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How much air necessitates purging?

harrycw Member Posts: 3
edited July 2015 in Radiant Heating
Hi folks. I'm new here, but I was thrilled to see Dan Holohan's name on the mast-head. Dan, your "Hydronic Radiant Heating" got me rolling on our home system, thank you very much for an informative and interesting (and inspiring) read! We didn't install the system (except the tubing), but understanding it has been a great help in dealing with our installation.

We have a radiant system, zoned with circulator pumps and check valves. I recently pulled one of the circulator pumps, a B&G NRF-25, to replace it. It has the nice flange valves on either side, so it was easy to isolate it from the system for removal, with little water spillage.

Reinstalling the new one was easy too. It's oriented vertically, so the outgoing port's flange is on top.

I connected the bottom first, and was able to fill the pump with distilled water before I connected the top flange. That left only a very small amount of air in the joint when I was finished, maybe only a teaspoonful of air remained after all was closed up.

There are 2 automatic air-bleeders at each manifold, and one more on the main circulator's 'mixing' loop.

Do I need to flush this bit of air out of the system manually, or can I count on the air bleeders to take care of it?

I'd prefer not to introduce any more new oxygen-rich water to the system.

Thanks for any input.


  • JStar
    JStar Member Posts: 2,752
    I'd feel safe letting the system get rid of the air on its own. You'll know if there's a problem if you hear water trickling sounds in the system while it's running, or if it fails keep up with the heating demand.
  • harrycw
    harrycw Member Posts: 3
    Thanks. No trickling sound, but the main 'mixing' circulator has a bit of a rattle sound, which I think the manufacturer said could be the sound of cavitation due to air.

    The system works ok, so I'm hoping it works itself out...
  • hot_rod
    hot_rod Member Posts: 20,828
    Is there an air purger near the boiler? That is the best option for air removal. if so, check and make sure the cap is lose so air can vent. The manifold vents will eventually get the air IF it makes it's way to the manifold.

    Keep an eye, or hand on that circ. if it gets really hot it could be air locked. Sometimes just loosening the flange bolts on the discharge side will burp the air bubble out.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • harrycw
    harrycw Member Posts: 3
    The only bleeder near the boiler is on the expansion tank "tee", relatively low on the system.

    The main circulator pump (the large one in the mixing loop) is on top of the boiler, and there are no bleeders up that high, so I like your idea of loosening the flange a bit.

    Thanks very much.