How to clean a boiler condensate trap

In the UK, all new gas and oil boilers are now required to use condensing technology. This is designed to make them more efficient, saving you money and cutting your carbon footprint. Condensing boilers operate at more than 90 per cent efficiency, which is a significant improvement on past models. For example, compared to an old G-rated non-condensing gas boiler, an A-rated condensing model can save a detached home over £300 a year in energy costs.

To experience the ongoing benefits of a gas condensing boiler, you’ll need to make sure it stays in good condition - and among other things, this means keeping the boiler condensate trap clean. Over time, these traps can become clogged up with sediment, and if this happens, your boiler may stop working.

Read on to learn more about why this design feature is needed and how you can ensure the one in your boiler stays clean.

Condensing boilers are designed to not only use the heat generated by the combustion process, but also the heat of water vapour content. Older, non-condensing models could waste up to 50 per cent of the heat produced due to gasses escaping up the flue. 

In new appliances, the energy in these gases is captured and used to preheat the cold water that enters the boiler. During this process, flue gases drop rapidly in temperature from around 130℃ to just 50℃. This fall in temperature causes the gases to condense.

Every hour, an appliance can produce as much as two litres of condensation - also known as boiler condensate. This condensate is then drained via your waste water outlet through a condensate pipe.

Generally speaking, there is nothing dangerous about boiler condensate. Some of the carbon dioxide created during the combustion process dissolves in the condensate, making it slightly acidic. However, because it is only mildly acidic, it is not dangerous to health - so this isn’t something you need to be concerned about.

However, it is worth noting that the liquid can be corrosive to certain materials, including metals. This is why the condensate pipe leading from your boiler will be made of plastic rather than metal.

The boiler condensate produced by your appliance will be safely drained with your household waste water, just like your washing machine and sink.

So, how does a boiler condensate trap work? The job of a condensate trap is to safely collect and release the condensation produced by the boiler, preventing any vapours created during the combustion process from escaping with it. These components vary in design depending on the model of boiler you have, but they are all fairly simple. Yours may be a basic ‘U’ bend trap or a slightly more complex siphon trap.

Over time, sediment can start to build up in these components. For example, tiny bits of the heat exchanger may be corroded and can end up in the sump and ultimately in the condensate trap. 

If there is enough sediment, this will eventually stop the flow of condensate, causing this liquid to back up into the bottom of the heat exchanger. If this happens, your boiler will automatically stop working for safety reasons.   

The first point to note about maintaining condensate traps is that as long as you ensure your boiler is serviced regularly in line with manufacturer guidelines, you shouldn’t experience any problems with this component. Your heating engineer will clean the trap out during these services, preventing sediment from building up. Clogged condensate traps usually only occur in appliances that have not been serviced properly or for a long time.

The second point to note is that if your boiler is failing to light and you suspect that a blocked condensate trap is to blame, you should call an engineer to clean it rather than trying to do this job yourself. Signs that your condensate trap is blocked can include a faint gurgling noise coming from your boiler, or an unusual sound coming from the flue outside your home.

If the trap does need to be cleaned, your heating engineer will remove it, rinse it out thoroughly, refill it and then refit it. This is a fairly quick job and shouldn’t be expensive, but it requires a Gas Safe registered expert so is not something you should attempt yourself.

If you have a Viessmann boiler and suspect you may have a blocked condensate trap, or another issue with your appliance, visit our support page for information and to get the contact details of our customer service and technical support teams.