Quote:
Originally Posted by longmian  When the circulator pump is running, if I open the cap on a tee (I do not know what this thing is called) upstatirs, sometimes I hear air getting into the pipe instead of going out |
You have just described an absolutely textbook case of what happens when the near-boiler piping is not configured as "pumping away."
Search around here for threads on pumping away. There's also a fantastic, simple book called Pumping Away by Dan Holohan that provides explanations and diagrams.
What is happening is that your circulator is probably mounted on the return to the boiler. Meanwhile, the expansion tank is mounted on the bottom of the air eliminator, which is on the supply. The expansion tank is the point in your heating system where the pressure is constant. Usually around 12-15 psi.
When the circulator runs, it creates a pressure differential. Because the expansion tank is on the outlet side of the circulator, the pressure differential required to move water is achieved by creating negative pressure at the top of your system. That's why you hear air sucking in when you loosen a cap upstairs. This also creates long-term air problems that don't go away. Every time the pump turns on, the pressure drops and any microbubble of air in your system expands (because of the decreased pressure) into a macrobubble, which then gurgles around seemingly forever.
In a 'pumping away' configuration, the point at which the expansion tank connects to the system is on the inlet side of the circulator. In this configuration, the circulator must create its pressure differential by adding positive pressure to the system. This has the effect of keeping microbubbles as microbubbles, and moves them around the system to the air eliminator.
There are two ways to fix the problem:
1) move the circulator to the supply side, just downstream of the point where the expansion tank connects to the system. Or
2) move the expansion tank to just upstream of the circulator. For example, remove the tank from the bottom of the air eliminator and install a plug. Cut in a tee just before the circulator and attach the expansion tank off the tee.
Both require some thought and attention to how you want the finished product to look, but both achieve the same result (my personal preference is to move the circulator, but sometimes that requires a lot more work). Both require draining most or all of the system. Plenty of help here should you choose to attempt it, or call a local professional.