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Return Pressure in sizing HVAC Equipment

Pango
Pango Member Posts: 1
When packaging Hydronic units, I have always found that return pressure has been dictated by the pressure setting of the PRV on the Make-Up water assembly, which most often comes directly from city water. I am rarely provided than information, making it difficult to size the PRV required. My question is this: is it important to know the make up water pressure (and as such your return pressure), or is the return pressure always looking to be reduced to minimize the overall pressure of the system; only to be sized for the pressure drop over the HVAC components plus pipe/static losses. Another way of looking at the question, is it safe to assume that the desired return pressure remains low (in the 15-50 PSI or so range)?

Thank you in advance for your help.

Comments

  • SWEI
    SWEI Member Posts: 7,356
    I'm not entirely sure what you mean by "return pressure" above, and there seems to be some confusion between the required static fill pressure (to maintain the system at positive pressure and reduce air ingress) and the required pump head (to overcome friction losses in piping and components.)
    Rich_49
  • hot_rod
    hot_rod Member Posts: 19,278
    You have several pressures at work in a closed loop hydronic system.
    The static or fill pressure is what you put into the system to assure you have water at the highest point in the system, ideally with 5 psi pressure at the very top. .433 psi to lift water a foot, so a building with a radiator up 17 feet would require 8.66 psi + 5 psi about 12 psi fill pressure.

    When a circulated starts spinning, if properly installed, it adds some additional pressure, called head. So a gauge at the discharge side of the circa pump would show a slight increase in pressure.

    Fill pressure, and dynamic pressure with circulator running. Here is a graphic. Red line indicates static fill pressure, green shows head energy added by the circulator.

    Gauges around the piping circuit show how the flow resistance in the piping circuit "scrub" away that head energy.
    ponc.png 108.4K
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream