CI rad loop pressure drop

kensheets2

Hello, 
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?

Thanks

Leon82

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

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...

motoguy128

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.