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Primary/Secondary Piping

KeenGreen Member Posts: 27
In recent months i have been given 2 different ways to size my primary loop using a Mod/Con boiler.
some say to use the output of the boiler to size the primary while others say to size it to the heating load required. Anyone know which way is best practice?
Learning as much as I can to be better than I am.


    HVACNUT Member Posts: 5,738
    As long as the piping is sized to handle the load of the primary pump. The manufacturer should have everything spec'd for the near boiler piping.
    The secondary manifolds can be generously sized.
  • Tinman
    Tinman Member Posts: 2,808
    The net of the boiler should be the same as the load (heat loss).
    Steve Minnich
  • Ironman
    Ironman Member Posts: 7,345
    It depends what you mean by "primary loop". Most of the time, pro's refer to the loop to which the expansion tank is connected as the primary. Most diagrams and manuals ID it that way too. Is that your meaning or are you referring to the boiler loop?

    In either case, ANY loop has to be sized based upon the flow rate (gpm) that's required in it. That's based on how many btus are required at the delta T required. The universal hydronics formula: Btus = (gpm X 500) X delta T. Any of the components of the formula can be moved to find the values of the others. 500 is a constant that stays within the parenthesis.

    Thus, at a standard delta T of 20*, an 80k btu boiler would need a flow rate of 8 gpm. That would require a 1" pipe.

    Look at the diagram below. I would generally consider what is flowing straight through the run of the tees as primary and what's going through the bull (side) of the tees as secondary. The boiler could be in either loop and therefore could be in either the primary or secondary.

    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
    EBEBRATT-Ed Member Posts: 15,307
    Either way size the boiler loop for the maximum boiler output at the TD of the water 20, 30 or 40 degrees. Size the heating loop for the heat loss or better yet the heat emitter output
  • Rich_49
    Rich_49 Member Posts: 2,766
    500 is only constant in systems using 100% water as the fluid . That number can change when Glycol is used and at different concentrations . gallon of water 8.33 pounds
    Hour x 60 minutes
    499.8 pounds (500)
    You didn't get what you didn't pay for and it will never be what you thought it would .
    Langans Plumbing & Heating LLC
    Serving most of New Jersey, Eastern Pa .
    Consultation, Design & Installation anywhere
    Rich McGrath 732-581-3833
  • Tinman
    Tinman Member Posts: 2,808
    edited May 2018
    When I talk to customers, the words primary and secondary rarely get used. I just use boiler loop and system loop, makes it easier for them to wrap their head around. But yeah, primary loop is usually the system side.
    Steve Minnich