Do this first
- Turn off the internal stop tap. Clockwise. If the pipe has already split, this is what decides how bad the next hour is.
- Open the tap the frozen pipe feeds, and the cold taps nearest it. Two reasons: it relieves the pressure building between the ice plug and the closed tap, and you will hear it the moment the pipe clears.
- Take the lagging off the frozen section and move anything you want to keep dry out of the way. Insulation keeps warmth out as effectively as it keeps it in, and a pipe you are about to thaw may not be intact.
- Warm it slowly, starting at the tap end and working back. A wrapped hot water bottle, towels soaked in hot water and re-soaked as they cool, or a heat wrap. A hairdryer on its lowest setting works but must be kept well away from any water. Thames Water and the Met Office both recommend the slow methods.
- Never use a naked flame, a blowtorch or a hot air gun. The Met Office puts it plainly: direct heat can permanently damage the pipe and can start a fire. Heating one spot hard also turns the ice plug to steam faster than it can escape, which is its own way of splitting a pipe.
- Check the whole run before you turn the water back on. Run dry kitchen paper along the pipe, section by section, and look for damp patches on the paper. Then open the stop tap a quarter turn and watch before opening it fully.
Why the burst shows up on the thaw, not the freeze
People search for "frozen pipe burst" in a particular order of events: nothing at all for two days of cold, then a ceiling comes down on the afternoon it warms up.
The reason is that ice expanding in the pipe is not what usually splits it. What splits it is the water trapped between the growing ice plug and a closed tap or valve. That water has nowhere to go, pressure climbs, and the pipe fails — often well away from the ice, sometimes on a fitting rather than the pipe. While everything stays frozen the split is plugged by its own ice. The moment the plug melts, the split is a hole with mains pressure behind it.
That is why the stop tap goes off before you start warming anything, and why you look at the whole run afterwards rather than just the bit you thawed.
Where London pipes actually freeze
- Loft pipework laid on top of the insulation rather than under it. Modern loft insulation is thick and does its job, which means the loft above it is now as cold as outside.
- Unheated garages, outhouses, cellar steps and external utility rooms.
- The run through an unheated void in an empty flat — this is the classic Christmas-holiday burst in a converted terrace.
- The boiler condensate pipe, where it runs outside to a drain. This is a different problem with an easy fix, below.
- The private supply pipe under a shallow drive or path. Not one you can thaw.
The boiler condensate pipe, which is the one you can fix
If the heating has stopped in freezing weather and the boiler is gurgling, and there is a white plastic pipe running from it to an outside drain, that pipe is probably frozen. Worcester Bosch lists an EA code on its boilers for exactly this. The remedy, on their own advice, is to pour warm — not boiling — water along the outside pipe, concentrating on the most exposed section and any bend or elbow, or to hold a hot water bottle or heat wrap against it, then reset the boiler. Boiling water on frozen plastic can crack it.
This genuinely does not need an engineer. What does need one is a boiler that locks out again the moment it is reset, or a condensate pipe that keeps freezing every winter, which is usually a pipe run in too small a diameter or with too little fall.
What you cannot thaw yourself
- Anything inside the boiler casing. Removing the case is gas work, restricted under the Gas Safety (Installation and Use) Regulations 1998 to Gas Safe registered engineers.
- The buried supply pipe between the boundary and the house. There is nothing to apply heat to. A frozen supply usually clears on its own as the ground warms, and if it does not, or if it turns out to have split, it is a supply pipe repair and an excavation.
- A frozen communal riser or a tank in a shared roof space. That is the managing agent's, not yours.
- A pipe you cannot see in a wall or under a floor. Heating a wall to reach a pipe inside it does not work and is a fire risk.
And if there is standing water anywhere near the electrics while you are working, stop and isolate the supply at the consumer unit first, if the fusebox is dry and you can reach it safely. Electricity and a thawing pipe are a bad combination in a cold loft.
Stopping it happening again
Lag every pipe in a loft, a garage and any unheated void, and lag the condensate pipe outside. Leave the heating on a low background setting rather than off when the property is empty in winter, and if it will be empty for weeks, turn the water off at the stop tap and drain the system down — see how to drain down your system. Find your stop tap now, while nothing is wrong, and turn it off and on gently twice a year so it does not seize. WaterSafe repeats that advice every time the clocks change, because the emergency is a bad moment to discover the tap has not moved since 1998.
If it has already split
Leave the water off and follow the burst pipe sequence. A split you can see is a section cut out and replaced. A split you cannot see — in a loft you have not been into, in a wall, under a floor — is traced before anything is opened, using a pressure test to confirm the loss and thermal imaging or tracer gas to place it. That is a burst pipe repair, charged at £150 per hour agreed in advance, with no fee for the time spent looking if we do not find it.
If the frost has left a hairline crack that weeps rather than runs, see what to do about a cracked pipe.
