic boilers built since April 2005 are condensing boilers, so it might be worth digging out your paperwork to find out when it was installed.

If you find that your boiler is an older non-condensing model, it may be time to think about having it replaced. Many boiler manufacturers and fitters recommend that you change your boiler every 15 years.

Another clue that will confirm whether you have a condensing boiler is checking outside your property to see whether there is steam coming from the boiler flue. Condensing boilers will also have a white plastic ‘condensate pipe’ which gets rid of any excess water from the condensing process. 

What Comes Out of a Condensing Boiler Flue?

A boiler condensing flue is a duct or piece of pipework that safely releases those gases outside. The easiest way to understand how your flue works is thinking of it as a chimney for your boiler, getting rid of all the carbon dioxide and water vapour created by the heating process. 

If your boiler isn’t working efficiently, it can generate harmful substances like carbon monoxide. As such, it’s important to get your boiler serviced regularly to check it’s working safely. Having a carbon monoxide alarm in your home will also provide you with greater protection in case your boiler does start to have problems.

How Much Condensate Water Does a Condensing Boiler Produce?

According to the HHIC (Heating & Hot Water Industry Council), a condensing boiler that’s working efficiently will produce around two litres of condensate water every hour. This water is safely expelled through your boiler’s condensate pipe, which should be directly connected to an external drain.

What Temperature Should a Condensing Boiler Be Set At?

The temperature your condensing boiler is set at will have a direct impact on how efficiently it works. For your boiler to perform at its best, your central heating should be set to a temperature of between 70C to 75C and your hot water should be set to 60C.

Bear in mind that this is the temperature for the water for your radiator and taps, so it’s not the temperature you set at your thermostat. There are very few situations where the temperature of your condensing boiler would need to be changed, for example if it had been set incorrectly during installation. 

Do I Need a Hot Water Tank with a Condensing Boiler?

Condensing boilers don’t necessarily need a hot water tank or cylinder in order to work — that will depend on the type of boiler you have in your home. Combi boilers do not use a hot water tank, but heat only or system models do. 

A combi boiler heats up and supplies water as and when it’s needed, while system and heat only boilers use tanks to store hot water ready for when it’s needed. That’s why boilers that use hot water tanks are better suited to larger properties, where several people will need to access hot water at the same time.

Where Can I Install a Condensing Boiler?

Condensing boilers need to safely get rid of any waste gases and excess water that are produced as they work to heat our homes. That means they need to be installed on an exterior wall, where these substances can be disposed of safely and efficiently.

There are still plenty of options for where you locate your boiler however, whether it’s safely hidden away in a boiler cupboard in the kitchen or up in the loft. You just need to make sure there’s plenty of space for servicing and maintenance, as well as taking a look at the manufacturer’s instructions for information around flueing and ventilation.

Are Condensing Boilers Safe and Reliable?

Modern condensing boilers go through rigorous and extensive safety testing before they are installed in our homes. Any waste gases and excess water that are produced during the condensation process are also disposed of safely through flues and condensate drains.

Like any appliance however, they need to be taken care of to work at their best. That’s why it’s recommended to have your boiler fully serviced at least once a year so essential checks can be carried out by a Gas Safe registered engineer.

Do Condensing Boilers Give Off Carbon Monoxide?

Modern condensing boilers are very safe and produce less harmful gases than older models. Although they can naturally produce small amounts of carbon monoxide, this gas will be safely disposed of outside through the flue pipe — this is why a condensing boiler should be installed on an external wall.

However, any boiler that falls into bad condition can leak carbon monoxide if it’s failing to burn fuel properly or its flue pipe becomes blocked.  That's another reason to make sure your boiler is serviced regularly, making sure essential safety checks are carried out. It’s always a good idea to have a carbon monoxide alarm in your home for extra piece of mind.

How Long do Condensing Boilers Last?

Boiler manufacturers and installers often recommend that modern condensing boilers are replaced every 15 years. Over time, boilers will naturally become less efficient as they work hard to keep us and our homes warm. That means you’ll actually save money in the long-run by replacing an older model that is struggling to operate efficiently. 

Have other boiler-related questions? Take a look at our Tips and Advice page for further guides.