The Physics of Freezing Pipes

Burst water pipes are caused by the expansion of freezing ice against the wall of the pipe, right? Wrong.

In a remarkable series of laboratory and field experiments, researchers at the University of Illinois have demonstrated that the actual cause of damage to freezing water pipes isn't the force of expanding ice on the pipe, but rather an extreme rise in water pressure downstream from the blockage. According to the study, the water pressure can rise in these situations from about 40 pounds psi (per square inch) to more than 4,000 pounds psi.

A typical scenario might go like this: A stretch of copper pipe is exposed to unusually cold temperatures, and ice begins to form on the pipe's inside walls. Since water volume expands by about eight percent as it turns to ice, the ice formation eventually can completely block the water flow. Water upstream from the blockage is able to flow back to its source, such as the street connection. But the water downstream is trapped because the faucets are closed. As the ice continues to form and expand, pressures downstream from the blockage skyrocket. Because this entire section of pipe experiences the same elevated pressure, the failure can occur at any point, even within the heated space of the building.

Now that researchers better understand the problem, they have devised a simple effective solution: a modified faucet washer that enables the faucet to leak a little under high pressure. This inexpensive device could eliminate much of the $400 million per year of insurance claims now paid to homeowners for freeze-related plumbing damages.

Source: Ned Nisson, Popular Science Magazine, February 1997

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