why do water pipes sometimes burst in the winter

Bursting water pipes are a major concern when winter temperatures drop below freezing. But why do frozen pipes burst? Perhaps surprisingly, freezing pipes don t burst because of
expanding in place. Instead, it has to do with pressure inside the pipes. When water freezes, its molecules crystalize into an open hexagonal form, which takes up more space than when the molecules are in their liquid form that is, the expand as they freeze.

As the ice expands, it pushes water toward the closed faucet. This causes an immense amount of water pressure to build between the ice blockage and the faucet eventually, the pipe ruptures under the pressure, usually at a spot where there s little or no ice. The pipe areas upstream of the ice clog aren t typically in danger of bursting because the pressure isn t great enough.

In this case, the water isn t blocked, and can always retreat back to its source. Pipes that are the most at risk of bursting are those located outside of building insulation, or those in unheated interior areas, including basements, attics and crawl spaces. Heating or insulating these vulnerable pipes with fiberglass or foam sleeves can help prevent them from bursting.

Additionally, keeping the facet open and letting the water run even at just a trickle will help prevent the water pressure from building to dangerous levels. Follow Joseph Castro on Twitter. Follow us @livescience, Facebook Google+. When water freezes it expands, as opposed to practically all other liquids. (For this reason ice fortunately forms on the TOP of ponds and lakes. ) The expansion causes pressure to build up that could eventually burst the pipe.

However, most water pipes burst because, as the ice builds up in the direction of the pipe, the water pressure continues to build up downstream from the growing volume of ice, that is between the ice and the faucet's. (Water pressure does not appreciably increase upstream as there is much more water on this side. ) This enormous buildup of pressure can eventually cause the strongest of pipes to burst.

It is a good idea to leave a faucet dripping if the pipe leading to it is in danger of freezing. Not only does this significantly reduce the chance of the water freezing in the pipe, but it keeps the water pressure from building up to dangerous levels that can cause bursting.