"Mulan" wrote in message
ups.com...
Why do plumbers leave a tap on with a trickle of water when they turn
off the water to the house?

I don't know why plumbers do that, but I did just the same thing myself
because the shutoff valve does not shut off the water completely. So I open
the lowest faucet (closest to the ground) I can find so that water would
come out of that faucet instead of the one I'm working on.

Then when I'm ready to test my new plumbing addition, I turn off that faucet
but leave the house shutoff valve untouched. This would still stress the
plumbing system, and if there is a leak, only a little water would come out.
If there is no leak, then I turn on the shutoff valve.