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Primary/Secondary or Primary Only

Corys Member Posts: 1
I am trying to figure out what situations require a primary/secondary system and which situations a primary only loop would work.

Do you always need a primary/secondary system on in-floor? The house will only have about 1000 feet of 1/2" pex installed. Can't I just use a variable speed delta-t circulator on a primary loop with a modulating boiler?

I guess I need to get clear on the basics of both systems.


  • SWEI
    SWEI Member Posts: 7,356
    it depends

    On the flow requirements (load size) and the boiler HX design.  Low restriction fire-tube heat exchangers can be direct piped much of the time.
  • Gordy
    Gordy Member Posts: 9,514
    edited January 2013

    P/S allows you to have two different flow rates with two,separate circulators. The boiler circulator is sized for the proper flow rate to the boiler hx, and the system circulator is sized to the required flow rate of the emitters.

    With primary only your boiler flow rate, and system flow rates are going to be tied together so unless system, and hx flow rates happen to be identical , and meet the minimum for the hx design one or the other will not get the proper flow. As SWEI said fire tube designs are less restrictive than other designs so there are situations you could get away with it, and save a pump. But you need to do the math.

    I suppose you could use a dt circ on the boiler sized correctly, but this would not guarantee proper flow in the emitters, only the boiler. Modulating condensing boilers like cool return temps so the delta could be 30- 40* where the emitters having that high of a delta may sacrifice comfort.

    That all puts us back to,it depends
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