No. Cold makes ice, but the break usually comes from pressure. Once ice blocks a pipe, continued freezing squeezes the trapped water between the blockage and a closed faucet, and that pressure splits the pipe, often where there is little or no ice. An open faucet gives the pressure somewhere to go.
What actually bursts a frozen pipe?
It's pressure, not the ice pressing outward on the pipe wall. A University of Illinois Building Research Council study says the commonly held view, that ice growth simply pushes against pipe walls, "is not correct." It found that pipe bursting occurs when freezing temperatures create ice blockages, then further ice growth applies dangerously high pressures to a confined water volume. South Carolina Farm Bureau Insurance describes it this way: "Surprisingly, ice forming in a pipe does not typically cause a break where the ice blockage occurs." Following a complete ice blockage, the insurer says, continued freezing and expansion inside the pipe causes water pressure to increase downstream, between the ice blockage and a closed faucet at the end.
Utica National, an insurer's risk management group, gives the same picture. It says the fact that water is freezing within a pipe does not, by itself, endanger the pipe. The damage comes from the hydraulic pressure in the part of the water that freezes last. That's why a burst often shows up in a spot with little or no ice.
Does the temperature still matter?
Yes, because cold is what starts the chain. Our page on whether pipes can freeze in Phoenix covers how cold it has to get here and which pipes are most at risk.
Wind matters too. Utica National notes that pipes in attics, crawl spaces and outside walls are vulnerable, especially where cracks or openings let cold outside air flow across them, and that wind chill can play a major role in accelerating freezing and bursting. A pipe in a windy gap in an outside wall can freeze on a night when the air temperature alone wouldn't seem to threaten it.
Why a dripping faucet helps
If the pipe has an open outlet, the water has somewhere to go. Utica National's advice is to let faucets drip slowly to keep water flowing through pipes that are vulnerable to freezing. It explains that ice might still form in the pipes, but an open faucet allows water to escape before the pressure builds to where a pipe can burst. If the dripping stops, it may mean ice is blocking the pipe, and keeping the faucet open provides pressure relief.
What to do on a freezing night
- Seal openings where cold air can reach exposed pipes, especially in garages, attics and outside walls.
- Insulate exposed lines with sleeves or wrap.
- Let a faucet on the vulnerable line drip slowly.
If a faucet is already down to a trickle, follow our guide on how to thaw a frozen pipe safely.
A burst pipe may not flood the house right away. Utica National notes that a rupture may be initially sealed by the ice blockage, and flooding happens when the ice thaws. If you find a split, when to call an emergency plumber helps you decide how fast to move, and the first step is shutting off the water. HQ offers 24/7 emergency service, and we're licensed and insured.