"Ed" wrote in message
. net...
Should I put a back flow preventer or check valve on the water inlet on a
water heater ?

That depends. A house that I owned up until 6 years ago was on top of a
hill. When the water supply failed (e.g. when there was a fire on my
circuit and the pumpers went whole hog) the water from the tank got sucked
back into the mains and the heating elements on my heater burned out ...
happened several times ... I was forced to install a check valve.