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I want to try and bleed the air from my hydronic ( baseboard ) heating system. It's a natural gas system, I think I understand the basic functionality but my system seems to be a bit unusual and I'm looking for some advice and information.

The furnace appears to have the automatic fill line feeding into the back left of the furnace. On the rear right, there is a large pipe going into the furnace that is attached to a manifold with 4 valves and an expansion tank, and also has the computer and pressure valve mounted to it. I'm assuming this is the return of the 4 zones. On the front right, there is the gas feed and also what I believe is the main output of the furnace. This feeds into the circulation pump and the output is split into the 4 zones. each zone has a servo valve, followed by a manual valve, followed by a drain valve.

Normally I would assume you close the manual valves and then open the drain valve for each zone to purge the air out of the system, the automatic feed valve should replace the water in the system automatically pushing the air out of each zone.

The thing that confuses me about this system is that the circulation pump is right before that 4 manual valves for the zones. If i shut those calves and open the drain valve, am I not just draining water from the system and not replacing it? or does the automatic fill valve fill the system from the back side? I cant really tell what the pipes do inside the furnace. I'm also concerned ( for no particular reason ) that the fill valve may not be working.

Am I doing this correctly? or am I making some wrong assumption about the setup? I've done this before on a different furnace and it was no problem, but this one is a bit more confusing.

fill valve enter image description here enter image description here

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Well... the way that my father does it is that he isolates the zone from the other zones by shutting off the manual cutoffs. Then attaches a pump to the drain cock on both sides of the zone, and runs the pump -- with luck you'll get the air out of the top of the line and back to the pump, and the auto-fill should replace the water.

Alternatively, you may have air separators at the top of each run, and you can open up the valve at the top to bleed those.

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