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Adjusting boiler K factor and pump delta T

Jerhrof16Jerhrof16 Posts: 3Member
I have a 150K Navien combination condensing boiler feeding 3000' of 5/8" pex in a concrete slab circulated by a Taco HEC-2 "bumblebee" pump. I am currently running the navien using the outdoor sensor at a K factor setting of 1.6 (6 is max) and the pump is currently running in constant speed mode on highest setting. I am about to change the pump over to variable speed Delta T mode but am unsure how to set it all up for maximum efficiency. The current delta T's are very high (30-40 degrees) but the system is heating the building. To lower the delta's do I want the pump to run near constantly at a lower speed and should I lower the boiler K factor and thus lower the output temp? Thanks!


  • Dave H_2Dave H_2 Posts: 371Member
    Delta T

    Is the delta T you are reading across the boiler or across the radiant?

    Most radiant applications would like a 10 to 15 degree delta T across the manifold

    If this is the delta T across the boiler, you are rockin along and it should be condensing nicely.

    So, if you were to change the pump from constant to Delta T mode and the setting is 20 degrees and you have a 30 degree across the manifold, then the circ will run full speed until the delta T drops below 20.

    Dave H.
    Dave H
  • Jerhrof16Jerhrof16 Posts: 3Member
    Delta T

    Thanks Dave. That Delta T was across the manifold. The delta across the boiler is more like 50 degrees. I think lowering the outdoor reset curve on the boiler will require longer cycle times on the pump which should get the manifold delta T's closer to the 15-20 region. I'm really looking to keep the boiler condensing well at an efficient level with the pump throttling back as well.
  • Dave H_2Dave H_2 Posts: 371Member

    did the system start up? That delta T for a radiant slab is large.

    If you are in outdoor reset and the system just started up, it is going to take a while to warm everything up.

    But I have to start asking questions as to how long are the loops, what is the spacing, one zone or many?


    Dave H
    Dave H
  • Jerhrof16Jerhrof16 Posts: 3Member

    Dave, it is a 1 zone system with 6 500' loops using 5/8" pex. After lowering the reset k factor on the boiler the delta's across the manifolds are decreasing (28-30) now. I just lowered it a bit more which I think should put everything where it should be. Bottom line, the system works well but I think 3000' of 5/8" pex on one loop with one pump is probably pushing it. The system was designed by Blue Ridge company but if I did it again I would go with a slightly larger boiler and 2 pumps but that would be viewed by most as overkill I guess.
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