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OK, here's the pic: new mod-con

28W28W Member Posts: 141
This is a tough crowd, so I post this with some hesitation.

The insulation is stuff I've started adding.


  • icesailoricesailor Member Posts: 7,265

    I'm sure it works fine. Some hair splitters may split hairs and I may have done a thing or two different but those are decisions, made in the field at the time.

    Some PNOC extremists may complain about the location of the fill and expansion tank but my feeling is that if you run it at 16# to 18#, and it isn't making water and dripping out the relief valve, you should have many years of happy service.

    But that's my opinion and not worth a lot.
  • R ManninoR Mannino Member Posts: 431

    manual shows the fill and expansion tank on the circulator line to the boiler. I'll bet that location would work just fine, it's on the correct side of all the pumps.
  • 28W28W Member Posts: 141
    edited August 2012
    That's correct

    You guys are the experts, not me, but FWIW, in systems with low-loss headers it is fine to put the expansion tank on the secondary loop return. For example, see the last section of this:

    I emailed Viessmann tech support, and they said the diagram in their instructions is provided as a suggestion only.  They had no concerns with the placement of the tank.
  • Jean-David BeyerJean-David Beyer Member Posts: 2,565
    I have a different boiler

    (W-M Ultra 3), and they, too, put the expansion tank and makup water on the secondary loop (the loop without the boiler). And my installing contractor did follow the near-boiler rules in the installation manual. The microbubble absorber is where the expansion tank is connected there.

    For me, that was not a good location only because the water temperature in my secondary loop is very low (main heating zone is in-slab radiant), the air is eliminated only very slowly (months, rather than days or weeks. Once it is gone, it stays gone, but it is inconvenient. Big bubbles go right away, but a circulator for one of the zones is noisy for quite a while each time some air gets in (as in when a circulator had to be changed a coupla years ago. I wish that had all been connected in the primary (boiler) loop, so when the indirect was running (it is across the primary loop), the higher water temperature would have gone through the microbubble absorber. Actually, there is not enough room to do that in my boiler area.
  • 28W28W Member Posts: 141

    You are clearly more knowledgeable than I am about these systems. There is an air vent atop the low loss header.  Is that different than a micro-bubble absorber?
  • icesailoricesailor Member Posts: 7,265
    Blowing Bubbles:

    Raise the system pressure to 18# and let her rip. Raising the pressure makes smaller bubbles and they are absorbed faster.

    Cuts down on cavitation issues too.

    At least that's how it works for me.
  • 28W28W Member Posts: 141
    It is at 14# right now

    but of course the heat is not on. Am I correct in assuming that when things are running full blast in the winter, the pressure will increase?
  • Chris_110Chris_110 Member Posts: 3,056
    Tank and Feed

    Should be on the boiler side of the LLH. Velocity slows to 1.7ft per second in the LLH which is why the mixing action and why sediment will settle to the bottom of the LLH. Your greating pressure drop is at the boiler and the greatest place in the system for the introduction of air..Will it work yes and maybe even trouble free but you do leave yourself open to potential air issues..Issues may be more of a nuisance to the guy that has to drain down the system and refill.

    I'd also like to see a little run of pipe instead of that elbow directly off the supply side of the LLH. Another point of air introduction..
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • 28W28W Member Posts: 141
    edited August 2012
    Over my head

    That's for sure! I don't know enough about the physics involved to understand why it makes a difference in terms of air issues. How does air get in, anyway? Also, I naively assumed that any air would come out at the vent on top of the header.
  • Chris_110Chris_110 Member Posts: 3,056

    You want the run of straight pipe (12") so you can eliminate the turbulence that is going to occurr in that elbow.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • 28W28W Member Posts: 141
    OK, that I understand

    But the location of the expansion tank is over my head, I think.
This discussion has been closed.


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