Condensate trap

The condensate trap is a device inside a condensing boiler which collects water from the condensing process before it is expelled into the sewer via the condensate pipe. The purpose of the trap is to prevent toxic gases from entering the condensate pipe, and to manage the flow of condensate fluid out of the boiler.

Condensate trap overview

The condensing process

When a boiler burns a fuel, one of the end products of the combustion process is water vapour. In a non-condensing boiler, this water vapour is simply expelled out of the flue. However, the water vapour is hot: it carries heat energy. This is why non-condensing boilers generally only have an energy efficiency rating of 50-80%. For every £1 you spend on fuel, 20 to 50p’s worth of heat is simply being pumped outside.

This is where condensing boilers come in. Condensing boilers draw the heat energy out of that water vapour to the extent that it condenses and turns into a liquid – water. By recovering this heat which would otherwise be wasted, condensing boilers typically have an energy efficiency rating of around 90%.

More on this topic: Condensing vs. Non-condensing boilers – what’s the difference?

The condensate trap itself

This water, or condensate as its known, is then expelled into the sewer via the condensate pipe. But before this happens, the water is collected in the boiler’s sump. It then flows into the condensate trap.

The trap has two purposes. Firstly, the water forms a barrier between the condensate pipe and the combustion areas inside. This is important because noxious fumes should always be expelled via the flue, not the condensate pipe. Think of it in the same way as the trap in your toilet or sink, which also uses a body of water to prevent the movement of fumes – in that case, to prevent bad smells from the sewer entering your your home.

The other purpose of the trap is to control the flow of condensate exiting the boiler. This is important because in the winter months, condensate pipes have a tendency to freeze and block. The boiler will then detect that it cannot expel condensate, and will shut itself down.

Frozen condensate pipes are often a result of a constant drip-drip-drip of water. To prevent this, condensate traps have an auto-siphon. Once the water in the trap reaches a certain level, the siphon is activated and all the water is siphoned out of the trap. By releasing the condensate in bursts like this, it’s much less vulnerable to freezing, and the warmth of the water should shift any ice which has started to form.

Condensate trap types

Generally speaking, there are two different kinds of condensate trap – those with an auto-siphon, and those without. The ones without may simply be in the form of a U-bend shape of pipe.

However, regardless of type, condensate traps are always made out of plastic, like other boiler components which handle condensate. This is because condensate itself is slightly acidic and would corrode metal-based fittings.


The boiler should never be operated without the condensate trap in place, as it prevents noxious combustion gases from travelling down the condensate pipe. Should the trap have to be removed from the boiler, it must be topped up with water when it’s replaced. This will ensure the presence of the water barrier between the condensate pipe and the boiler’s combustion chambers.

Note that it is illegal for someone who is not listed on the Gas Safe register to work on a gas appliance, such as a gas boiler.