Just checking that my understanding is correct - when cities/towns add check valves at the water meter, the check valves will not allow expansion back to the mains so an expansion tank is recommended as a way to absorb excess pressure. Correct?
Correct, as long as it is understood that the "excess pressure" being absorbed by the expansion tank is from the water molecules expanding due to being heated by the water heater. The expansion tank isn't designed to manage any pressure fluctuations in the city supply. If pressure fluctuations from the city supply range higher than 75-80 psi, then a pressure regulating valve would be needed.
The expansion tank, on the other hand, can contribute to a longer water heater tank service life, since the flexible air bladder in the expansion tank is the path of least resistance for the expanded water to act upon. Otherwise, if the water system truly is a closed system with a check valve present, the incremental pressure increase from heated water has to be absorbed by the glass lined tank itself... not a good thing. In fact, any cyclic expansion contraction cycles in the plumbing can fatigue metal, wear out elastomeric washers in flexible lines with threaded connections, crack glass linings, and put poorly sweated connections to the test.
In many, if not the majority of jurisdictions, the expansion tank is now a code requirement. But even if not required, it is certainly a good idea, even if the city water system hasn't installed check valves... because if they haven't yet, they might still, and they won't ask your permission, and may not even warn you when they do.
