enter image description hereI have a newly replaced two zone boiler system and am having issues. One zone is the basement only, the other zone is first and second floor. I have basement zone at 70 and it functions as expected. However, even with the upstairs zone thermostat at 62 the temp reads 73-75, some radiators in the upper zone are HOT as are the return pipes from all zones near the boiler. Basement pump is warm or hot, other zone pump is room temp. Air whooshing is ongoing despite bleeding. Water pressure goes between 12-17 poounds. Contractor seems stumped so open to feedback/ideas. Thanks! enter image description here

  • Where does your thermostat wiring go to ? Do you have zone valves or actuators? or the thermostats are simply activating the boiler and a pump for the individual zone. you absolutely should not hear air in the system - did you bleed until you don't see bubbles or are you hoping your air scoop / air eliminator would remove it all? where is your expansion tank? do your radiators have bleed valves? you should add a dirt mag to your system. Commented Feb 7 at 20:08
  • Thanks for reply. Thermostat wiring goes to 2 thermostats one in basement and one on first floor. Both seem to be functioning correctly. I have been bleeding til I don't hear air, I can do again until bubbles are gone. Not sure if we have air scoop or air eliminator. Expansion tank is on ground in the right rear of the boiler. And then I don't think we have a dirt mag but have city (not well) water (chicago) that is fairly well regarded. Commented Feb 8 at 17:49
  • dirt mag gets rid of magnetic particles from expansion tank / boiler / radiators / pipes degrading over time. Commented Feb 8 at 21:05

1 Answer 1


Probably thermo-syphoning (heat delivery without pumping. Hot water rises and cold water falls. Might be aggravated by the comically large iron lines (old steam system, converted to hot water? Probably even an old gravity steam system, so everything is designed to thermo-syphon?)

You can verify by turning off both thermostats for a while while the boiler remains hot (unless you have a "rapid-start" or "cold-start" boiler that's only hot when there's a call for heat.)

Check valves that permit pumped flow but won't open on thermo-syphoning micro-pressures, or deep U-bend heat traps are typical approaches to limit that.

It's sadly common for HVAC folks to be blithely ignorant of the problem and solutions. They shouldn't be, given how common it is, but since they never install gravity systems these days they don't think about it, as far as I can tell.

  • Thanks @Ecnerwal! This makes sense! 110 year old house so very likely the original radiant heat was steam. Two more questions based on your answer. First off would changing the pumps from PULL to PUSH be adding to the issue? And, if I turn off both thermostats I'd be verifying the issue if the upstair zone got cooler, correct? We do not have a rapid-start or cold-start boiler, it is one that turns on when the water in the boiler gets to 165 degrees and it stays on until 180 degrees. Thanks again. Commented Feb 7 at 21:11
  • I'd fully expect the upstairs to continue getting heat, because the pumps don't have to be running for thermo-syphoning to happen. Just need a hot boiler and a cooler radiator above it with nothing blocking the path. As such I doubt pump irection has much to do with it, though it might have an impact on possible cavitation as I don't see the expansion tank on the inlet side of the pumps, but that's a whole other issue.
    – Ecnerwal
    Commented Feb 7 at 22:28
  • So given what you are seeing check valves on the returns would be best course of action? Thanks again! Commented Feb 8 at 16:26
  • Might be needed on supply and return. It's possible (especially with comically large pipes) to have both hot and cold flow in opposite directions in the same pipe. But if the returns get hot with the pumps shut off, the returns alone might be enough.
    – Ecnerwal
    Commented Feb 8 at 16:33
  • I found a picture of our old setup and since I couldn't add it to the comment I added it to the original post at the top. it's the gray boiler with the expansion tank on the ceiling. Any chance you can tell if there are any check valves on this setup? Trying to figure out if the installer and/or engineer who quoted this missed something. Thanks! Commented Feb 8 at 16:38

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