During winter the UK has experienced prolonged spells of extremely cold weather - down to minus 15oC in some areas. This resulted in a significant increase in the number of calls to boiler manufacturers and heating installers from householders where the condensate drainage pipe has frozen and become blocked with ice, causing boiler shutdown. In most cases, problems occur where the condensate drainage pipe was located externally for some part of its length.

A condensate drainage pipe gets rid of any condensate water produced during combustion, transferring it to a drain outside your home. While your boiler is working, it produces a lot of condensate water. An efficient boiler will generate around 2 litres of condensate water an hour at a temperature of around 30-40oC. If your condensate becomes blocked and this water cant be disposed of, a signal is sent to your boiler to prevent it working for safety reasons. If this happens, your boiler may display a fault code such as 133 or 501. These could be signs that your condensate pipe is frozen, which can be easily fixed! 

Defrosting a Froze Condensate Pipe 

1. Locate your condensate pipe, it will run from inside the house through the wall to an external wall, it's the external part you need to look for. 

2. Remove any insulation that is fitted to the outside pipe.

3. Apply a hot water bottle, a microwavable heating pack (the sort used for muscular aches and paints) or cloths soaked in warm water to the exterior pipe, close to the likely point of blockage. This is usually at bends and joints or the connections to the soil stack. 

4. Warm water can also be poured onto the pipe from a watering can or similar container. Do not use boiling water. Note: You should not attempt to thaw a condensate drain pipe if you cannot easily reach it from ground level. Be aware that any water used can quickly freeze if it falls onto pathways - causing a possible slip hazard. 

5. To help prevent any further freezing, refit the insulation sleeves onto your pipe. Dry the pip before refitting. 

6. In most cases, once the condensate drain pipe is cleared and a reset has been carried out, the boiler will re-ignite using its automatic operating sequence. 


If you have any doubts or concerns, seek assistance from a Gas Safe engineer. 

Winter advice 2021 v3 1

The Heating and Hotwater Industry Council (HHIC) have also published guidance for householders on frozen condensate drainage pipes.