Condensate Traps and Pipes: Winter's Most Common Boiler Fault
Every condensing boiler produces acidic condensate that drains away through a trap and pipe, usually to an outside gulley or internal waste.
Every condensing boiler produces acidic condensate that drains away through a trap and pipe, usually to an outside gulley or internal waste. When that pipe freezes or the trap blocks, the boiler locks out completely, and it's one of the most common winter call-outs there is.
What it does
The trap holds a small water seal that stops flue gases escaping through the drain, while the pipe carries the mildly acidic condensate away from the boiler to a suitable discharge point.
How it fails
Sludge or debris blocks the trap, or the external pipe run freezes in cold weather if it wasn't installed with enough fall or insulation.
Repair or replace
A blocked trap is usually cleared and flushed rather than replaced; a badly routed external pipe is often re-run internally or insulated to stop it recurring.
Fix a boiler that's locking out in cold weather
Tell us whether the fault started with a cold snap and we'll check the condensate route as a priority alongside the trap.
Questions we get asked
Is clearing a condensate trap gas work or plumbing work?
The internal trap is part of the gas appliance and is Gas Safe registered work; clearing an external pipe blockage is often plumbing work, though we check both together.
Can I thaw a frozen condensate pipe myself?
Pouring warm (not boiling) water over an externally accessible pipe is a reasonable first step many people manage themselves; anything requiring access to the boiler itself is best left to us.
Why does this only happen in winter?
External condensate pipes are the ones exposed to frost; if the run has too little fall, is too narrow, or lacks insulation, freezing is far more likely.