MLD wrote:
Once again you responded by sticking to the simple easy solution of
an easy problem and completely ignored and tip toed away from the
initial question (and my questions) that started this whole
discussion. First, to address your comments: You must be a very
unusual plumber--I don't ever recall one that actually measured
anything--typically, they replaced or cleaned out, nothing scientific
or mind boggling.
Thermal expansion: Your comment:*** "The thermal expansion causes
the spike in pressure equally throughout the system as long as the
PRV is working and is closing the system.***

Well, let's see....

Dribble, and it surely indicates that you really don't understand what
you're talking about. Think of thermal expansion as it applies to the
cooling system in your car. As the coolant temp increases, volume
increases, pressure increases until it gets to the radiator cap
setting and then the coolant spills over to the expansion tank.

Not true. Pressure builds until it reaches the pressure limit of the
radiator cap, THEN it spills over to the expansion tank. Haven't you ever
seen a radiator doing an "Old Faithful" or read the caution on a radiator
cap?" In days of yore, there was no expansion tank and the coolant spilled
onto the street.

No
spikes in pressure--- it increases along with the temperature. With
the PRV and all faucets closed you have a volume of water trapped
between them--aka, a hydraulic lock. As the temp is increased, up
goes the pressure-what do you think would happen to your piping if
there was no escape route via a relief valve built in to the PRV or
the water heater T&P valve?

PRVs do NOT allow water to back up into the city's mains. Water heater
valves do NOT drip or flow at the first increase in pressure. It takes a
substantial increase in pressure to trip a water heater relief valve,
radiator cap, or PRV. And I do mean substantial.

No different than if the water
froze--something is going to break!

It's not going to break. Pipes can handle a goodly amount of pressure.

The pressures involved in hot water (or even steam) are much less than the
pressures exerted by freezing water. By a couple of orders of magnitude.
Steam pipes are just regular pipes and none has ever been known to break
from steam pressure. But they will break if exposed to freezing temperatures
and not drained beforehand.