Well, our water heater's playing up (I'm suspecting lower element of the
pair), but I noticed that there's no check-valve on the cold inlet; should
there be? We're on a private well with a pressure tank on the cold side,
if it makes any difference. Sometimes the hot water output seems to cough
and splutter a bit when the hot tap's first turned on, and I wondered if
the lack of valve means that water's somehow getting sucked back out of
the tank when "at rest" via the cold line, dropping the tank level below
the hot water output.

I discovered yesterday when I went to drain the system that
the shut-off valve on the cold water inlet to the heater is utterly
buggered - so I need to do some putzing around with a new valve there
anyway, and I suppose it'd make sense to add a check-valve if needed at
the same time.

Oh, the upper heating element in the tank uncrews nicely. The lower one
that I want to change has currently had a 4ft breaker bar put on it and
isn't shifting. I'm leaving it to soak in a well-known penetrating oil for
a bit before I try again :-)

cheers

Jules