Dan Listermann wrote:
Our cold water faucets always produce an initial stream of hot water
before the cold starts. I believe it has to do with the expansion
tank on the heater. There is a check valve on the water line that
feeds the heater, but the backflow seems to indicate that it does not
work. It is the flapper type and it is pointed down. Is there a
problem with trying to use this sort of check valve in a downward
position? I checked the arrow marking and it appears to be installed
in the right direction.

That kind of check valve needs gravity to be naturally closing the flapper to
work properly at low flows. The flapper should be hanging down from it's hinge
point if the valve is horizontal, or the flapper should be at the top if
vertical, so it is always closed at no flow. The simplest solution for you might
be to replace it with a spring check valve that doesn't depend on gravity.