On Jun 9, 9:50 pm, "Edwin Pawlowski" wrote:
"hvacrmedic" wrote in message

You don't have an air buffer. What do you think the purpose of the
expansion tank is? It's supposed to have air in it. If you'd quit
letting the air out of it, then your problem might take care of
itself.

I'm not letting air out of the tank, it is in the water side of the system.
The only way to expel air is to add water or something else to replace it.
The expansion tank is to allow for the change in volume of the water as the
temperature changes, but does nothing to expel air from the copper tubing.
I'm sure you know better.

http://www.watts.com/pro/_productsFu...4&parCat=251&p...
Description:
Series FV-4M1 Automatic Vent Valves are used in commercial and
residential hydronic heating applications to provide automatic air venting
for hot or cold water distribution systems. It consists of a brass body
construction with female NPT connection, brass cover, air vent with silicone
rubber seal, polyethylene float with valve plug, and automatic vent. Series
FV-4M1 has a high temperature rating and is ideal for use with glycol
systems or for use as an anti-vacuum device. It can also be installed to
permit the separation and dispersal of air while fluid is circulating in the
system. Can be disassembled for inspection and cleaning. Maximum Pressu
150psi (10 bar).

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.