What is a gas condensing boiler?

Understanding gas condensing boilers

So firstly, what is a condensing boiler? A condensing boiler is one that uses condensing technology to become more efficient and reuse some of the heat that is generated by burning gas, oil and other fuels. They are more efficient because they use the heat from the flue gases that would otherwise be wasted and put it back into the system to further warm the cold water. In doing this, they can be up to 99% efficient.

How does a condensing boiler work?

A condensing boiler works much the same as any other boiler:

1. Gas enters the combustion chamber and is burned to create heat.
2. As the gas heats up, it travels into the heat exchanger.
3. Within the heat exchanger is a series of pipes surrounded by cold water. The heated gas moves through the pipes, gradually heating the water as it moves. 
4. The heated water can be pumped around your home or delivered to a hot water tank for storage. In the case of a combi system, it is delivered straight to your taps/showers when they’re turned on.
5. As the water moves round your heating system and begins to cool, it is pumped back to the boiler to be heated again. 

densing boiler works specifically by capturing the hot flue gases and ‘recycling’ them back into the heating system. When this happens, the flue gases can drop in temperature from 130℃ to 50℃, causing condensation. This is where the name ‘condensing boiler’ comes from, and as much as two litres of water an hour can be created from the process. It exits the system via your home’s condensate pipe.

What’s the difference between a combi boiler and a condensing boiler?

A combi, or combination, boiler is one that provides both heating and hot water within a single unit. These systems don’t usually require a hot water tank, as the boiler heats the water you need on demand. The combi boiler is able to heat the mains water internally and deliver it straight to your taps and shower when you turn them on.

While perhaps not ideal for larger households with a big demand for hot water, they’re ideal for smaller homes with fewer people. With a combi boiler, you can never run out of hot water, so say goodbye to cold showers for good. Their lack of a water tank could save you space, too. 

All modern combi boilers are condensing, so there’s no ‘difference’ as such between the two. You can get condensing conventional and system boilers too, and so the term simply explains the effective condensing process these boilers undergo. 

Do I need a condensing boiler?

Since 2005, UK building regulations have made it compulsory for all new gas boilers to have condensing technology, so if you’re purchasing a new boiler, it will need to be a condensing one. 

This is no bad thing, however. Thanks to their increased efficiency, these boilers use less gas to heat your home and can therefore save you money in the long run. According to the Energy Saving Trust, you could save over £300 a year by replacing an old G-rated non-condensing boiler with a new high-efficiency condensing boiler and improving your heating controls.

It’s also thought they may be safer to run than older, non-condensing boilers. 

Why is a condensing boiler safer?

Boilers require oxygen in order to safely burn the gas that is used to heat the water. While condensing boilers take this oxygen from outside the room it’s located in, non-condensing boilers take it from inside the room. This increases the risk of anything being sucked into the boiler, making them slightly less safe. 

A condensing boiler may be considered safer as it pulls in oxygen from outside, but also because it disposes of the condensed liquid via the condensate pipe. This reduces your risk of coming into contact with toxic substances. 

Common condensing boiler problems

As with any heating system, condensing boilers come with their own set of problems and so there is a list of pros and cons to consider. However, what problems are specific to only condensing boilers?

These boilers produce a fair amount of condensation, which generally drains via the condensate pipe. As this pipe exits via an external wall and is often uninsulated, it can freeze over and stop the water from getting out. This can prevent your boiler from working altogether, so can be a reason for a boiler fault code or cold radiators. 

What size condensing boiler do I need?

When sizing any boiler for your home, you need to consider the number of rooms/radiators you have, as well as how much hot water you’re likely to use. 

Generally, if you live in a one-bedroom, one-bathroom house or apartment and have fewer than five radiators, a 9 to 18 kW condensing boiler should be sufficient. For a two to three-bedroom house with two bathrooms and between five and 10 radiators, a 24 to 27 kW condensing boiler should be a good size. 

If you’d prefer a combi boiler, a 24 to 30 kw combi is likely a good option for most standard two to three bedroom houses with one bathroom. 

For more information on boiler sizing and additional help, you can read our guide on what size boiler you need.