Frozen Pipes: What to Do, and What Splits Them

How do I know a pipe has frozen?

No water at one outlet while others still run, during or just after a cold spell, points to a frozen section rather than a supply failure. Frost or condensation on an exposed run confirms it. A boiler locking out in the cold usually means the condensate, not the main.

The pattern separates a freeze from every other cause of no water. A supply failure takes every cold outlet in the property at once. A frozen section takes the outlets fed by that one run, so a bathroom basin runs and a loft-fed shower does not. Timing narrows it further: the fault appears on the coldest night of a spell rather than gradually over a week.

Physical confirmation is quick where the run is visible. A frozen length carries a ring of frost or a band of heavy condensation, and it is noticeably colder to the touch than the pipe either side of it. A run boxed in or buried in insulation gives none of those signs, which is why the outlet pattern does the work instead.

Why the damage appears when it thaws

Water expands about nine per cent as it freezes and the pipe wall yields before the ice does. The split is already there while the ice plugs it. Water escapes only when the ice clears, which is why nothing appears until the thaw.

Ice occupies roughly nine per cent more volume than the water it formed from, and that expansion happens inside a closed length of pipe with nowhere to go. Copper and plastic both yield rather than compress, so the wall opens. The ice that opened it then sits in the opening as a plug, and the pipe holds pressure against that plug for as long as the temperature stays below freezing.

The consequence is a timing one and it catches people out every winter. A property is inspected during the cold spell, found dry, and left. The thaw arrives two days later, the plug clears, and the split discharges at mains pressure into a building nobody is watching. The damage belongs to the freeze; the water arrives with the warmth.

What should I do before it thaws?

Close the stopcock and open the lowest cold tap. Move belongings from beneath the run and put a container under it. Photograph the position. Isolating first means the split releases what is in the pipe, not what is in the main.

1. Close the internal stopcock clockwise, before anything is warmed.
2. Open the lowest cold tap in the property and leave it open, so the run empties through the tap rather than through the split.
3. Move furniture, stored boxes and anything absorbent out from beneath the frozen run.
4. Put a container under the run and photograph the position and the surrounding area.

Isolating first is the step that changes the size of the incident. A split thawed on a live main discharges continuously at supply pressure. The same split thawed on an isolated, drained run releases the few litres left in the pipe and then stops.

How do I thaw it safely?

Warm the pipe gradually from the outlet end backwards, using a hot cloth, a hairdryer on low, or ambient heat. Never use a blowtorch or any naked flame: boxed-in runs sit against timber, and thawing this way starts fires every winter.

1. Start at the outlet end — the tap or fitting the run feeds — and work backwards towards the frozen section.
2. Apply gentle heat: a cloth wrung out in hot water, a hairdryer on its low setting, or simply warming the room and waiting.
3. Keep the outlet open throughout, so meltwater and released pressure have somewhere to go.
4. Watch the run as it clears, because the split shows itself the moment the plug releases.

Working outlet-first matters mechanically. Heating the middle of a frozen length first melts a pocket of water trapped between two ice plugs, and that pocket has nowhere to expand. Clearing from the open end backwards gives every drop of meltwater a route out of the pipe.

Which pipes freeze first in a London property?

Four runs freeze before the rest: loft pipework laid above the insulation, garage and outbuilding runs, external taps, and the boiler condensate. All four are unheated, and the condensate is small-bore, which is why it blocks first.

• Loft pipework above the insulation line. Insulation laid between the joists keeps heat in the rooms below and leaves the loft at outside temperature.
• Garage and outbuilding runs. No heating, and frequently an outside tap feed left charged through the winter.
• External taps and their feeds. The tap body sits outside and the feed passes through the wall, so the coldest point is inside the structure.
• The condensate pipe. A condensate pipe is the small-bore plastic waste run that carries acidic water away from a condensing appliance. It is frequently 21.5mm, frequently run externally, and it blocks with ice before any supply pipe does.

When does this need an engineer today?

Call the same day where the frozen run serves a boiler, where water has already appeared, or where the stopcock will not close. A property left unheated with a frozen main is the case that becomes a ceiling collapse rather than a repair.

Two of the three are about losing control of the water rather than about the ice. A stopcock that will not close removes the property’s only isolation point, and water already appearing means a split has already opened. The third is about the building: an unheated property with a frozen rising main freezes further with every hour, and the eventual thaw discharges into rooms nobody is in.

Rillwater Plumbing & Heating attends Barnet, Bromley, Camden, Croydon, Ealing, Greenwich, Hackney, Hammersmith and Fulham, Islington, Kensington and Chelsea, Lambeth, Lewisham, Southwark, Tower Hamlets, Wandsworth and Westminster, 24 hours a day on 07446 375079.

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