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CI rad loop pressure drop

kensheets2 Member Posts: 36
edited November 2020 in THE MAIN WALL
I have a 3/4" pex loop for cast iron rads.  Im pressure testing before I close everything up.  Piping stubbed out with caps, no rads attached.  Have a valve on return side and was able to bleed air out.
I pressurized with garden hose and was able to get to 78psi.  Over two days it has dropped approx 6 psi.
I see no leaks at fittings so is this drop acceptable?



  • Leon82
    Leon82 Member Posts: 684
    So both sides are closed with a valve or the water is constantly pushing?

    Of so your pressure readings would be affected by pressure drops on the city water line.

  • Jamie Hall
    Jamie Hall Member Posts: 17,111
    Possibly. It does suggest a very slight leak somewhere -- but if you really got all the air out, the leak could be truly tiny. Water is almost -- but not quite -- incompressible, and the radiators are also, of course, so the main volume change would be from the PEX, which isn't all that elastic itself. It could also be, now that I really think about it, from the PEX taking a set under pressure, with no leak at all...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • motoguy128
    motoguy128 Member Posts: 378
    Did the temperature drop by chance? 10 degrees might cause a 6psi drop. With PEX, air can tend to leak out where water wont (bigger molecule). If it’s stubbed up and capped I don’t see how you got all the air out.