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



As the others have asked, why? Siphoning the water tank dry sounds pretty
unlikely if all taps are closed.

But anyhow, your water meter may already have one as I believe most newer
meters are required to have them (5-10 years). If your heater has an
expansion tank hanging from the ceiling on the hot side, then you already
have one.