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Primary secondary pumping

I have a customer who has a pole barn set up for radiant floor heat that I am going to be installing the boiler for. I normally use primary secondary pumping with a “pumping away” module for piping layout on job with multiple zones and radiant floor heat. This however will be just one big zone of only radiant floor heat. I am trying to find out what would be the pros and cons of just piping as one zone the boiler is a high efficiency I can set water supply temp at 120° and the return temp will be what it will be. From what I’ve seen the lower the return water temp on high efficiency boilers the more efficient they are, so I’m not seeing the purpose of primary secondary pumping in this instance. Manufacture says the recommend primary secondary but can not give reason why..any thoughts?


  • Zman
    Zman Member Posts: 7,447
    What boiler?
    What is the flow rate for the radiant?
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • thegreatcornholio
    thegreatcornholio Member Posts: 25
    Navian Nhb-110
    11 gpm 
  • ewang
    ewang Member Posts: 77
    Primary/Secondary piping "ensures" (if done right) that flow across the boiler is independent of flow happening in the heat distribution system. You have a lot more flexibility in remedying issues with Primary/Secondary.
  • Ironman
    Ironman Member Posts: 7,119
    Directly pumping the system like that will add the resistance of the boiler’s heat exchanger to that of the system. You may need a larger circulator which usually costs more than what a standard one and a couple of Tees would. And then you still wouldn’t have the benefits of p/s.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • thegreatcornholio
    thegreatcornholio Member Posts: 25
    I suppose I might go to 1 NRF36 vs 2 NRF25’s if I decide to not do p/s. 
  • hot_rod
    hot_rod Member Posts: 19,286
    Their manual shows the pressure drop for the various models.
    I'd prefer a hydraulic separator as the first choice.
    If you can assure flows you could direct pipe. Easier done with a non-zoned system like yours.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Ironman
    Ironman Member Posts: 7,119
    You'll have at least 22' of head if you direct pump that system. At 10+ gpm, that's stretching the upper limit of an NRF36. And it costs 3 times as much as an NRF 25. To me, it just makes more sense to do it p/s like the manual calls for.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • GroundUp
    GroundUp Member Posts: 1,574
    11 GPM? How big is this building? Personally, I'll pipe a single zone pole shed like this direct every time as long as the MFG allows for it. 16ish feet of head at 11 GPM assuming 300ft loops of 1/2", but unless this is a 6000 sq ft building with an incredibly low heat loss, it doesn't need 11 GPM.
  • thegreatcornholio
    thegreatcornholio Member Posts: 25
    3200 square feet (40X80) 13 parallel 250’ 1/2 circuits
    EBEBRATT-Ed Member Posts: 13,456
    Mod cons have high pressure drops. So P/S piping ensures the boiler has the flow it needs.

    If you pipe it direct you need a high head pump (system + boiler). Any glitch in the system calculations and the boiler doesn't see enough flow or you oversize the pump and choke it down

    Those boiler don't hold hardly any water. I like to feel like I am not going to scorch the HX and want to know the boiler has what it needs. Weathe r the system does or not becomes a separate issue