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Boiler Lockout (or fault) during freezing cold temperatures.

Updated: Apr 28, 2023

In winter, temperatures can fall below zero degree Celsius. Often boilers with poor installation and on odd occasions boilers with correct installation can have a freeze up. This occurs in the condensate pipe.



In boilers a condensate pipe is usually a white plastic pipe ( 21.5mm in diameter ) that comes out at the bottom of the boiler. How this particular pipe terminates is what determines if your boiler condensate pipe has the potential to freeze up.



If the boiler condensate pipe is terminated internally in the property (into to the kitchen sink waste, toiler sink waste, or any internal pipe) this pipe will not freeze up in winter. This pipe can remain 21.5 mm if terminated internally.



However, a large number of boiler installs will require the condensate pipe to be terminated externally and this is where a risk of condensate pipe freeze up exists. The pipe size plays an important role here.

As per MIs and Guidelines, the pipe must be at least 1 1/4 inch ( 32mm) '' before '' it exits the property and then terminated into the pipe stack or soakaway. 1 1/4 inch or 32mm pipe is significantly larger than 21.5 mm diameter pipe. Hence very unlikely it would freeze up. Further insulating this pipe with external lagging/insulation will almost eliminate chances of condensate pipe freeze up.




It is much easier and a lot quicker to drill a 22mm hole with a drill bit instead of '' coring '' a 32mm hole. Hence, a lot of installers will choose to drill a 22mm hole and opt for a thinner pipe that would generally freeze up in winter.




In case you are in the situation where boiler has locked out due to frozen condensate pipe, you have some options.


  1. Disconnect the condensate pipe under the boiler. Some of the water will leak through but it will be easily caught into a rag, small container or towel. This will help you get the boiler started. Leave a container under the boiler to catch condensate water until condensate pipe is reconnected. Its extremely important that pipe is reconnected after heating up your house or once hot water is used. An operational CO alarm must also be placed around the boiler to ensure you are made aware in case of any CO gas escape via condensate pipe.

  2. Another option is to heat up a kettle and pour over the external part of a condensate pipe. 1 kettle may not be enough, try 2 or 3...

  3. If unsure, call out a gas safe registered engineer to deal with the issue. Only carry out any works if you feel competent and comfortable to do so.



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