Frozen condensate pipe: how to thaw it and restart the boiler

Why a condensate pipe freezes

Every modern condensing boiler produces water as it burns gas. The boiler collects this slightly acidic water, called condensate, and drains it away through a plastic pipe, normally white or grey and about the width of a thumb. It goes to an internal waste pipe, a drain outside or a soakaway.

Where part of that pipe runs outside, the condensate sits in it or trickles slowly, and in a hard frost it freezes. The ice blocks the pipe, the water backs up into the boiler, and a sensor shuts the boiler down to protect it. You lose heating and, on a combi, hot water as well.

Pipes are most likely to freeze where they run a long way outside, go through an unheated garage, have a dip or a flat section that holds water, or end in an open drain exposed to wind.

Signs the condensate pipe is the problem

A frozen condensate pipe is one of the usual causes of a winter breakdown, and it has a recognisable pattern.

• The boiler worked yesterday and stopped overnight or first thing in the morning after a frost.
• It shows a fault code or a flashing light, and a reset works only briefly or not at all.
• You hear gurgling or bubbling from the boiler or the pipe.
• The pipe outside feels solid and cold, or you can see ice at the open end.
• Neighbours with similar boilers are having the same problem.

Fault codes for a blocked condensate pipe differ between makes. Look up the code in your boiler manual, or on the manufacturer's website, before you assume it is the condensate.

How to thaw a frozen condensate pipe

Only do this if you can reach the pipe safely from the ground. Do not climb a ladder in icy conditions to reach a pipe high up the wall.

Warm water from the kettle that has been left to cool for a while, or from the hot tap, is ideal. Boiling water can crack the plastic, scald you, and refreeze as ice on the path underneath.

• Turn the heating off at the thermostat so the boiler is not trying to fire.
• Trace the pipe from where it leaves the wall to where it ends. The blockage is usually at the lowest point, a bend or the open end.
• Pour warm water slowly along the pipe, starting at the end and working back towards the wall, or hold a hot water bottle or a warm cloth against the frozen section.
• Repeat until you hear the water draining or see it come out of the end.
• Reset the boiler using the method in its manual, then put the heating back on.
• Put a bit of grit or salt on the ground where you poured, so the water does not turn into a slip hazard overnight.

If the boiler still will not restart

Give the reset one or two tries after thawing. If the boiler locks out again, there may still be ice further along the pipe, or the fault may be something else, such as low pressure or an ignition problem. Our guide on how to reset a boiler lists the other simple checks, and the one on boiler lockouts that keep returning explains what happens next.

Do not remove the boiler casing or disconnect the condensate trap inside it. That work is for a Gas Safe registered engineer. The same goes for any smell of gas: stop, leave the controls alone and call the National Gas Emergency Service on 0800 111 999.

Stopping it freezing again this winter

A pipe that freezes once in an unusual cold spell may not need any work. A pipe that freezes every winter is a design problem, and each lockout costs you a cold morning. Current manufacturer guidance generally asks for external condensate pipework to be kept as short as possible, run in wider pipe (usually 32mm) with a steady fall, and insulated with waterproof lagging.

The options an engineer will look at, roughly in order of cost:

• Adding weatherproof insulation to the outside run.
• Replacing a narrow external pipe with a wider one and removing dips or flat sections.
• Rerouting the pipe to drain internally, into a kitchen or bathroom waste, so less of it or none of it is outside.
• Fitting a trace heating cable where the route cannot be changed.
• Checking a condensate pump, where one is fitted, and its discharge route.

When to call us

Call if the pipe is out of reach, it has cracked or come apart, the boiler will not restart after thawing, or the same thing happens every cold spell. We have 10 vans covering London and everywhere inside the M25, 24 hours a day, and boiler work is done by Gas Safe registered engineers. Our call-out charge is £60.

Tell us the boiler make, the fault code and where the condensate pipe runs. If it freezes regularly, we can quote to reroute, upsize or insulate it while we are there, so the next frost does not take your heating out again.