How do I stop my boiler condensate pipe freezing again?

A condensing boiler produces a small, steady trickle of slightly acidic water, and that water has to leave. Because it trickles rather than flows, a length of narrow plastic pipe on a north wall in a hard frost fills with ice, the boiler senses that its condensate cannot drain, and it locks out. It is why boilers all over the country stop on the same cold morning — and it is entirely preventable.

Safety

What you'll need

• 32mm waste pipe and fittings, if upsizing (optional)
• Weatherproof external pipe insulation
• Pipe clips
• A hacksaw and a solvent or push-fit system (optional)

Step by step

1. Understand why it freezes.Condensate leaves as an intermittent dribble rather than a flow, so it lines the pipe and sits there. Anything that slows it — a dip, an elbow, a long horizontal run, an undersized bore — gives ice somewhere to start. Once a plug forms, everything behind it backs up and freezes solid.
2. Consider moving it inside first.The best fix by a wide margin is an internal termination: into an internal soil stack, or into a sink or appliance waste on the house side of that fitting's trap. A pipe that never goes outside cannot freeze. This is boiler work and needs a Gas Safe engineer, but it is a permanent solution rather than a mitigation.
3. Upsize the external run.Indoor condensate pipe is commonly 22mm. Anything running outside should be at least 32mm outside diameter. The larger bore takes much longer to bridge with ice and is the single most effective change if the pipe must stay outside.
4. Shorten it and simplify it.Every extra metre outside is more exposed pipe. Bring it through the wall as close to the termination as possible. Remove unnecessary bends: each elbow is a place where flow slows and ice starts.
5. Get the fall right and remove every dip.A continuous fall the whole way, with no sags between clips and no low points where water can stand. A pipe that dips and rises again is holding water permanently, and that standing water is the first thing to freeze. Clip it properly at close intervals so it cannot sag over time.
6. Terminate with a cut end, not an elbow.An elbow or a bend at the very end restricts the outlet and is the most common single freezing point. A plain angled cut discharging into a gully below the grating but above the water line is the correct arrangement.
7. Insulate with the right material.Weatherproof external pipe insulation, UV stable and properly taped, over the whole external run including the elbow at the wall and right down to the termination. The grey foam sold for indoor pipes degrades within a season outside and does very little.
8. Consider trace heating for a persistent case.A self-regulating electrical trace heating tape run along the pipe under the insulation, on a frost thermostat. It needs an outdoor-rated supply and it is an electrician's job, but on an exposed north wall with no internal option it is the reliable answer.
9. Keep the emergency method for now.Until the permanent fix is done, know how to thaw it: warm — not boiling — water poured slowly along the pipe from the outside end back, or a hot water bottle held against it. Never a naked flame, never boiling water on a cold plastic pipe.

Tips

• Do this work in September, not in January. The pipe is accessible, the solvent cement cures properly and nothing is frozen.
• If the boiler is due to be replaced within a couple of years, get the condensate re-routed internally as part of that job rather than paying twice.
• Photograph the external run so you can describe it accurately to an engineer over the phone during a cold snap.
• A boiler that locks out in a frost with no other symptoms is the condensate nine times out of ten. Knowing that saves a call-out fee.

If it doesn't work

Keeping on top of it

• Check the external condensate run and its insulation before winter (Every autumn)
• Clear leaves and silt from the gully or soakaway the condensate discharges into (Twice a year)

Questions people ask