Tim Smith wrote:
On 2007-04-10, MLD wrote:
Going back (way back) to my mother's time, the one-lever shutoff
valves have lasted about 20 years. Besides the obvious hose-burst
that may occur by leaving the valves on, there's an additional reason
for shutting them off when not in use: The water pressure, over time,
deforms the mixing valve in the machine, eventually causing the
solenoids to stick open or closed, resulting in either no water
delivery or no temperature selection.
When you shut the valve off what do you think happens to the water pressure
that's trapped between the S/O valve and the machine solenoids (aka,
hydraulic lock)? Unless there is a way for this pressure to bleed down it
will remain high and will remain the same (or near) as the supply pressure.
Disaster prevention is the best reason for keeping the shutoff valve closed
except when the machine is in use.

I'm confused by the physics here. Water isn't compressible, so what is
the source of this trapped pressure?
Think of one of those long balloons, cut the end off and pinch it
(washing machine valve), now blow into the balloon from the other
end until you have a pressure equivalent to your water pressure.
Pinch off that end (shut off valve). What happens to the air in the
balloon? Its still at pressure isn't it? If you want to relieve the
pressure, then you have to open the unpressurized end. The only way
to do that is to either unscrew the fitting at the machine and deal
with the drip, or cycle the machine through a fill cycle - the only
water it will get is what's in the hose.
--
Grandpa