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Primary/Secondary quick question

fatty_2 Member Posts: 54
1" supply/return, 50k output cast iron (WM cga3) boiler. taco 404 zvc w/ outdoor reset & 4 zones.

i did a heat loss and my maximum possible temp. drop (allzones) is 11.25 degrees.

would it be worthwhile by any means to do P/S pumping on this circuit? the zones can easily do 8gpm max, which matches what the 1" supply can manage in the primary loop. so it would not be infrequent that the primary loop would get taken up entirely into the circuit.

with such a modest delta T (11.25) is the outdoor reset enough protection?

note: the slowest flow across the boiler with only one ZV open would be 4 gpm.


  • tim smith
    tim smith Member Posts: 2,738
    Thermal shock still an issue

    You still would have a problem w/ thermal shock and or condensation unless there is return protection built into the zvc.
  • hot_rod
    hot_rod Member Posts: 21,841
    What type of emitters?

    baseboard, panel rads are what i consider low mass. really no need for P/s with those style.

    If you have a high mass radiant slab, or large cast iron radiators the potential to keep the boiler in a "cold" return situation increases.

    A cold concrete slab start up, for instance could keep the boiler in a condensing mode for many hours. you need protection to prevent that, regardless of the piping you chose.

    Or if you need to supply multiple temperatures P/S work best to pipe in mixing devices.

    hot rod
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • fatty_2
    fatty_2 Member Posts: 54

    thanks guys
    i have cast iron radiators, of a medium size, i shrank them all by 50% when i rebuilt the house and insulated it. i'm coming to the end of year two - and this heat will be the icing on the cake. i'm going to go with p/s

    i see! if the mass is great, the boiler temps are low for some time and that itself can cause condensing. i thought it was the DIFFERENTIAL between the supply temp and the return that made condensation. but no thats where thermal shock is the risk! i see i see. its the low temperature itself. ah ha! all i have to do is balance the isolation valves to ensure some bypass in the primary loop at all times - even in an all-zone call situation. i'll be back with some pictures in a few weeks.

    many thanks
  • Tim_39
    Tim_39 Member Posts: 12

    I am trying to understand a similar issue--which is how to achieve boiler protection in a P/S piping set up with multiple secondary (system) zones at different temperatures.

    In setting up my primary (boiler) loop, do I need ONLY the single-pipe primary loop itself and throttle down the isolation valves, as fatty suggests, to ensure some flow in the primary loop at all times?

    OR do I need to add a bypass leg to my primary loop as in the schematic attached here? And, if so, does the bypass need a TV or only a pair of balancing valves?

This discussion has been closed.