Category - Boilers
Guide author photo

Will Scholfield, Engineer

20 Oct : Updated 18 Apr ● 5 min read

Boiler condensate pipes - What are they and what do they do?

If you’re stuck when it comes to knowing your flues from your thermostats and your combi boilers from your condensing, you’re definitely not on your own. Knowing how to identify every pipe, button, and outlet that's attached to your boiler can be tricky.

A repeat offender when it comes to confusing homeowners is condensate pipes. What are they? What do they do? And why are they important?

In this guide, we’ll walk you through everything you need to know about your boiler’s condensate pipe, cutting through the jargon so you can feel confident about the inner workings of your home’s central heating system.

Firstly, what is a condensing boiler?

Before we take a closer look at condensate pipes, we first need to understand what condensing boilers are. 

Condensing boilers are a highly efficient type of home heating system. They are designed to maximise the heating potential of the energy they burn by minimising the amount that’s lost when generated as a byproduct. The result? Condensing boilers are able to give you more hot water per penny spent, improving your home’s energy efficiency and helping to reduce your carbon footprint in the process.

To do this, condensing boilers capture the products of combustion (fumes) that are generated through the combustion of natural gas, LPG, or oil (depending on the boiler’s fuel type) via the heat exchanger. On condensing boilers, these heat exchanges extract more heat from the products of combustion due to their design. So much so that the gases turn, or ‘condensate’, back into liquid form. This condensate water is mildly acidic.  Condensing appliances can produce up to 3 litres per hour. All of this water has to be collected and removed from the boiler’s system, and this is where condensate pipes come in. 

What does a condensate pipe do?

The condensate pipe is an outlet, usually made from plastic or PVC, that transports acidic wastewater (condensate) from your boiler to drains located outside your home.

When it’s fired up and running, your boiler can generate an impressive 2-3 litres of condensate waste per hour, all of which need to be removed from the system to keep everything operating smoothly and safely. 

The condensate pipe is designed to release water once the volume hits a certain threshold and is usually released in around 300ml increments. This is to reduce the risk of the water freezing in the part of the pipe that's outside your home, which could then cause a blockage.

The condensate pipe will usually come out of the back of your boiler, down the external wall, and into the drain. It could also terminate into a waste pipe internally (a sink waste pipe, for example). It's a small, usually-white pipe, so It should be easy to spot amongst the metal pipes.

What happens if a boiler’s condensate becomes blocked?

Over time, sediment can start to build up in your condensate sump pump and/or pipe. The U bend or syphon-shaped 'trap' part of your condensate pipe, which is responsible for safely collecting and releasing the wastewater, is usually where the sediment will sit. Build-up can come from things like tiny bits of the heat exchanger if it's corroded, which can ultimately end up in your condensate trap.

If there is enough sediment build-up, condensate wastewater will cease to flow, causing the liquid to back up into the bottom of the heat exchanger. Your boiler will then automatically stop working for safety reasons.  

You shouldn't have to worry too much about this happening if your boiler is regularly serviced. Your heating engineer will clear the condensate trap during your service. Cogged condensate pipe traps are usually only to be expected in appliances that haven't been serviced in a long time.

Telltale signs that your condensate pipe has a blockage include the boiler making a gurgling sound when it’s fired up due to wastewater struggling to pass through the pipework and, if the blockage is bad enough, the boiler stopping working altogether.

If you suspect a clogged condensate pipe trap is to blame for your boiler failing, you should contact an engineer rather than trying to fix the problem yourself, as the job requires a Gas Safe registered expert. Your engineer will clean the trap by removing it and rinsing it out thoroughly before refilling and refitting it.

If you'd like us to come and take a look, you can book a boiler repair here by answering a few simple questions about your home and boiler.

Need a new condensate pipe or condensing boiler?

Since the legislation was introduced way back in 2005, all new boiler installations must be condensing boilers. This is because condensing boilers are far more efficient than old-style non-condensing boilers, reaching energy efficiencies of above 90%.

At BOXT, we supply a huge range of A-rated condensing boilers from brands such as Ideal, Baxi, Navien and Worcester Bosch with industry-leading home cover boiler service plans to keep things running smoothly.