Balancing flow through primary / secondary on Navien NCB-240

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Amir Eyal

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I have a Navien NCB-240 piped Primary / Secondary. The secondary loop is feeding a couple of manifolds that split off the flow to half inch PEX-AL_PEX home runs to 12 cast iron radiators. The boiler is set to run on reset curve with max temp of 140 at 5 degrees outdoor temperature and 104 at 60. House is in the Detroit metro area.

I am concerned that I am over pumping the secondary loop hence increasing the return water temperature beck to the boiler.

I installed a new circulator pump on the secondary loop today. It is the Taco 0018e which has Bluetooth connectivity. I set it to pump about 5GPM at 5 Feet of Head - I can see these values on the app.

Today at an outdoor temperature of 29 degrees the supply temperature on the boiler primary loop was 120 (A value) the return water temperature stabilized at 108.

The reason I think I am over-pumping the secondary loop is because the temperature on the supply side of the secondary loop is reading about 114 (not 120) so it seems like there is mixing of return water across the closely spaced Ts.

When I reduce the flow in the secondary loop to about 3.5GPM I see the temperature increase on the supply side of the secondary to about 117-118 (I expect I will never be able to see 120 degrees exactly) Return water temperature stays stable. This leads me to believe that the Primary circulator is pumping at a rate of 3.5GPM as well.

Now to my question - Does the Navien NCB-240 internal primary circulator run on a fixed speed or is it a variable speed circulator?

Is it a fixed rate flow of 3.5GPM on the primary loop (seems too low) If variable, how should I go about balancing flow between primary and secondary loops?
 

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