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Quetion regarding ciculator pump sizing

glifen
glifen Member Posts: 22
I have a Navien NHB-080 that was just recently replaced under warranty. After further issues with the new one is appears that the original installation done a few years ago was the cause of the initial failure, an incorrectly sized pump for the DHW to an indirect tank. How do I figure out what the correct pump size should be? The installer is of course not claiming any responsibility in this and I need to try and gather as much as I can.

The indirect tank wants 8gpm, currently its getting under 3. The boiler is constantly throwing errors due to this while DHW is being called. Navien techs said its the boiler turning itself off to try and protect itself from overheating.

Navien NHB-080
Buderus S32 Indirect Tank- wants 8gpm, adds 1.9 feet of head
https://www.bosch-climate.us/files/Bosch_Indirect_Tanks_Sizing_Guide_07.2018_US.pdf
Taco 007-15-71FC Pump
40 feet of 3/4ths copper
16 90degree elbows
1 T

If i look at this chart - http://www.taco-hvac.com/uploads/FileLibrary/100-1.8.pdf - and work backwards. 007 pump with 3gpm would be ~9.5 feet of head. Does 9.5 feet of head make sense with the above specs? or is it possible the taco pump i have is not working properly?

Then, assuming I want 8gpm with 9.5 feet of head, that would be roughly a taco 008? does that make sense?


Comments

  • Ironman
    Ironman Member Posts: 6,308
    How about posting some pics of the boiler and it's near piping?

    A taco 007 may be slightly under-sized, but it shouldn't cause the boiler to go off on high limit or cause it to fail. More than likely, there's a piping issue.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • glifen
    glifen Member Posts: 22
    you can see the piping at the boiler in my original post from a few years ago:
    https://forum.heatinghelp.com/discussion/158773/critique-navien-nhb-80-installation#latest

    i'll get a picture of the indirect in a few
  • glifen
    glifen Member Posts: 22
    Here's some images of the indirect and piping




  • ScottSecor
    ScottSecor Member Posts: 384
    Tough to get 8gpm through 3/4 inch copper pipe. It can be done, but you may get noise and other issues, much better off with one inch. Need to know what the pressure drop (in feet of head) for the Navien unit and the other items that may be in this loop.
  • ScottSecor
    ScottSecor Member Posts: 384
    edited October 2019
    just looked at the 007 pump curve, it will produce 8gpm at 8 feet of head. The catch is this assumes you head loss is less than 8 feet. Assuming your pumping away and not operating the indirect at the same time as the heating system (aka dhw priority), you may not have enough oomph to pass enough gpm. Yes, the 008 should do it, I would try it fist (again assuming everything is pipes properly) before up-sizing the pipe to 1 inch pipe.
  • Ironman
    Ironman Member Posts: 6,308
    I just checked the specs and that Navien heat exchanger has 17' of head @ 8 gpm. You'd need a Grundfos ups26-99 or Taco 013 to do that.
    Yes the 007 is too small, but I doubt that it alone is the problem. During space heating, with only one zone calling, you're probably only moving < 4gpm and the boiler will modulate to accommodate that with no issues.
    Check the 007 to see if it's pumping towards the boiler. There's a directional arrow in the casting.
    And as Scott mentioned, 3/4" is only good for 4 gpm.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
    ZmanAlan (California Radiant) Forbes
  • glifen
    glifen Member Posts: 22
    The pump is currently set up to pump towards the boiler.

    How do you determine what size piping is required and how does that play into the pump/feet of head calculations?
  • Ironman
    Ironman Member Posts: 6,308
    From piping tables. You need a 1" pipe to carry 8 gpm at under 4 spf velocity. Hydronic piping should kept at 4 spf or less.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
    ZmanAlan (California Radiant) Forbes
  • SuperJ
    SuperJ Member Posts: 602
    Can you lower the boiler temp dhw priority setpoint? 8gpm becomes less important and it will be more efficient if you're not running at 180f.
  • Zman
    Zman Member Posts: 6,430
    The above advise is spot on. Some boilers will let you limit the output during a DHW call. If the Navien will, you could solve this with a simple adjustment of the controller. A call to tech support would give you that answer. I you want full output, 1" pipe would be the best option.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Jamie Hall
    Jamie Hall Member Posts: 15,334
    glifen said:

    The pump is currently set up to pump towards the boiler.

    How do you determine what size piping is required and how does that play into the pump/feet of head calculations?

    Head loss in pipe is, to a first approximation, inversely proportional to the square of the inside diameter, and directly proportional to the length.

    For example -- a 3/4 inch pipe will have, to a first approximation, almost twice the head loss for a given flow and length as a 1 inch pipe would have.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.
    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • glifen
    glifen Member Posts: 22
    thanks everyone for the great advice. The Navien does allow me to set the DHW temp, although right now i'm only getting about 2.7gpm so not sure if just lowering it would really solve the issue?

    How do you calculate what water temp the DHW should be set to to maximize efficiency?

    I looked at the Taco 007 specs and chart again, it maxes out at 10' of Head and on the chart with it, 10' of head is right around 2.8gpm. Seems to make perfect sense why this thing is operating the way it is.
  • ScottSecor
    ScottSecor Member Posts: 384
    How are you calculating the gpm?
  • Zman
    Zman Member Posts: 6,430
    I am suggesting lowering the boiler firing rate not the temp. That would protect the boiler from the low flow condition and prevent damage to exchanger.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • glifen
    glifen Member Posts: 22
    The GPM is displayed on the Navien unit. A Navien tech walked me through displaying the info.

    As far as the boiler firing rate, I'll check the manual.
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