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Primary / Secondary Pump size's

kpc_73 Member Posts: 26
what was the boiler, output, pipe size add zone load and pipe size....were you using for circs.


  • David Lambright
    David Lambright Member Posts: 2
    Primary / Secondary Pump size's

    We had an application where the secondary pump was larger then the primary (boiler) pump. The Lochinvar Rep said we needed to install a pump that was 10% larger then the primary pump in the secondary loop. I don't feel this is correct. My understanding is that the pumps are sized independent of eachother. Could you please clarify this or direct me to where I can get the information. Thanks.
  • don_205
    don_205 Member Posts: 66
    Could it be

    That he wants to used the tee connection as some what of a mixing valve?

    Thats what going to happen you know when the secondary circ is bigger then the primary.May even have to increase the temps on the primary loop.
  • Pyro
    Pyro Member Posts: 1

    Primary pumps should be only sized for the needs of the boiler PERIOD! OFC you need to keep in mind that to do the job right, you need to size it vs. GPM needed at the boiler, Delta T intended (condensation or not), and pipe size.

  • Andrew Hagen_2
    Andrew Hagen_2 Member Posts: 236
    Flow Rate vs Pump Size

    For a condensing boiler, there is no reason to have a boiler flow rate higher than the system flow rate. That would unnecessarily warm the return water and raise the average temperature of the boiler.

    You are correct that it is the flow rates, not the pump sizes, that are important.
  • DanHolohan
    DanHolohan Member, Moderator, Administrator Posts: 16,490
    I think you would enjoy

    my book, Primary-Secondary Pumping Made Easy

    It's all about the whys and hows of pipe sizing for these systems.
    Retired and loving it.

  • just to back up Dan, that's a great book and it should definitely be purchased and read.
  • David Lambright
    David Lambright Member Posts: 2
    Add it to my order

    Dan, can you add that book to my order I made earlier for the IBR training manual? You should have all the info.

    In regard to the pump question, I have to get with my technicians to find out more details. The part that had orginally got me was the comment from the Lochinvar rep that the primary pump needed pump 10% more then the secondary loop pump. When I get further information I will post it. Thank all of you for such a prompt reply. I'll have to spend some time reading past threads.
    David Lambright.

  • you have two choices. As others have said, it's really about flow, not about pump size.

    if boiler flow is greater than secondary flow, then secondary gets boiler temp supply water... and return water is a blend of boiler supply and secondary return, which is warmer than it could be.

    If boiler flow is less than secondary flow, then secondary gets a mixture of boiler supply and secondary return, so it's cooler than it could be otherwise, but the return to the boiler is coldest possible.

    This means you might need to turn up your boiler temp to get the same temp out to the zones... but your return should be cooler, which is better for your boiler efficiency.

    that's my take at least.
  • DanHolohan
    DanHolohan Member, Moderator, Administrator Posts: 16,490

    Retired and loving it.
This discussion has been closed.