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air bleeder on LLH, open all the time?

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sunlight33
sunlight33 Member Posts: 378
There is an air bleed plug on my Viessmann LLH, I understand it's for any air in the closed loop to escape but should the cap be turned loose all the time? Will air find its way into the system?

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  • Jamie Hall
    Jamie Hall Member Posts: 23,675
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    It's used for bleeding or purging. If it's loose all the time, air shouldn't get in, true -- the system pressure should be positive. But... water might come out, no? And you don't want that...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • sunlight33
    sunlight33 Member Posts: 378
    edited January 2019
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    Thanks, here's a related question: If I change the pump on the secondary loop, what's the best way to purge the air out of the system? Right now I have a Grundfos 15-58 and want to replace it with the Alpha2. I am guessing the air bleed on LLH won't do much to air (if any) on the second floor, since the boiler is on the first, right?
    Edit: the Alpha2 manual says it's self-venting, that means it can do the purging itself right?
  • Jamie Hall
    Jamie Hall Member Posts: 23,675
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    No. Purging a drained loop of air and venting are not the same thing at all. Venting will take care of small bubbles of air -- or sometimes even fairly large bubbles -- but to really purge the piping of air you not only need to vent the air from the high points in the system, using vents at those points (and then closing them) but also to run water through the system at fairly high velocity -- best bet is use your domestic water feed at one end to an open drain at the other -- until it runs clear with no bubbles.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • sunlight33
    sunlight33 Member Posts: 378
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    Just to confirm, I will connect a hose to the drain valve of the secondary return, then open up the pressure reducing valve (all the way up?) to let 50 psi city water flush the system, then restore the pressure reducing valve to normal position, and monitor the pressure until it drops back to 15 psi then turn off the return drain valve. Anything I missed?
  • Jamie Hall
    Jamie Hall Member Posts: 23,675
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    May work... provided the pressure reducing valve can flow enough. Some can't.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,849
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    You should have air vents on the baseboard. When you refill the system purge it with a hose as discussed above. I wouldn't let the pressure go above 30 psi. After purging, bleed the baseboards. Then start the pump and lower the pressure back to normal. The velocity in the pipe should carry any remaining air back to the LLH where you can vent it