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Air problem

joe amadio
joe amadio Member Posts: 41
We recently installed a new Weil McLain gas hot water boiler. It is installed in an old house with cast iron radiators on the 1st and 2nd floors. In the basement we ran a zone of radiant heating. There is also a zone for an indirect water heater. We used a primary/ secondary loop to protect the boiler from thermal shock from the cast iron. We have been back twice in the last 3 weeks because the indirect water heater zone and the radiant loops became air bound. When I got there the second time I noticed that my tech installed the expansion tank on a tee off the purge tapping off the top of the boiler and then installed a taco auto vent off the top of the tee. We have the system running close to 20 psi because there are 2 radiators up in the attic. What pressure should we have the expansion tank set at? And could the expansion tank being connected off the purge tapping that close to the auto vent be making the auto vent suck air in as the diaphragm contracts? Could there be any other reason for the air troubles?


  • Gordy
    Gordy Member Posts: 9,514
    The x tank should be set to the same psi the system is. The x tank psi should be set isolated from system pressure.
    Pics of near boiler piping is always helpful.
  • hot_rod
    hot_rod Member Posts: 16,575
    does the system have a central air eliminator? Just a float vent into the boiler is not enough.

    remember P/S piping alone may not protect the boiler without a temperature sensing and reactive control at the return.

    Whats the pump, and where is it piped in relationship to the expansion tank? Pumping towards the tank can sometimes cause air to be sucked into a float type vent.

    Pictures tell the story.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • dandifabio
    dandifabio Member Posts: 4
    did you use an x tank rated for radiant? most of the air should just settle up in the radiators. did you bleed all the rads? i have had luck with a main spirovent with the x tank below it and the fill in between tank and vent.