Hydronic system fill PRV setting?

[Takket3]

I know from the following that a PRV should be set to maintain 4PSI at the top of the system for "positive venting".

https://www.deppmann.com/blog/monday-morning-minutes/07th/

But what if the PRV is actually at the top of the system? (Same elevation as my chilled water piping). Do i just set it at 4PSI? Or should I use a minimum of some other pressure?

 

[LittleInch]

You should set it so that you don't exceed the design pressure of your system at the lowest point. It would be better to set it higher than 4 psig. You always need to allow some margin from normal/max operating and relief pressures.

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[lilliput1]

In addition to the PRV on the water makeup line set to maintain minimum 4 psig above the highest piping static water level pressure provide a relief valve vented to floor drain that would open before the maximum allowable water pressure in the system is reached if the water makeup PRV fail open.