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Boiler Condensate pipe and trap: What It Does and Signs It's Failing

Condensing boilers wring extra heat out of the flue gases, and that turns water vapor into a liquid called condensate that's mildly corrosive to metal. Vaillant puts the average at around 2 liters (about half a US gallon) an hour. A trap (or siphon) inside the boiler collects it, and a white plastic pipe carries it to an inside waste pipe or out through the wall to a drain.

How it works

The trap holds a water seal so flue gas can't escape down the drain, and Worcester's large siphonic trap empties in one go every 15 to 20 minutes. Everything downhill of it relies on gravity. Outside sections freeze in hard frosts, the condensate backs up, and the boiler shuts down to stop water reaching the burner.

Signs it is failing

• Gurgling or bubbling from the boiler in freezing weather
• Ignition lockout or failed-to-light code during a cold snap
• Ice at the open end or at a bend in the outside pipe
• Water dripping from the bottom of the boiler casing
• Pipe cracked, sagging or disconnected from the drain

Ignition and flame-loss codes in winter are often a frozen condensate pipe. Worcester shows EA for it on digital-display models and Vaillant points to F.28 and F.29.

Can you replace it yourself?

You can thaw the outside section: pour warm water (not boiling, which can crack the plastic) along it, or hold a hot water bottle on the frozen spot, then reset the boiler as your manual shows. The trap inside the casing is cleaned and refilled by an engineer at the annual service, so don't take it apart yourself.

Buying the right part

Rerouting a pipe inside, upsizing it or adding lagging outside is a job for the engineer or plumber who fits it. Condensate pipe should be plastic, because the liquid corrodes metal.

Error codes linked to the condensate pipe and trap

• E133Gas supply interrupted or flame failed to establishBaxi
• E128Flame failureBaxi
• E119Low primary water pressure (Platinum Combi)Baxi
• F.28Boiler failed to lightVaillant
• F.29Flame went out and the boiler couldn't relightVaillant
• EA / EA 227Burner didn't light (no flame detected)Worcester Bosch
• FAFlame still present after the gas shut offWorcester Bosch

Last reviewed 2026-10-21.