| We've got a 007 here. Typical residential circulator. Nothing outrageous. There's a long 2nd-story circuit. Let's suppose it flows at 4 gpm, which might be about 10 ft of head in a long 2nd story circuit of 3/4" piping. The pressure differential required to do 4 gpm at 10 ft of head is about 4.3 psi.
Two things are happening. One, that 4.3 psi differential is being achieved by dropping the pressure at the top of the system. Two, that pressure reduction is allowing small bubbles to expand into larger ones that get pulled through the system. When they get back to the air eliminator where the expansion tank is, they are seeing the point of maximum system pressure and are compressed once again.
It's a real pain to get a system like this purged of air. Every purge introduces fresh, oxygen-rich water. Lather, rinse, repeat....
Yes, many, many, many older systems (and even new ones) work fine without pumping away. But old or new, there is a limit to how far you can push it. Sounds to me like this system is pushed too far.
Jacking up the system pressure to 18-20 psi cold won't help much, if at all. You still have a 4+ psi pressure drop, just from the circulator. When hot, the system is also probably in the 22-24 psi range, which is getting close to starting the relief valve weeping.  |