Loss of water pressure in freezing weather often means that water has frozen in one or more locations in your water pipes. You may be able to do something about it, depending on where the blockage is, and it may take some detective work to find it. It's important to know the location for another reason -- the frozen section of pipe may leak when the water thaws, and you have to fix the leak.
Identifying a Frozen Pipe
Water expands when it freezes, and the outward force it exerts is strong enough to distend a copper pipe and make it bulge noticeably. Frozen pipes also get frosty, so look for one or both of these symptoms. Galvanized steel pipes aren't as malleable as copper and won't bulge so much as turn frosty. You can also find an ice blockage by feeling the pipe or using an infrared thermometer -- the frozen part is noticeable colder than normal pipes, which are usually at a temperature of about 50 degrees. The most obvious symptom of freezing is a crack in the pipe from which dripping water has formed an icicle.
Checking the Main Pipes
When one fixture, such as a sink faucet, stops working, it's likely that other fixtures have also stopped, and determining which are frozen and which aren't is the first step in finding the blockage. If none of your fixtures work, look for an exposed section of frozen pipe in the basement or crawlspace near the water meter. Failing to find one there, follow the pipe outside from the point it emerges from the house until it goes underground. If you don't see signs of freezing in any aboveground pipes, the blockage may be underground. This is unusual, and if it happens, the easiest way to find it may be to wait for the water to thaw and watch for signs of a leak.
Finding a Blockage in a Branch Pipe
If only some of the fixtures in your house are affected, you don't need to check the main pipes, but the branch that services those fixtures. This pipe may be in a poorly insulated part of the wall or running underneath the ground floor, and knowing which fixtures are affected helps pinpoint it. For example, if none of the fixtures in an upstairs bathroom work, but the rest of the house has water, the frozen section could be in a wall or just beyond a tee in the basement to which the upstairs bathroom is connected.
Thawing Frozen Pipes
Once you find the blockage, you can hasten thawing in metal pipes by the application of gentle heat, using a hairdryer, or wrapping the pipe in rags soaked in hot water or a heating pad. Don't try to hurry the process by using a torch or heat gun, because the ice can turn to steam and cause an explosion. As an added precaution against this eventuality, open one or more faucets to relieve pressure while you're thawing the pipe. Allow plastic pipes to thaw by themselves; heat can weaken the plastic and increase the likelihood of a crack. Prevent a recurrence by opening one of the affected faucets to allow a drip of water until the weather warms up.
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