Pretty sure I've got this, but would like confirmation.

One zone of 4 on my boiler is not putting out heat. Gas company came out, gave boiler, valves etc. the thumbs up and say they believe the line has air in it and is not flowing. They can't do anything because they are not plumbers. So it's going to be -14 this week, and I have no heat in 1/4 of the house.

I did this many years ago, but would like to confirm what I remember from looking at my system.

  1. Zone feeds, just beyond the zone control valves, with positive stops and drains on each: ![zone feeds](https://imgur.com/QHltdfq)

2.Common return to boiler, with drain valve. ![common return](https://imgur.com/tbyBGyJ)

If I: a. Shut down boiler. b. Shut positive stops to all zones (orange handles in pic 1). c. Attach drain line to offending zone drain (in pic 1) into bucket of the glycol mix. d. Attach pump to drain line in 2 and feed from glycol bucket. e. open both valves and start pump

I should push glycol up through the return line, through the offending zone and back out it's drain, pushing out air and refilling line.

Correct? Or will I have a problem with the pump in 2 being in the return line I am trying to push the glycol through?

I don't want to screw up my boiler with even colder weather coming, let me know if I am overthinking this or missing anything.

THANKS for any help!

  • imgur.com/QHltdfq Dec 27, 2017 at 21:28
  • imgur.com/tbyBGyJ Dec 27, 2017 at 21:29
  • 1
    First - is this a glycol system at all? Many are plain water. Second, have you looked (carefully, they are not always convenient to find and often hidden under baseboard covers or in closets, etc.) for air bleeders on the affected loop? Third - what does the pressure gauge on the boiler say? It's a bit hard to make out in the second picture.
    – Ecnerwal
    Dec 27, 2017 at 22:49
  • Yes, it's glycol. No, there are no bleeders on that loop, I've been over it multiple times, and grew up with such systems. I only have a single bleeder valve in the house. Pressure and temp are normal for the boiler, I look at those pretty regularly, around 170 and 15 psi. Dec 28, 2017 at 2:08

3 Answers 3


I had something similar happen here in NJ. Basically, the ground level zone had no hot water circulating nor could I flush out the water from that zone. It felt like something was stuck/clogged. The other 2 zones (2nd / 3rd floor were fine. After 6 hours of YouTube videos etc, it occurred to me that maybe the zone had a frozen section and due to the weather dropping to 7 degrees overnight.

Surely that was it. Waited until the weather went to over 40 and basically everything works fine now and flushes fine and heat is restored.

Lesson: Since I have my boiler in the 1st floor next to the thermostat, this zone essentially never turns on overnight especially if I have it set to 65 since the water heaters and boiler naturally keep the room above 65. So I will set it to 73 when the weather drops below freezing so the zone activates and doesn't freeze.


Sounds right for a glycol system with no bleeders. The circulating pump is not an issue, it won't block flow unless it has a check valve built in and your connection is opposite that (which I don't think it is) - in which case pump the other way. Want to run your other end back into the bucket below the surface and run the fill pump until you have flow out of the drain hose with no bubbles in it. i.e. push out the air and keep flowing for a bit to make sure all air is out.

If it's a recurrent problem you might want to have bleeders installed (or install bleeders, depending on your DIY-ness) on the high points over the summer. You may also want to check your expansion tank.


You can sometimes flush air pockets out of pipes by pumping city-pressure water through them. Your circulator pump is not powerful enough for this. To the circulator, an air pocket appears as a constriction in the pipe and it just pushes tiny amounts of water through past the air bubble.

This video is an example, but you have to modify the procedure to suit your system.

You may not have the right control points ... you have to isolate the zone from the boiler and other zones, and have a way of pumping in and expelling water, then shutting it off without draining it. In the video, the fill valve and various other existing valves are used. If you don't have all those you'll have to improvise, or drain the system and add them.

I have seen plumbers do this by disconnecting two radiators and pumping water between them, when they know the air pocket is in that part of the line. This is partly art, and takes experience.

If your system is filled with glycol I don't know how this procedure could be adapted to it. It's worth some thought.

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