What the condensate pipe does
Almost every modern gas boiler is a condensing boiler. It recovers extra heat from its flue gases, and that process produces water: a steady trickle, slightly acidic, that drains away through a plastic pipe, usually white or grey.
Often that pipe runs through the wall and down the outside of the house to a drain. In a long spell below freezing, the water in it can freeze, usually at the open end or at a bend, and block it. The water backs up and the boiler shuts itself down.
The signs
- The boiler stops working during freezing weather
- A gurgling or bubbling noise from the boiler or the pipe
- A fault code or flashing light on the display. Your manual will say which one means a blocked condensate
- The boiler fires briefly after a reset, then locks out again
Thawing it safely
- Find where the plastic pipe leaves the boiler and follow it outside. Feel along it for the coldest, hardest section: usually the lowest point, a bend or the open end
- Pour warm water over that section, from a watering can or a kettle that has been left to cool for a while. Not boiling water, which can crack the pipe and scald you
- Or hold a hot water bottle or a microwavable heat pack against it
- Repeat until the ice clears. You may hear it gurgle as the water starts to flow
- Reset the boiler the way the manual describes. It may take a few minutes to fire
- Grit or salt the path afterwards. The water you poured will freeze too
Stay on the ground
If the frozen section is above head height, or can only be reached from a ladder, leave it and call an engineer. A ladder on icy ground turns a boiler problem into a trip to A&E.
The same goes for the boiler itself. Do not open the casing to get at the trap or the pipework inside.
Stopping it happening again
A pipe that has frozen once will probably freeze again. The fixes are usually small jobs that can be done on a service or repair visit:
- Insulating the outside run with waterproof, weather-resistant lagging
- Re-routing the pipe inside to a sink waste or soil stack, so little or none of it is outdoors
- Fitting a wider pipe for the outside run, which takes much longer to freeze solid
- Keeping any outside run short, with a steady fall, and ending it inside the drain, below the grid, rather than above it