"Winston" wrote in message
I was thinking about equipping the end of the hot water
plumbing with a timed 'hot water' solenoid valve that
would re-direct this 'warming' water into a tank outside,
on the push of a button. Conceivably, I could fill a
55 gallon tank in 18 days or so.

I could use that for my garden or to wash my
car using a solar powered pump, for example.

I live in a very temperate desert climate, so I'm not
too concerned about freezing.

I heard that it was illegal to make use of 'grey water'
so I certainly don't want to run afoul of the law in
case this qualifies, in some way.

I don't see how it qualifies as gray water. Using it is far better than
dumping it.

My run is considerably shorter, but aside from the shower, I don't wait for
the hot water to wash my hands or similar uses, but I don't want to step
into a shower not properly set.