Why does the hot water slow to a trickle after initially blasting out?

Dear Cecil:

I have an old apartment complete with old pipes and I've noticed an annoying phenomenon when I turn on the hot water. At first it comes blasting out, but within a short time it slows to a trickle, as though a poltergeist were under the floor squeezing the pipe. What gives? The pipe stays the same and I assume the water pressure does too. Is there some law of fluid dynamics that causes this? It doesn't seem to happen to cold water pipes.

Cecil replies:

Cecil would love to tell you there’s some mind-boggling phenomenon at work here involving gravitons and the strong nuclear force, but, unfortunately, no can do. According to my Plumbing Repairs Made Easy — I love books with titles like this — the hot water slows to a trickle because the washer in the faucet expands when it gets hot. Solution? “Replace with a proper, nonexpanding washer.” Short, simple, and unlikely to inspire much backtalk from the troops. Sometimes I think I went into the wrong line of work.

Send questions to Cecil via cecil@straightdope.com.

Comment on this Column