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When required GPM Is Too Slow for Rec. Flow Rate @pipe size

D107 Member Posts: 1,814
As HO, hydraulics seems to be one of the most perplexing aspects of HVAC. Our Turbomax--used only as indirect--stipulates 5 gpm for the dedicated circulator with our Peerless MI-03 50K net output boiler. Got about 18ft of 1.25" steel pipe plus elbows etc. Head is negligible. I recall that generally 2fps is a minimum for flow speed to keep air entrained in system. According to a table I have 5gpm in 1.25" pipe will run at 1.31fps. Our VR1816 circ is set to fixed speed mode, first notch up--which is equivalent to taco 006. No real complaints with hot water, but the system psi is running 5psi above the 15 setting and just want to see if a)slow flow speed might be causing higher pressure via released air and b) we are operating at best efficiency. Those pump curves are purely about gpm and head, I guess it's up to the installer to figure out pipe size/fps. If our speed setting is equiv to 006, then at 4ft head we get 5gpm.


  • SuperJ
    SuperJ Member Posts: 605
    edited October 2019
    Don't get too hung up on low velocities if you've followed best practices with pumping away and a proper microbubble separator and proper system pressure expansion tank setup.
    You can always temporarily boost pumps to a higher speed for initial flush and fill.
    Also it wouldn't hurt to put an air vent on top on the TurboMax tank.
  • D107
    D107 Member Posts: 1,814
    edited October 2019
    @SuperJ thanks. Yes there is hygroscopic air vent on turbomax, good separator, pumping away and on paper at least, a properly sized exp tank. This is new boiler from last March. Full heating season will tell the story.

    Heating circ pumps away, zone valves are on return. Turbomax circ also pumps away from Caleffi Discal air separator. Due to size of reverse indirect Turbomax (48 gallons) it was speculated that we may find the Amtrol-Extrol EX-30 undersized under certain cold startup conditions--e.g. startup from power or other outtage in dead winter. Summer we run cold start; perhaps in December we should switch to 140-160 swing.