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How to remove air from hydronic system / buffer tank

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TopocalmaWychwood
TopocalmaWychwood Member Posts: 7
edited November 17 in Radiant Heating

Hi,

I have had my system running for one season and just burned out the Grundfos UPS 26-99 I believe due to not addressing cavitation in time. Good reminder for me to put more focus on removing air from the system.

My question is the following: 1) How can I improve the setup to allow for better removal of air and 2) what process should I follow to allow for best results.

I am pasting below a sketch of the system including vertical distances. I am less worried about removing the air from the underfloor heating loops (I believe I know the process well). My main issue is how do I remove air that is trapped in the top of the buffer tank - it does not have a vent valve at the top, and the in- and outflow ports are approx. 8 inches below the top of the tank. Second how do I make sure the supply and return piping to the outdoor air-to-water heatpump is fully vented.

The system has an air separator installed on the warm / supply side for the underfloor manifolds. (Caleffi 551041A, however in the mean time it seems Caleffi updated the design (?). I have a version that looks different than what is being sold under this model number right now). I believe the layout is according to classic recommendations and manufacturer specs.

Any thoughts on how to vent the buffer tank fully and any other recommendations would be much appreciated!

image.png

Comments

  • hot_rod
    hot_rod Member Posts: 26,254

    is the heat pump mounted above the buffer? There should be an air vent in or on that. A manual vent is fine my Viessmann shipped with a Caleffi 551 to mount at the unit

    Is there glycol in the system? It takes some time to get air out of glycol , especially at low operating temperatures

    Glycol usually is pumped in and that should be enough flow for a good purge

    Got a pic of that Caleffi 551

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • TopocalmaWychwood
    TopocalmaWychwood Member Posts: 7

    Yes, buffer tank is in the basement and the heat pump is outside a foot off the ground so about 10 feet higher. So putting a manual vent at the pump would make sense - I shall do that!

    No glycol just water.

    I am not with the system and was trying to find a picture of the caleffi online but did not find one. Will post a pic as soon as I can.

  • TopocalmaWychwood
    TopocalmaWychwood Member Posts: 7

    Ok I have a Taco 4900 air separator

    image.png
  • Kaos
    Kaos Member Posts: 816

    The primary pump should be mounted near the bottom of the tank. The tank needs an air vent at the top as air will always collect there and get into your secondary loop.

    If you are doing re-pipe, a 3 port buffer works much better with air to water units as you get less mixing inside the buffer tank.

    P.S. Looks like you are also suffering from wall of pumps syndrome. You have to be careful with that type of setup with AWHP units as you can over-pump the secondary loop. This has the effect of mixing the emitter supply down with the return water which reduces the COP of the heat pump. With all pumps running, the secondary loop flow should match or be slightly less than the primary loop flow.

  • hot_rod
    hot_rod Member Posts: 26,254

    Viessmann HP has you put an air sep right at the outdoor unit. Discal insulate jacket added after pic was taken

    Screenshot 2025-11-17 at 7.58.33 AM.png

    I did it a little different than their drawing and put the relief valve inside. Their piping is a bit cluttered and would be hard to insulate.

    Screenshot 2025-11-17 at 8.02.02 AM.png

    No harm in having a purger at the HP and on the secondary side of the buffer.

    Not all buffers have a place for a top air vent, mine doesn't. So dual air purgers should do a good job.

    If a tiny bubble traps at the top of the buffer, not a big deal, it will stay there.

    If you find a buffer tank with a large top connection pipe the air purger up there, then it is both a purger and high point vent.

    Three pipe buffer for reference. Return from system crosses bottom of tank to hp for lowest return temperature.

    Screen Shot 2020-07-06 at 10.06.41 AM.png
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • TopocalmaWychwood
    TopocalmaWychwood Member Posts: 7
    https://forum.heatinghelp.com/discussion/comment/1872678#Comment_1872678

    Good point - My graphic is not correct, I have 4 variable speed Taco 0013 e3s (not 8 as shown), one for each floor, hydronic separated via closely spaced Ts… I am optimistic that should avoid "wall of pumps" issues.

  • TopocalmaWychwood
    TopocalmaWychwood Member Posts: 7

    This make sense. I take it these Caleffi air separators are fit for outdoor use? I see you installed it on the supply side.

  • leonz
    leonz Member Posts: 1,709

    Your vertical buffer tank has no top tapping????

  • TopocalmaWychwood
    TopocalmaWychwood Member Posts: 7
    https://forum.heatinghelp.com/discussion/comment/1872922#Comment_1872922

    No unfortunately not.

  • Kaos
    Kaos Member Posts: 816
    edited November 18

    picture=1000 words.

    Check if your heat pump has a built in auto vent. Mine had one by the plate HX that I had to open to purge air during initial fill.

    Four 0013s are pretty big pumps, that doesn't feel right, are you sure about the part #.

    In either case, exact pump count and size aside, you want to check the flow meters on all your manifolds with all zones running, sum them up and you want to make sure that total flow matches your primary loop.

    By closed spaced Ts, I'm assuming a bunch of Ts to feed the pumps from the main 1 1/2" copper pipe. There isn't a set of closed spaced Ts like this:

    image.png
  • leonz
    leonz Member Posts: 1,709

    Perhaps you should seriously look at placing this buffer tank on its side and then add manual boiler drain and an automatic air vent with a ball valve with to enable you to change it and replace it when it goes bad and starts leaking.

    The only other option is to turn all the thermostats all the way up to move more air bubbles to the existing air vent.

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