Over the past several weeks I have encountered a very noticeable drop in the pressure of the hot water in the hose. Previously pressure was sufficient to run three showers at one time, now only barely one. The hot pressure problem is experienced in all bathrooms. The natural gas tank has been flushed and no sediment was found. There appears to be an atmospheric back flow valve on the cold water supply to the tank and water seems to be dripping out the vent constantly. Cold water pressure is around 100 pounds. Could the back flow valve be faulty and causing the low hot water pressure coming out of the tank? Any suggestions would be appreciated.