Can't help but respond to your comments: Let's take them one at a time:
1. The car coolant:
I said: ****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.***
You said:***Not true. Pressure builds until it reaches the pressure limit of
the radiator cap, THEN it spills over to the expansion tank.
One of us has a reading comprehension problem--I don't see any basic
difference between the two comments.

2. You said:***PRVs do NOT allow water to back up into the city's mains.
If you're a plumber, than get with it because that's not true. PRV's will
let allow water back to the main. For your info, below I've pasted a
manufactuer's description of his PRV. Note the comment on thermal expansion.
So you don't miss it----under certain conditions it allows the expanding
water to escape back into the supply main before it can affect the relief
valve.-----

3. Pressures developed by expanding water. A car radiator cap is set for
about 15 psi. T&P's I think somewhere between 100-150 psi, steam systems
anywhere from 100-200 psi (only a guess).
Expanding water in a closed system. Not too hard to come up with a pretty
good number.
Assume the water temp goes from 50 to 150F. The Specific Gravity (or
density) changes about 1.87%.
Use the following expression:
Delta P=(delta V/V)*BM
Delta P= change in pressure
Delta V=change in volume
V=trapped volume
BM= Bulk Modules and for water=312,000psi
So: Delta P= .0187*312,000=5834 psi
If you think that the pressure required to trip a T&P valve was substantial,
what do you think of 5800 psi?
How will the typical household components (water heater, washing machine and
hoses etc) deal with that level of pressure? That's why you have T&P valves
on the water heater and because you don't want water spilling all over the
floor all the time that's why expansion tanks are installed so as to absorb
the volume changes without the accompanying rise in pressure.
MLD


*******The use of a water pressure-reducing valve normally creates a closed
system.
When water is heated in a closed system, it expands, causing an increase in
pressure. This pressure may increase to the set pressure of the relief valve
(on the water heater) causing it to drip, thus releasing the expanding water
and protecting the system against excessive pressure, This increase in the
system pressure over that regulated by the reducing valve is called "thermal
expansion pressure".

No. 25AUB by-pass Model water pressure reducing valves, are an economical
solution of this annoyance, since under certain conditions it allows the
expanding water to escape back into the supply main before it can affect the
relief valve.

Effectiveness of the Thermal Expansion by-pass featureis limited to systems
where the street main pressure is less than the setting of the heater relief
valve. Therefore, the highest allowable pressure setting for the relief
valve should be selected for widest effectiveness of a Thermal Expansion
by-pass






"HeyBub" wrote in message
...
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.