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LLH pumping

DC123 Member Posts: 69
I'm curious about how LLH pumping works practically. I've read that the primary loop flow is supposed to be lower than the system loop flow for the LLH to work properly and supply the lowest return water temperatures to the boiler. However, to take the Veissmann Vitodens 200 B2HA19 as an example, at a deltaT of 20, the primary loop is supposed to flow at over 6 g/min whereas my understanding is that system flow probably shouldn't go above 4 g/min. How is a LLH effective in this situation?


  • hot_rod
    hot_rod Member Posts: 18,190
    The flow rate on either side can and will vary depending on zoning, V/S pumping, etc

    unless the flow rate on both sides is identical, there will be some temperature mixing in the LLH

    Here is a good explanation and some examples

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • STEVEusaPA
    STEVEusaPA Member Posts: 5,966
    edited December 2016
    The LLH's job is to isolate the boiler loop from the distribution loop. Neither will (should) affect the other. Through your heat loss/load you will determine flow rates and temperatures. You have to heed manufacturer's minimum flow rate through their boiler. If it turns out you're not meeting your delta T, you can adjust water temperatures, or on the distribution side, temps and flow rates.
    If you haven't read this, it has some great info:

  • DC123
    DC123 Member Posts: 69
    Ok, thanks. So I think my concern is that most low-mass boilers, such as the vitodens, are going to most often be in the 3rd scenario in those case examples from iDronics 1, where the boiler flow is greater than the distribution flow. It looks like the boiler flow, with that boiler as an example, only goes below 4 GPM at deltaTs above 30, which seems impractical. So in most cases, the boiler efficiency will take a significant hit due to high return temps from the distribution side. Am I thinking about this correctly?
  • DC123
    DC123 Member Posts: 69
    Oh, I think my confusion may be that while an individual radiator may only see 4 gpm, the main distribution pipes will see all of those return flows combined by the time it hits the LLH again, which will presumably be much more than 4 gpm, especially if multiple zones are calling. Is that right?
  • Gordy
    Gordy Member Posts: 9,537
    edited December 2016

    System side flow rates should always be higher than boiler side flow rates for efficiency at the boiler. This gets to be hard when micro zoning takes place, and a zones load is less than the minimum modulation of a modulating /condensing boiler.