| Longmain, one item not mentioned is what type of expansion tank is installed: bladder or non-bladder? There is a difference in how the air gets vented from the system. With a non-bladder tank is should be returned back to the tank.
But at the same time, I think the air problem is a little more basic. You mention that when the circulator is running there is negative pressure on the upstairs rad's (or the 2nd floor Tee). IOW, air will enter the system when the bleeder is opened.
This may be the real issue, as it will allow air to be pulled into the system. Air being less dense then water can seep into a system while the water can't seep out.
It may be worthwhile measuring the lengths of pipe along with the diameters on each of the zones. Then calculating each zone's head loss, then a combined head-loss for each mix of zones (1 & 2, then 1 & 3, then 2 & 3, then all 3).
You may find that the 007 pump isn't the best choice. Or that the upstairs zone has a head loss that doesn't match the pump. Which could cause problems when only that zone is active.
TACO has several technical documents (their name) on circulator sizing. TD10 and TD09 seem to cover the requirements and how to figure them.
Al.  |