How to defrost a frozen condensate pipe

Even the best-tended boiler will struggle to function if its condensate pipe freezes. Luckily, it's not a difficult job to put things right if it does happen. This handy guide will help you get your boiler up and running quickly and easily.

What is a condensate pipe?

A condensate pipe carries condensation from your boiler to your outside drain. It's usually a white or grey plastic pipe that travels from your boiler through the external wall directly outside where your boiler is located.

During cold weather the condensation in this pipe may freeze and cause a blockage causing the condensate to back up into the boiler and cause a shutdown.

 Diagram of condensate pipe emerging from house.
Condensate pipe emerging from the external wall.

How to defrost the pipe

If you suspect that your boiler condensate pipe has frozen, following steps should help you get things back to normal quickly. Remember, if you don't feel competent to follow this guidance then you should contact a expert boiler engineer for help.

  1. Confirm the condensate pipe is frozen
    Depending on the make of your boiler, a frozen condensate pipe may be indicated by a 'fault code' or warning light on the boiler's display. Gurgling or bubbling sounds coming from the boiler or the condensate pipe are another sign that the condensate pipe has frozen.
  2. Locate the blockage
    The pipe is probably frozen at its most exposed point. This may be the open end of the pipe, or at a bend or elbow. Running your hands over the pipe until you find a section that feels colder than the rest should help you identify the blockage quickly.
  3. Thaw the pipe
    Using a jug or watering can, pour warm water along the length of the pipe, repeating the process until the pipe has thawed (don't use boiling water as this can crack or damage the pipe). Alternatively, you can use a hot water bottle or a heat pack to slowly defrost the pipe.
  4. Restart your boiler
    Once the frozen section has been melted and cleared, check your boiler manual for instructions on how to reset the boiler correctly. Your boiler should now restart correctly. If you boiler doesn't restart you will need to call out a qualified boiler engineer.

 Diagram of a condensate pipe that is being defrosted.
Pouring warm water along the length of the pipe.

How to prevent the pipe from freezing again

You can avoid future stress by taking some steps to help prevent the condensate pipe from freezing again.

If your condensate pipe isn't already insulated, wrap it in some old towels immediately to prevent it freezing again. As soon as you can, get down to your local DIY store and buy some foam pipe insulation to wrap the condensate pipe in. This insulation comes in a variety of sizes, so measure the pipe's diameter before you buy.

 Diagram of a condensate pipe that has been insulated.
Condensate pipe wrapped in foam insulation.

During extreme weather conditions, even proper insulation may not be enough to prevent the condensate pipe from freezing. It may help to temporarily run your boiler with the boiler thermostat set as high as possible for as long as the cold spell lasts. But if you continue to experience problems, it's best to call out a qualified boiler engineer for help.

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