Navien NCB 240E - Space Heating heat exchanger flush difficulty

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Lawrence, MA
Today I attempted to flush the space heating side of the heat exchanger on my NCB240E. See attached picture for piping.

Here are the steps I took:

1) Closed valves marked with the red arrows in the picture. (yes, the picture shows them as open...ignore that as I took the pic at a different time).

2) Attached a hose to the green arrow fitting with the other end draining into a bucket.

3) Attached a hose to the blue arrow fitting with the other end attached to a submersible pump in the same bucket.

4) Opened the valves at the green and blue arrows...waited for boiler to drain.

5) Filled bucket with white vinegar, plugged in circulation pump.

Problem: there was literally no flow coming out the hose attached to the green arrow side, thus heat exchanger not being flushed. I would see a trickle of vinegar come out only.

I verified the pump was working, made sure it was primed.

Question: Is there something I need to do with the 3-way valve so that it is set correctly to allow for flushing? Any other suggestions?

(Note: I used this same setup on the DHW side and it flushed just fine)

Hey, wait a minute.

This is awkward, but...

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