What is the difference between a condensing and a non condensing boiler?

A condensing boiler recovers heat from its flue gases by cooling them until the water vapour condenses, and drains the resulting water through a condensate pipe. A non condensing boiler sends that heat out of the flue. Condensing boilers use noticeably less fuel for the same heat.

Is a combi boiler a condensing boiler?

Almost every combi fitted in the UK since 2005 is a condensing combi. Condensing describes how the boiler burns fuel; combi describes how it supplies hot water. System and regular boilers are condensing too.

Why is there steam coming out of my boiler flue?

It is water vapour, not smoke, and it is normal for a condensing boiler, especially on cold days. It becomes a concern only if the flue is damaged, blocked, or the plume is accompanied by fumes, soot or a carbon monoxide alarm.

Why does my condensing boiler stop working when it is freezing?

Most often because the condensate pipe outside has frozen. The condensate cannot drain, so the boiler locks out. Thaw the pipe with warm, not boiling, water and reset the boiler. If it keeps happening, the pipe needs rerouting or insulating.

Can I still get a non condensing gas boiler?

Not as a normal replacement. Building regulations have required new gas boilers in England and Wales to be condensing since 2005, and exemptions are rare.

Do condensing boilers work with a hot water cylinder?

Yes. A condensing system or regular boiler heats a cylinder through the coil inside it. It condenses a little less while reheating the cylinder, because the water has to be hotter, but it still works efficiently overall.

What Is a Condensing Boiler? And Why Yours Is Probably One

A condensing boiler is a gas or oil boiler that captures extra heat from its own exhaust gases by cooling them until the water vapour in them turns back into liquid. That recovered heat goes into your heating and hot water instead of up the flue, which makes a condensing boiler noticeably more efficient than the older kind. Every new gas boiler fitted in England and Wales since 2005 has had to be a condensing model, so if yours is less than about twenty years old it almost certainly is one. Condensing is not a type of boiler in the way combi or system is: a combi, a system boiler and a regular boiler can each be condensing.

How a condensing boiler works

When gas burns it produces hot exhaust gases that contain a lot of water vapour. In an old boiler those gases went straight out of the flue while still very hot, taking a good share of the heat with them.

A condensing boiler has a larger, more efficient heat exchanger, and the cooler water coming back from the radiators is passed through it first. That cool return water chills the flue gases until the vapour in them condenses on the metal. Turning vapour back into water releases heat, and the boiler picks that heat up too. Our guide on what is a heat exchanger explains the part doing that work.

The by product is a steady trickle of water, called condensate, which is mildly acidic. It runs out of the boiler through a plastic condensate pipe to a drain, a waste pipe or a soakaway. That pipe is the one thing every condensing boiler has and older boilers do not.

Condensing boiler or combi: a false choice

People often ask whether they should get a condensing boiler or a combi, but the two words describe different things. Condensing describes how the boiler burns fuel and recovers heat. Combi, system and regular describe how the boiler gives you hot water.

A condensing combi heats radiators and makes hot water on demand with no cylinder. See how does a combi boiler work.
A condensing system boiler heats radiators and a hot water cylinder, usually an unvented one, with the pump and expansion vessel built in. See what is a system boiler.
A condensing regular boiler, also called heat only, heats radiators and a cylinder fed from tanks in the loft. See our guide on the heat only boiler.

So the real decision when a boiler is replaced is combi, system or regular. Whether it condenses is already settled, because a new gas boiler has to. Our guide on combi or system boiler with a cylinder helps with that choice.

How to tell if your boiler is condensing

You can normally tell in a minute or two, without opening anything.

Look for a plastic pipe, usually white or grey, coming out of the bottom of the boiler and running to a drain, a sink waste or outside. That is the condensate pipe.
On a cold day, look at the flue outside. A condensing boiler gives off a visible white plume of vapour while it runs. That is normal and not smoke.
Check the age. A gas boiler installed since 2005 will be condensing unless it was a rare exemption.
Check the data badge or manual, which will usually say condensing, or give an efficiency figure of around 90 percent or more.

Is a condensing boiler actually more efficient?

Yes, but only when it is allowed to condense. The key is the temperature of the water coming back to the boiler. It needs to be cool enough, roughly below the mid fifties in degrees C, for the flue gases to condense. If the boiler is set to send water out very hot, the return water is too hot as well, and the boiler behaves much like an old non condensing one.

That is why turning the boiler flow temperature down on the heating side often saves gas without making the house any colder, especially in milder weather. Our guide on what temperature should I set my boiler covers the sensible settings for radiators and hot water.

Hot water is a partial exception. A hot water cylinder needs to reach around 60 degrees to stay safe from bacteria, which means the boiler has to run hotter while it reheats the cylinder, so it condenses less during that time. Our guide on what temperature should a hot water cylinder be explains why that figure matters. A combi making hot water for a tap does condense well, because the incoming mains water is cold.

The winter problem: a frozen condensate pipe

The most common condensing boiler breakdown in a cold snap has nothing to do with the boiler itself. Where the condensate pipe runs outside, the slow trickle of water inside it can freeze in sub zero weather. The ice blocks the pipe, the condensate backs up into the boiler, and the boiler shuts down to protect itself. You lose heating and hot water at exactly the moment you need them.

Typical signs are a boiler that locks out or shows a fault code on a freezing morning, sometimes with a gurgling sound from the boiler or the pipe. If the pipe runs outside and it is below zero, this is the first thing to suspect.

Follow the plastic condensate pipe from the boiler to where it goes outside, and find the section most exposed to the cold, usually a bend or the open end.
Pour warm water, not boiling, slowly over that section, or hold a hot water bottle or warm cloth against it. Boiling water can crack the plastic.
Once it has thawed and water can flow, reset the boiler following our guide on how to reset a boiler.
Do not try to thaw it with a flame or a heat gun.

If it freezes every winter, the fix is permanent: reroute the pipe inside where possible, or upsize and insulate the outside run. That is a small job compared with repeated winter breakdowns, and worth doing before the cold weather arrives.

Other faults that are specific to condensing boilers

Because a condensing boiler handles water in its exhaust side, it has a few weak points an older boiler did not.

A blocked condensate trap. The trap inside the boiler collects debris over time and can block, which causes the same lockout as a frozen pipe. Clearing it is part of a proper boiler service.
A failed condensate pump, where the boiler is too far below a drain for the condensate to run by gravity.
A clogged main heat exchanger. The narrow waterways that make condensing possible are also easy for sludge to block, which leads to kettling, overheat lockouts and poor hot water. Our article on how long should a boiler last covers why clean system water matters so much.
Flue and plume problems. The plume is harmless, but a flue terminal positioned where it blows vapour at a window or a neighbour can be a nuisance, and a flue must never be boxed in or blocked.

If your boiler stops this autumn or winter

The first cold week of the heating season is when a lot of boilers show the faults that built up quietly over summer. If your boiler has stopped, start with the simple checks in our guide on no heating or hot water, then look at the condensate pipe if it is freezing outside. It is also worth making sure frost protection is working if you have pipes or a boiler in a cold loft or garage; our guide on the frost stat explains how that is meant to work.

Anything inside the boiler casing on a gas boiler is work for a Gas Safe registered engineer, and you can check anyone you use on the register before they arrive. If your boiler has locked out, keeps faulting or your hot water has stopped and you would like it looked at properly, call us on 020 4652 5111 or send an enquiry. Tell us the boiler make, whether you have a cylinder and what the display shows. We have 20 engineers across London, and our rates are on the pricing page, agreed before anyone attends.

Questions we get asked

What is the difference between a condensing and a non condensing boiler?: A condensing boiler recovers heat from its flue gases by cooling them until the water vapour condenses, and drains the resulting water through a condensate pipe. A non condensing boiler sends that heat out of the flue. Condensing boilers use noticeably less fuel for the same heat.
Is a combi boiler a condensing boiler?: Almost every combi fitted in the UK since 2005 is a condensing combi. Condensing describes how the boiler burns fuel; combi describes how it supplies hot water. System and regular boilers are condensing too.
Why is there steam coming out of my boiler flue?: It is water vapour, not smoke, and it is normal for a condensing boiler, especially on cold days. It becomes a concern only if the flue is damaged, blocked, or the plume is accompanied by fumes, soot or a carbon monoxide alarm.
Why does my condensing boiler stop working when it is freezing?: Most often because the condensate pipe outside has frozen. The condensate cannot drain, so the boiler locks out. Thaw the pipe with warm, not boiling, water and reset the boiler. If it keeps happening, the pipe needs rerouting or insulating.
Can I still get a non condensing gas boiler?: Not as a normal replacement. Building regulations have required new gas boilers in England and Wales to be condensing since 2005, and exemptions are rare.
Do condensing boilers work with a hot water cylinder?: Yes. A condensing system or regular boiler heats a cylinder through the coil inside it. It condenses a little less while reheating the cylinder, because the water has to be hotter, but it still works efficiently overall.