Radiator / furnace problem

[Doug Miller]

How is air going to be sucked into a system that's at 10 to 20 psi *above* ambient pressure?

[hvacrmedic]

If the expansion valve is correctly sized, and correctly charged with air, then the system will never drop into a vacuum, and thus never suck in air. Under and overpressurization is the very purpose of the expansion tank. If yours doesn't work, then the solution is to fix it, not to shut off the make-up valve. Shutting off the valve will introduce problems of its own. As air bleeds off through the airtrol valves (or equivalent) then the valve must be open in order to allow the pressure in the system to be maintained at the regulator setting. With it closed, all of the air in the expansion tank will begin to bleed out of the system as it diffuses into the water, where it makes its way to the airtrol valves. The pressure in the system will steadily drop until the low side is at or below atmospheric. After that air will be sucked into the system and subsequently released from it during each full heating and cooldown cycle. Water will then have to be added to the system manually, and the air cushion in the expansion tank reset. Where the water is leaving the system is at the expansion tank, i.e. when you blow it down to refill it with air.

[hvacrmedic]

In retrospect, I think you said that you only had problems when the valve was closed. In that case, what I went over above is why you had problems before, i.e. with the valve closed. The valve is supposed to be open, so you didn't do anything special to your system, you only set it back the way it was supposed to be. Pardon the oversight.

[Edwin Pawlowski]

Thank you for a good explanation. I keep my valve open.