The rise can be considerable. I would not be alarmed to see 100 PSI. It depends on how big the WH and how much temperature rise there is between cold and hot. It also depends on the size of the expansion tank. I would not be surprised to hear that you see a lot more rise when you use more hot water, and then stop using water.
If you use a little water, such as flush a toilet or get a glass of water, that could remove enough water to make the rise much smaller.
http://www.engineeringtoolbox.com/expansion-tanks-d_885.html is the non-simple version.
Now my PRV has a 10psi backflow feature, meaning if the pressure is 10 psi greater than what it is set to it will back flow.
Are you saying that it pushes the water back to the city water pipe? In that case, that would require the pressure to be higher than the city water pressure. If you are saying there is a port to release water to a drain, then it could work as you described.