What to do if you have a Blocked Condensate Pipe

A blocked condensate pipe often happens when you need your boiler the most - during cold weather. Failing to unblock it can leave you without hot water or heating. Read on to find out how to tell if your condensate pipe is blocked and what you can do to unblock it.

Written by Kojo Prempeh

Last updated - 20/10/2022

Estimated reading time - 6 minutes

What is a condensate pipe?

A condensate pipe discharges condensate from a condensing boiler to an outside drain. It is a small, usually white, plastic pipe that is connected to the bottom of a boiler. As condensate is slightly corrosive to metal, the condensate pipe is made out of plastic. In fact, it is the only plastic pipe connected to a boiler.

What is a condensing boiler?

A condensing boiler recovers heat from flue gases and recycles it back into the heating system to preheat the cold water entering the boiler. During this process, the temperature of flue gases is reduced from around 130℃ to 50℃. This drop in temperature causes condensation. A boiler can produce as much as 2 litres of condensate water per hour. This water is disposed of via the condensate pipe.

Instead of capturing heat from the gases in the flue, non-condensing boilers expel it, which means this heat is wasted.

Due to the condensing process, condensing boilers use less fuel and power than non-condensing boilers, which could save homeowners up to £300 a year on their energy bills while also reducing CO2 emissions.

As of the 1st of April 2005, it became compulsory for all new boilers installed in domestic properties in the UK to be high efficiency condensing boilers.

Where is my condensate pipe?

In some properties, the condensate pipe runs from the boiler to an internal kitchen or bathroom waste pipe. In other properties, it exits behind the boiler through an external wall and into a drain. The section of the condensate pipe that is located outside is more prone to blockages.

Why is my condensate pipe blocked?


As some parts of a condensate pipe may be located outside, leaves and debris may get blown inside it and clog it. The most common cause of a blocked condensate pipe is it freezing. During times of very cold weather, the water flowing through the condensate pipe could freeze. If this happens, the wastewater will not be able to discharge from the boiler into the drainage and will cause your boiler to stop working.

How will I know if I have a blocked condensate pipe?

A clear sign of a blocked condensate pipe is your boiler shutting off and displaying an error code. If you have a blocked condensate pipe, condensate water will have no way of leaving the boiler and could back up into the boiler. If condensate backs up into the boiler, it could cause damage to its electrics. Your boiler will shut off to prevent this from happening.

Different makes and models of boilers will display different error codes. You will need to refer to your boiler’s user manual to find out what error code relates to a blocked condensate pipe.

Another sign that you have a blocked condensate pipe is a gurgling noise coming from your boiler when you first turn it on.

You can also conduct an inspection of the pipe for signs of freezing. You should be able to tell if it is frozen by sight or touch.

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How to unblock a condensate pipe

Thaw a frozen condensate pipe

If your condensate pipe is blocked due to frozen water, you will be able to unblock it simply by thawing the ice with hot water.

To thaw a blocked condensate pipe, follow these steps:

  1. Locate the blockage. The condensate pipe usually freezes at the most exposed points, such as the open end of the pipe or at the bend.

  2. Boil a kettle and let it cool down slightly. Do not pour boiling water on the condensate pipe as it may cause it to crack.

  3. Pour the hot water over the condensate pipe. You may need to do this a few times until the ice has thawed. You will know that the blockage has been cleared if you see water leaving the pipe.

  4. Reset the boiler. If your boiler shuts down as a result of a blocked condensate pipe, it may have to be reset before it can start up again. This can normally be done by pressing a reset button on the boiler, or by turning the boiler off and back on again. Instructions on how to reset your boiler can be found in its user manual.

How to prevent a blocked condensate pipe

Insulate the condensate pipe

A good way to protect your condensate pipe from freezing is to wrap any part of it that is located outside in foam pipe insulation. This will keep the pipe insulated and help to prevent the water flowing through the pipe from freezing.

You can buy foam pipe insulation online or from a DIY shop for a few pounds, and it can easily be installed by yourself. When purchasing condensate pipe insulation, it is recommended to use a minimum thickness of 13mm insulation for a 32mm diameter condensate pipe.

Internal condensate pipes in cold areas of properties, such as in the garage, basement, or loft, also run the risk of freezing, so they should be treated like an external condensate pipe and insulated appropriately.

Turn the heating up


If the weather report suggests freezing temperatures are on the way, one way to reduce the chance of a blocked condensate pipe is to operate the boiler at a higher flow temperature; in other words, turn the heating up. Increasing the temperature will reduce the amount of condensate produced and reduce the chances of the condensate pipe freezing during the cold spell.

You can also leave your heating on overnight at a lower temperature to keep the condensate pipe warm. Although this will use energy, your boiler will take less time to heat up your home in the morning, and you will reduce the chance of waking up to a blocked condensate pipe.

Install a drain cover on the end of the condensate pipe


A drain cover helps to prevent leaves and other debris from entering the condensate pipe. As the area around the opening of the pipe is covered, it also helps reduce wind chill. Wind chill is a big factor in causing a frozen condensate pipe. If freezing cold air manages to get inside the pipe, it could freeze the condensate water flowing out of it.

Reroute an external condensate pipe internally

Condensate pipes that are installed inside a property are less prone to freezing than ones located outside. If insulating your condensate pipe isn’t enough to prevent it from freezing, you should consider getting it re-routed internally into one of your property’s waste pipes, such as the one underneath your kitchen sink, or into a soil stack.

As the condensate pipe is considered part of a boiler’s flue, rerouting it should only be carried out by a registered Gas Safe engineer.

Other things to look for

If any of the following statements are not true, your condensate pipe may be at a greater risk of freezing

  • The condensate pipe takes the most direct and vertical route to the drain

  • All horizontal parts of the condensate pipe run at a downwards angle away from the boiler towards the discharge point

  • The end of the pipe is as close to the discharge point as possible

  • The diameter of the condensate pipe is 32mm diameter

If one or more of the above statements aren’t true, you may want to speak to a boiler engineer to find out what can be done to rectify the issue and reduce the risk of a blocked condensate pipe.

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