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

2 Answers 2


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.


The problem is that the circulation pump develops very little pressure. When the system is full there is only an inch or so of water column pressure.

Air collects in high places, and if you get a big bubble in a line you have reduced or no flow. you can get automatic air bleeders to put at high points to catch this.

On my system there is a drain valve on both the main line from boiler to both the supply and return manifolds.

The circulation pump doesn't supply enough pressure to move the air.

Try this:

  • 5 gallon bucket.
  • Washing machine hose with female garden hose fittings on both ends.
  • Sump pump with a male garden hose fitting.
  • Another chunk of hose about 4 feet long with a female garden hose coupling. Can be the other washing machine hose.

Sump pumps typically have a working head of 12-20 feet, and can move a lot of water. As the pump runs, it will carry air with it. Since one end is free in the bucket, you can see bubbles coming out. Run it until you get mo more bubbles.

By isolating each zone one by one, you keep bubbles from hiding. If there are multiple paths and one is airlocked the water will tend to bypass the airlocked zone.

  1. Fill the bucket half full of water.
  2. Attach washer hose to output of pump, and to one of the drain valves on the supply line.
  3. Drop the sump pump in the bucket of water.
  4. Attach the other hose to the other drain valve.
  5. Put the free end in the bucket.
  6. Close off all the other zones.
  7. Open both drain valves.
  8. Turn on the pump.
  9. If you have to do several zones, just open the next and close the one you just did.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.