BoilerFaultCodesFrozen condensate pipe: fix it yourself in 10 minutes
In cold weather, the most common reason a boiler suddenly shows a fault code and stops working is a frozen condensate pipe. The good news: this is one of the few boiler problems you can safely fix yourself — no engineer needed.
Is this definitely a frozen condensate pipe?
- › It's cold outside (below 0°C)
- › Your boiler was working fine before the cold snap
- › You can hear a gurgling sound from the boiler
- › The boiler shows a fault code it's never shown before
If all of these apply, a frozen condensate pipe is almost certainly the cause.
What is a condensate pipe?
Modern condensing boilers produce water vapour as a byproduct of the heating process. This water (condensate) is mildly acidic and needs to drain away — usually into a drain, gully or soil stack. The condensate pipe carries this water outside, and in freezing temperatures the water inside can freeze solid, blocking the pipe and causing the boiler to shut down as a safety measure.
How to thaw a frozen condensate pipe
Locate the condensate pipe
Look for a white plastic pipe (usually 20–25mm diameter) leaving the bottom or back of your boiler and running outside — typically through an external wall. It's usually shorter than the gas or water pipes.
Find the frozen section
Follow the pipe outside. The freeze usually occurs where the pipe is most exposed — near bends, at the end of the pipe, or where it runs along an external wall. Look for visible ice or frost on the pipe.
Thaw with warm water
Pour warm water (not boiling — this can crack the pipe or injure you) slowly over the frozen section. A jug or watering can works well. Repeat until you hear the ice shift and water flows freely. A hot water bottle or warm damp cloth wrapped around the pipe also works if you can't easily pour water.
Reset your boiler
Go back inside and press the reset button on your boiler. On most models, hold it for 3–5 seconds until the boiler clicks and attempts to fire. It may take a couple of attempts. Consult your boiler manual if you're unsure where the reset button is.
Prevent it happening again
Once the boiler is working, insulate the exposed section of pipe with foam pipe lagging (costs under £5 at any hardware store). This is the most effective way to prevent a repeat. Alternatively, have a Gas Safe engineer re-route the pipe to a more sheltered location.
Fault codes caused by a frozen condensate pipe
Different boiler brands show different codes when the condensate pipe is blocked. Some common ones:
Search your brand above for the exact code shown on your boiler.
When to call a Gas Safe engineer instead
- › The pipe has cracked or split from the freeze
- › You've thawed it and reset the boiler but it still won't fire
- › The boiler shows a different fault code after the reset
- › You can smell gas at any point
- › You're not sure where the condensate pipe is
Need a Gas Safe engineer?
Search the official register for a qualified engineer near you.