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Lochinvar knight KHB085N with ECM pump P/S vs direct pipe

robjmc1 Member Posts: 7
With the new Lochinvar knight fire tube boiler and the ECM pump is there any benefit to using a primary secondary loop vs piping direct to the zones with zone valves. I am trying to get the lowest return temp and long run times without using the extra power that primarily secondary pumps use. I have three zones and one indirect hot water zone. The zones are cast iron rads supplied by manifolds so flow to each zone can change to accommodate the minimum flow of the boiler and not affect the heat balance in the house. My concern with the P/S loop is hot boiler water returning to the boiler raising the return temp and cold water returning to the system lowering the system temp. Lochinvar claims that it ok to use direct piping and their install manual shows it but I don’t know which way is best. I know how important P/S piping is with some boilers and why it is used but I don’t think it will give the lowest return temperature.


  • hot_rod
    hot_rod Member Posts: 22,394
    yes and also dial in the outdoor reset function so the boiler adjusts to the lowest SWT at all times

    if possible add an indoor feed back to modulate SWT based on actual room load requirements

    The key is to assure the min flow under any condition, that the boiler requires
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,646
    As long as your smallest (most restrictive) zone meets the minimum flow requirements of the boiler, go for it!

    It will definatly give you the lowest return temps.
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
  • ekubec
    ekubec Member Posts: 12

    Curious how this turned out.