- Q.What causes hot water
pipes to "bang" and how can this be prevented?
A.The usual cause of the
banging in domestic hot water pipes is water hammer. When the
pipe full of water is running, there is a considerable momentum
from the mass of water in motion. When the flow is suddenly
stopped, such as when a solenoid valve on a washer or
dishwasher closes, or when a faucet is turned off quickly, this
energy of the moving water is transferred to the pipe, causing
it to vibrate and move enough to hit the framing.
Plumbers often take off the supply from a tee, and add a
short length of capped pipe above the tee to serve as a shock
absorber. This works as long as air remains in the capped pipe.
When the air chamber becomes waterlogged, the system has to be
drained down to replenish the air. Since air dissolves readily
in hot water, however, you may have to drain the system
The best solution is to install a water hammer arrestor
(available from plumbing supply houses under a variety of trade
names) near the fixture. It contains a bellows and spring to
absorb some of the energy of the flowing water.