Should I let faucets drip during a freeze?

The trigger is the forecast, not the thermometer on your patio. The National Weather Service office for Phoenix issues a Freeze Warning when "Temperatures of 32F or lower are occurring or imminent" in major population areas. It can send a Freeze Watch 2 to 5 days ahead of the first expected freeze of the season. A watch is your cue to check hoses and covers. A warning is your cue to drip.

City of Phoenix Water Services lists "Allow a faucet to drip slightly to minimize freezing" among the steps to take "if a freeze warning is issued." The American Red Cross uses a similar trigger and says to do it "when the weather is very cold outside." Most winter nights in the Valley never need this, so the forecast is what tells you which nights do.

Pick the faucet farthest from your water meter or main shutoff. Mississippi State University Extension explains the reason in one line: if you know the farthest faucet, "you can run only that faucet and it will draw water all the way through the house." One open tap at the far end moves water through the whole run, so you do not need to drip every fixture.

Then choose the cold side. The same Extension guidance answers "hot or cold" directly: "Run the cold water faucet." If a single-handle faucet mixes both, set the handle to cold so you are not pulling hot water from the heater all night.

There is one exception worth knowing. If the lines at greatest risk feed a particular fixture, such as a kitchen sink on an exterior wall or a bathroom over an unheated garage, drip that one. The Red Cross says to let the cold water drip "from the faucet served by exposed pipes."

Think thin and steady, not a stream. Mississippi State's guidance is that "You need just a trickle of water to drip so that water is moving through the pipes consistently." Its estimate is that at that rate the faucet "will fill a gallon pitcher in about an hour." Over a ten-hour night that is on the order of ten gallons, a small amount of water next to what a burst line puts on your floor.

Leave it running until the cold snap is over. Mississippi State says to let it drip "until temperatures are consistently above freezing to be on the safe side."

The Red Cross puts the mechanics simply: water expands when it freezes, and "No matter the strength of a container, expanding water can cause pipes to break." Its advice on dripping is that "Running water through the pipe - even at a trickle - helps prevent pipes from freezing." Moving water is harder to freeze, and an open tap means the line is not a sealed tube if ice does start to form. Phoenix Water Services applies the same idea to outdoor bibs, saying to "Keep the outside valve open to allow any remaining water in the pipe to expand without causing the pipe to break."

It helps, it does not guarantee. Mississippi State says plainly that "Keeping pipes from freezing is not an exact science" and that results depend on how well insulated the house is and how long it stays cold.

This is where most Phoenix homes actually get hurt. A faucet drip protects the pipes that feed that faucet. Three places sit outside that loop:

• Outdoor hose bibs. City of Phoenix Water Services says to "Remove, drain, and store hoses used outdoors," close any inside valve that feeds the bib, and open the outside bib so the water drains. A hose left on traps water in the faucet. Our guide on winterizing a hose spigot in Arizona has the full sequence.
• Irrigation and pool lines. The Red Cross says to drain swimming pool and sprinkler supply lines following the manufacturer's or installer's directions, and not to add antifreeze unless directed.
• Pipes in the attic, garage or an exterior wall with no air movement. Open the cabinet doors under sinks on those walls so warmer air can reach the plumbing, as both the Red Cross and Phoenix recommend, and close the garage door if supply lines run through it.

For the bigger picture of which pipes in a Phoenix house are exposed, see can pipes freeze in Phoenix.

Do not rely on a drip for a house nobody checks. The Phoenix guidance for travel is to set the thermostat "no lower than 55°F" and have a friend or neighbor check on the house. For long stretches, shut off the water to the house and to the water heater where it enters the heater. Know where the house shutoff is before you leave; our page on testing your main water shutoff valve shows how.

If you turn on a faucet and only a trickle comes out, suspect a frozen pipe. Keep that faucet open, warm the pipe slowly, and use no open flame. The steps are in how to thaw a frozen pipe safely. If you cannot find the frozen section, or water starts leaking as it thaws, shut off the main and call a licensed plumber. HQ Plumbing and Air runs 24/7 emergency plumbing across the Valley, and a firm price comes before work starts.