[jnaas2]

I wouldnt have the city decrease the water pressure , but you may try turning the water supply off and opening all the valves in your house incase there are water hammer arrestors installed in the walls that have lost there air charge and they will allow the water heater to heat the water and keethe water pressure from rising to high As hayzee says that could be a siphon breaker but I have never seen one installed on a hot water heater before and you learn something new everyday