Condensate Pipes: What They Do and How to Maintain Them

What Is a Condensate Pipe?

All modern condensing boilers produce condensate — a slightly acidic liquid that forms when flue gases cool within the boiler and water vapour condenses. This liquid must be safely drained away. The condensate pipe is the plastic pipe (usually 21.5mm or 32mm white or grey PVC) that carries this liquid from the boiler to a drain.

Where Does the Pipe Go?

Ideally, the condensate pipe drains internally to a waste pipe (kitchen sink trap, soil stack, or dedicated drain). Where this is not possible, it may exit through an external wall and drain to an outside drain. External runs are less desirable because they are vulnerable to freezing.

The Freezing Problem

During very cold weather — typically when temperatures fall below -5°C for extended periods — the water in an external condensate pipe can freeze. This blocks the pipe, causing the boiler to sense backpressure and shut down with a fault code.

Symptoms: boiler displays a fault code (varies by brand, e.g., "EA" on Worcester Bosch or "F28" on Vaillant); a gurgling sound may precede the lockout.

How to Thaw a Frozen Condensate Pipe

1. Locate the external section of pipe (usually a white or grey plastic pipe running down an outside wall)

2. Apply a warm water bottle or warm (not boiling) wet cloth to the frozen section

3. Work along the pipe until the blockage clears

4. Reset the boiler using the reset button

Do not use boiling water — it can crack the pipe.

Prevention

For condensate pipe problems or insulation work, contact Halo Heating on 01483 676108.