Thaw a frozen pipe without splitting it

A frozen pipe is the one plumbing emergency where you have time to think, and where hurrying is what causes the damage. The ice is not going anywhere in the next twenty minutes, and the two mistakes that turn a frozen pipe into a burst one are both things people do because they are rushing.

Water expands by roughly nine per cent as it freezes. That expansion is what splits the pipe wall or cracks a fitting, and crucially it may already have happened. A pipe can be frozen and intact, or frozen and split, and it looks and behaves identically until the moment the ice melts.

Working out that it is frozen

• A tap that runs to nothing, or to a trickle, in cold weather, while other taps work.
• The cold tap upstairs is usually first, because it comes from the loft tank through the coldest run.
• Frost or condensation on a visible pipe, or a section that is noticeably colder to the touch than the rest.
• A bulge, a hairline crack, or a joint that has been pushed apart. Look before you warm anything.
• Gurgling, banging or no sound at all where you would expect flow.

Trace the run. The frozen section is almost always in an unheated space: the loft, an outside wall, a garage, an outbuilding, under a suspended ground floor, or the length that runs along a cold external wall behind a kitchen unit.

Check the whole house first. If several taps are dead, the freeze is upstream, possibly at the loft tank or the incoming main, and warming one visible pipe will achieve nothing.

Before you apply any heat

See the mains shut-offs if you have not located and exercised the stop tap. A seized stopcock discovered during a thaw is a bad afternoon.

Have towels and a bucket in position before you start, not after.

If you can see a split, do not thaw it at all until the water is off and you have a repair ready. See burst pipes.

Thawing it

Direction is the whole technique. Start at the tap end of the frozen section and work backwards along the pipe, away from the tap.

If you warm the middle or the far end first, you produce meltwater trapped between solid ice on one side and more solid ice or a closed tap on the other, and it has nowhere to go as it warms and expands. That is how a pipe that survived the freeze bursts during the rescue.

Gentle heat, kept moving:

• Hot water bottles laid along the pipe, refilled as they cool. Slow, completely safe, and the right answer in most lofts.
• Warm damp cloths, wrung out and re-warmed. Effective and equally safe.
• A hairdryer on a low setting, held a hand's width away and moved continuously along the pipe.
• A heat mat, a heated pad or trace heating, if you have it.
• Warming the whole space rather than the pipe: a fan heater in the garage, the loft hatch left open so house heat rises, a room heater near an internal run. Slower, and it thaws everything at once rather than creating a trapped pocket.

Be patient. A long frozen run can take hours. Heat applied hard and fast to one point is exactly what stresses the metal and cracks the joint.

Keep the heat moving. Concentrating on a single spot can boil the water inside locally, and steam pressure in a closed section is far more dangerous than ice.

What never goes near a pipe

Do not use boiling water poured over the pipe either. The thermal shock can crack a fitting, and you now have a loft floor covered in water.

When it thaws and water appears

The moment flow returns, two things can happen, and you should expect either.

Water comes out of the tap. Good. Leave it running gently for a minute, then check every visible inch of that run, and every ceiling below it, for weeping.

Water comes out of the pipe. The freeze had already split it, and the ice was acting as the plug. This is common and it is why the stop tap had to be ready.

1. Stop tap off.
2. Open every tap to drain the system.
3. Power off if water is anywhere near electrics.
4. Then burst pipes takes over.

Check the whole run afterwards, not just the bit you warmed. Freezing damage frequently shows up at a fitting some distance from where the visible ice was, and plastic pipe in particular can split well away from the obvious point.

The pipes you cannot reach

Some of the frozen run will be behind plaster, under a floor or buried in a wall.

• Warm the room, both sides if you can, and wait. It is slow and it is safe.
• Open the cupboard doors under sinks and in airing cupboards so household heat reaches the pipework behind them. This alone thaws a lot of kitchen freezes.
• Do not start cutting into the fabric to reach a pipe in a hurry. You will do more damage than the freeze.
• The incoming main under the drive or the front path is beyond what you can do. That is a water company job, and in the meantime you are on stored water.

Not having to do this again

Prevention here is genuinely cheap and it is the same list as the one on the burst pipe page, because it is the same problem caught earlier.

• Lag every pipe in an unheated space. Loft, garage, outbuilding, under a suspended floor, along an external wall. Foam sleeve costs a few pounds a length and fits in minutes.
• Lag the tank and its pipes, but never underneath the tank. The small amount of warmth rising from the house is part of what stops it freezing.
• Leave the loft hatch open during a hard frost.
• Keep a low background heat on rather than switching off entirely when the house is empty in winter. It is far cheaper than a burst.
• Isolate and drain outside taps before winter.
• Let a tap drip on the most vulnerable run during an exceptional freeze. Moving water is much harder to freeze, and the water lost is trivial next to a burst.
• If you are leaving the house empty for a while in winter, either keep the heating on low or drain the system down properly.