A pipe is frozen. What to do before and after it bursts.

Open the faucet that the frozen pipe feeds, then warm the pipe starting at the faucet and working back toward the ice. Leaving the faucet open is the important half: it gives the melted water somewhere to go and relieves the pressure that actually splits pipes.

Why an open faucet matters more than the heat

A pipe does not burst because ice expands against the walls of the pipe. It bursts because the ice plug seals the line, and then water trapped between that plug and a closed faucet has nowhere to expand to. Pressure builds in that short sealed section until the weakest point lets go, and the split is usually downstream of the ice rather than at it.

So the first move costs nothing and takes two seconds. Open the faucet that the frozen line serves, both hot and cold if you are not sure which side is frozen, and leave it open. Even a trickle relieves the pressure.

Find the frozen section

Frozen pipes are almost always in the cold parts of the building rather than in the heated middle of it. Look in this order:

Exterior walls, especially where a pipe runs to a kitchen sink on an outside wall.
Unheated crawl spaces, basements, garages, and attics.
Under a sink in a cabinet against an outside wall.
Outdoor spigots and the short run of pipe behind them.
A vacant or unheated part of the house nobody has been in.

You can often find it by touch. Run a hand along the pipe until you reach a section that is noticeably colder, or that has frost or a bulge on it. A pipe with visible frosting on the outside is the one.

If no faucet in the house produces anything at all, the freeze is likely at or near where the supply enters the building, and that is a job for a plumber or the water utility rather than a hair dryer.

Thaw it slowly, from the faucet end backward

Start warming at the faucet and work back along the pipe toward the frozen section. Doing it in that direction means the water you melt has an open route out. Starting in the middle of the ice can trap melted water against a plug that is still solid.

What works:

A hair dryer held a few inches off the pipe, moving constantly.
Towels soaked in hot water, wrapped around the pipe and replaced as they cool.
An electric heating pad wrapped along the pipe.
A portable heater aimed into the space (not at the pipe from inches away).
Opening cabinet doors under sinks so house heat reaches the pipes behind them.

Also do not use an electrical appliance while standing in water, and do not plug a hair dryer in next to a pipe that has already started to leak.

If the pipe is inside a wall

You cannot reach it, so you are raising the temperature of the whole space instead. Turn the heat up, open interior doors and any cabinet or access panel near that wall, and point a heater or fan at the wall surface. It is slow and it sometimes does not work.

If you are going to open a wall to reach a pipe, that is the point to call a plumber rather than to start cutting, because the pipe you are looking for is rarely where you think it is.

When it has already burst

You will know because water is arriving. The order changes completely:

Close the main shut-off valve. Everything else waits. Where it is and how it turns depends on the building, but it is on the pipe where the supply enters.
Open the lowest faucets in the house to drain the standing water out of the pipes rather than through the split.
Stay out of water near anything electrical, particularly if the split is over a ceiling or near a panel.
Photograph everything before you clean up.

Then leave the faucets open. A closed system with no flow is exactly how the next section freezes while you are dealing with the first one.

After a freeze, check for the leak you have not found yet

A pipe can freeze, split, and not leak until it thaws, because the ice is plugging its own hole. This is the reason a burst is often discovered hours after the weather has warmed up, sometimes in a house that seemed fine.

Once things are thawing, walk the building. Look at ceilings under bathrooms, along the base of exterior walls, and in the crawl space or basement. Check the water meter with every faucet closed: if the dial is moving, water is going somewhere. A wet patch with no obvious source after a cold snap is usually a split pipe that has just started to run.

Insurance treats a frozen pipe as sudden, with one condition

A pipe that freezes and splits is generally treated as sudden and accidental, which is the distinction most policies actually turn on.

The condition that trips people up is occupancy. Most policies expect you to maintain heat in the building, or to have shut the water off and drained the system, if the home is going to be unoccupied in cold weather. A house left empty and unheated over a winter break is the classic denied frozen pipe claim. If you are going away, either leave the heat on at a low setting or turn the water off at the main and drain the lines.

Keeping it from happening again

Let a faucet on the coldest run drip during a hard freeze. Moving water is much harder to freeze.
Open cabinet doors under sinks on exterior walls.
Disconnect garden hoses and close the interior valve for outdoor spigots.
Insulate pipes in crawl spaces, garages, and attics.
Know where the main shut-off is before a night when you need it in the dark.
If the heating itself has failed, protect the plumbing before you worry about the temperature.

The short version

Open the faucet first, then warm the pipe from the faucet end backward with electric heat only. Never a flame. If it has already split, close the main valve, drain the lines, and stay out of water near anything electrical. After a thaw, go looking for the leak that has not shown itself yet.