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Baxi condensing boiler - is this plumbed correctly?

jeds Member Posts: 13
I have a thread in the Radiant section where I received a lot of good advice that has helped me to understand and start making use of the Outdoor Reset function of my Baxi Luna HT380.  That led me to examine the plumbing and logically it seems as if the flow direction is opposite of what I would expect it to be.  My understanding is that the goal in a system such as this one is to maximize the temperature differential between the output water temperature going to the emitters and the return water coming back into the boiler in order to maximize condensation and therefore efficiency.  Correct?

If that is the case, then why is my system mixing the hot water coming from the boiler with the cool return water before the return is pumped back into the boiler?  The below picture illustrates the flow path.

<a href="https://picasaweb.google.com/lh/photo/MV4G6jCz0_srljjfm078F2ky2t3axPgBCwUA2sxHplg?feat=embedwebsite"><img src="https://lh4.googleusercontent.com/-2OE7162BRro/ULTP-keZArI/AAAAAAAADdI/oY5j7-a_9oY/s800/IMAGE_3C8EFB82-0B46-4A24-ACFE-FA3C6C0349C2.JPG" width="800" height="598" alt="" /></a>From <a href="https://picasaweb.google.com/jedsheckler/DropBox?authuser=0&feat=embedwebsite">Drop Box</a>

The yellow arrows indicate the direction the circulating pump is pulling the water.  Would it not be more efficient to pull the water in the opposite direction, maximizing the temperature of the water going to the emitters, and minimizing the temperature returning to the boiler?  Why is the piping connecting the output and return required at all?  (I think this might be in order to allow the water to continue to circulate when there is demand for DHW and the three-way valve inside the unit closes, but I'm not sure.)

Thank you very much for your input.

<a href="https://picasaweb.google.com/lh/photo/MV4G6jCz0_srljjfm078F2ky2t3axPgBCwUA2sxHplg?feat=embedwebsite"></a>


  • zacmobile
    zacmobile Member Posts: 211
    edited November 2012

    Unless there is a second separate pump circulating fluid to the radiators then yeah it's piped wrong. If the boiler pump is the only one on the system then you should be going straight out to the rads & straight back but with a bypass between the two; either a manual valve or a pressure activated bypass valve.
  • jeds
    jeds Member Posts: 13
    Yes, there is a circulating pump

    further down the line on the far left of the photo labelled 'to pump and radiators'.
  • Gordan
    Gordan Member Posts: 891
    RTM... the answer is in there in the form of schematics

    Someone could google the manual and post the answer but... so can you. :-) You seem like you have the requisite understanding of the issue to comprehend the installation manual. But, in short - the tees from the boiler are on the too-far-apart side and yes, one would expect the boiler supply tee to be downstream of the boiler return tee.
  • jeds
    jeds Member Posts: 13
    Thanks Gordon.

    I have read and re-read the manual extensively.  It is remarkably short on information, at least for someone who until a couple of days ago had never thought twice about his heating system.  ;)

    Anyhow, here is the only diagram that seems pertinent from the manual, and it is what triggered my concern that there was something wrong, but I'm not sure I'm fully understanding everything, and I'd like to prepare myself before I call the installer back out and potentially make a fool of myself. 

    Also, to be clear, in my situation this is a single-zone system.

    From Drop Box
  • Gordan
    Gordan Member Posts: 891
    My bad...

    Boy, that is TERSE. I had to check it out after seeing your reply.

    But yes, what you think is the boiler supply is the boiler supply, and what you think is the boiler return is the boiler return, and they should in fact be piped in the sequence system return - boiler return tee - (short nipple) - boiler supply tee - system supply.
  • Baxi Luna

    That looks like half-inch piping for the heating, no?  I know the Baxi's are sensitive to flow - they like a lot of it because the heat exchanger produces so much heat.  Even though the inlet and outlet are 3/4", I know an installer who pipes a 1-1/4" primary.

    Just sayin'.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
  • jeds
    jeds Member Posts: 13

    It measures .88" with my calipers.

    Thanks for the thought though.
  • jeds
    jeds Member Posts: 13

    I think I'll have him stop over soon and discuss it directly.  At least it doesn't appear I'm totally off my rocker.
  • OK

    Then it's 3/4" copper (but it sure looks like 1/2").

    On your original topic, you're absolutely correct: the boiler wants to see the coldest return water from the system to get full efficiency and I don't think you're getting it with the existing piping.  According to your diagram, the return water is blending with supply water.

    It looks he's tried to install a primary-secondary system, but it's not correct.  Does anyone have a diagram to show one that has been piped properly?  To begin with, closely spaced tees are absent.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
  • jeds
    jeds Member Posts: 13
    Another question

    Hi all. Thanks for the help so far. I have not yet had the contractor out to review this, but in the meantime I have another question.

    The external circulating pump was replaced shortly after the install of the boiler as some radiators were not getting hot. With the new, larger pump, they do, but there is quite a bit of resonant noise from the velocity of the water through the pipes. Upon further inspection I've now realized that the pump has three speeds, and it was set to max. At low speed, it still keeps the house warm, and the noise is gone, however the boiler is now running 17-18 hours per day vs ~12-13 with the pump on high. How much is too much? How should I balance pump rate vs. the temperature curve on the boiler?
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