I have a combination oil/hand fired coal boiler, and for the 6 years we've lived here, I always have had air in the 2nd floor zone. I've purged the system by isolating one zone at a time, hooking a garden hose and bucket up to the drain valve and opening the flow reducer. Always there were bubbles in at least one of the zones, and they always eventually stopped, leaving me to think I'd been successful, but then about a day an a half later we get woken up in the night sounding like Niagara Falls. With the system up to temperature, between 160 and 180, the pressure gauge on the boiler shows around 20 psi.
Originally there was a single circulator pump, it was on the left, and on the right were 3 zone valves, where the flow control valves are now. We had a leaking valve and hired a plumber to fix it, and he recommended and installed the system as it is in the picture.
Earlier today I drained and properly recharged the expansion tank, which was completely waterlogged, but that didn't seem to make any difference whatsoever.
I'm guessing what happens is air originates from somewhere in the system, but always makes its way to the 2nd floor zone, since it is the first one to come off the output side of the boiler, it's the pipe with the #3 label on it.
So problem 1, i'm not sure why the air isn't going to the expansion tank instead. I guess the boiler output is higher, so it goes there. I'm about ready to put an air separator right after the elbow on the output pipe.
Problem 2, the root cause, is where is all this air coming from? When I override the thermostat, I can sit and listen to the pump sound like it's pumping soda. It might quiet down for a minute or so, but then I hear a release of bubbles all at once, and about 45 seconds later the pump is making noise again.
There are air separators right before the downward elbows into the pumps, you can see one in the top right of the picture, but a) at that point, the air has already been through the house and woken everyone up, and b) they don't seem to be doing a damn thing anyway as I sat earlier today and listened to it pump air around and around for over an hour.
Any help and suggestions would be appreciated.