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Induced flow into an RMB

Rocky_3
Rocky_3 Member Posts: 236
in Radiant Heating
Taco RMB supplying mixed water to underfloor plate system. 3/4" primary loop at boiler, with 3/4" closely spaced tees off this loop supplying the variable speed side of the RMB. 1/2" of pipe between the two tees. Reemed. Boiler pump is inducing flow into the RMB to the point where the variable speed pump will not even come on because the mixed water out is over 145 degrees, and target temp is only 115. Only thing I can see that might be slightly suspicious is there is only 6" of straight pipe before the closely spaced tees. However, there is approx 45 feet (supply and return total) of 3/4" pex feeding the RMB. I can't for the life of me figure out why the water would rather flow through 45' of 3/4" pex, through the RMB rather than jumping the 1/2" gap between the tees. Would not having 12" or 18" of straight pipe before the close tees really make this much of a difference? RMB sits about 30" below the level of the close tees so this is not thermal convection; it is pumped flow. How about if I swelled up the 3/4" primary loop to 1" right at the tees, used 1X1X3/4" tees for the close tees; kinda like making a low loss header of sorts. Do you think this may alleviate some of the pressure differential that is obviously taking place?
Thanks in advance for your thoughts,
Rocky

Comments

  • hot_rod
    hot_rod Member Posts: 23,059
    Is there a check in the RMB? It may just need a little pressure resistance?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Rocky_3
    Rocky_3 Member Posts: 236
    Hi, Hot Rod. I have to issue a mea culpa. Went back out to do some finish wiring on this job and noticed that even when the telestats were not calling, the variable speed pump was still spinning. Huh? Note to self: when sticking the control back on the RMB after wiring, do not, I repeat, do not inadvertently hit the "manual" switch on the RMB's variable speed pump. It will "induce" ( wink wink) flow from the primary loop and make it seem like the boiler pump is pushing water into the RMB. Go figure.
    Flipped the switch back to auto and guess what? Perfect operation.
    Rocky
  • hot_rod
    hot_rod Member Posts: 23,059
    Nice that it was that simple. Hope it didn't take a dog sled trip :)
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream

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