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air in hydronic system despite automatic vents

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Posted by Dave on December 06, 2004 at 12:49:08:

We just had a new air handler/hot water coil installed in our attic (about 35-40 feet above our boiler).
We continue to have problems with air trapping in this loop. There is an Amtrol automatic air bleeder at the highest point, but when I push down on the schrader valve, nothing comes out (no air or water). The boiler pressure has been increased to 24 psi, and the contractor seems to think it must be increased to a working pressure of 30 psi to keep the air out.
He wants to swap out the boiler T&P valve and replace it with one that would release at 40 psi (instead of 30).
I was told that the thing to do would be to have someone in the basement to turn on the water inflow and then watch for when water comes out in the attic at the highest vent. This would give the working pressure for the boiler.Does this seem right?
How about the expansion tank. Say it is 40 feet from the boiler to the highest vent. What should the pressure be set at for this tank?
And how do I keep the air from returning? I thought the automatic vents were supposed to prevent this.

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