How to Thaw Frozen Pipes

Run the hot tap until it is properly hot, then fill a gallon zip-top freezer bag about two thirds full and seal it over the sink. Two thirds leaves the bag enough slack to fold around a valve body instead of sitting on it like a brick.

Hot, not boiling. Water straight off a rolling boil will scald you through thin plastic, and it shocks cold metal harder than the job needs. If the house supply is already frozen, heat bottled water in a kettle and let it stand a minute before it goes in the bag.

Carry the bag outside and lay it over the spigot so the plastic folds around the valve body and the short length of pipe behind it. You want contact on as much metal as you can reach, not one hot spot on the handle.

Hold it there, or tie it on with a strip of old cloth. A bag this size stays useful for about ten minutes, then it has given up its heat and wants swapping for a fresh one. Two or three rounds is normal on a spigot that has been frozen overnight.

Unscrew the garden hose and any quick connector from the spout, then open the spigot handle a full turn and leave it open. A hose left on over winter holds a slug of water hard against the valve, and that is a common reason the spigot froze at all.

An open outlet gives melting ice somewhere to go, and it tells you the second water starts moving again. Closed, pressure builds between the ice plug and the seat with nowhere to vent.

Keep the heat on the valve until water comes out of the spout. It starts as a dribble with slush in it, then opens right up. Let it run for a minute to push the last of the ice through, then close the handle.

Now go looking for damage. Run a dry hand along the valve body, the wall bracket and the pipe behind it. A pipe that split while it was frozen holds together on the ice and only leaks once the ice clears, so a weeping joint or a fine dark line along the copper is a call to a plumber rather than something to watch.

Inside the house, follow the incoming water line to the main shutoff valve. It is usually brass, it sits on the pipe coming up out of the floor or through the wall, and it hides under the kitchen sink, in a utility room, in the garage or in a crawlspace. Turn it clockwise until it stops. Our guide to shutting off water to your house walks through the valve types you might meet.

Do this before any heat goes on the line. A split that happened during the freeze does nothing while the ice is still in place. Close the valve first and the worst case becomes a bucket of water instead of a flooded room.

Point an infrared thermometer along the pipe a few inches at a time and watch the number move. The frozen stretch reads far colder than the rest of the run, and that gap is what tells you where the heat needs to go.

On this line the pipe below the main shutoff valve reads minus 11.6 Celsius, about 11F, while the rest of the run sits close to room temperature. The ice is low down, near the floor, where the line comes in from outside. No meter? Slide a bare palm down the pipe and stop where it turns painfully cold.

Stand an electric fan heater on the floor about a foot back from the wall and point it at the coldest stretch, below the shutoff valve. Leave it running. Moving warm air spreads heat along a long length of pipe, which is what the job needs, because an ice plug can easily run two feet.

A hair dryer or a heat gun on its low setting does the same work in a tighter space. Keep either one moving rather than parked. What you never use is a blowtorch or a propane torch. Flame turns the trapped water to steam faster than it can escape, melts the solder out of the joints, and sets light to the insulation packed around the pipe.

No power in the crawlspace, or no heater in the house? Hang a bag of hot water over the pipe so it drapes across the frozen section and gravity holds it against the metal. Tuck it in above the shutoff valve, where the line comes through the wall.

Swap it for a fresh bag every ten minutes as it cools. A rubber hot water bottle is better again, since it holds more water and stays warm for a good half hour. Slow going next to a fan heater, but it works, and it puts no flame anywhere near the copper.