Drip faucets during hard freezes to help prevent frozen or burst pipes. The exact temperature depends a bit on your climate and how exposed your plumbing is.
Key temperature guidelines
Many plumbing and home-maintenance sources recommend starting a drip when outdoor temperatures are expected to be at or below about 20°F (around −7°C) for several hours, especially overnight. Some safety organizations and home guides advise dripping when temperatures are below 32°F (0°C) if pipes are exposed or you know your home is vulnerable.
Special situations
Drip faucets if you lose power during freezing weather, because your heating will not protect indoor pipes. Also drip if your plumbing runs through unheated or poorly insulated spaces, like exterior walls, crawl spaces, attics, garages, or basements, even at temperatures just below freezing.
How long to keep dripping
Keep faucets dripping as long as temperatures stay at or below freezing, or below about 20°F if that is the guideline you follow. You can usually stop once both daytime and nighttime forecasts are consistently above 32°F (0°C).
Which faucets to drip
At minimum, let one faucet drip in each vulnerable area of the home, focusing on the faucet farthest from the main shutoff and those served by pipes in exterior or unheated spaces. If possible, drip both hot and cold lines slightly (or set a single-handle faucet to the middle) so both supply pipes keep moving water.