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Solar DHW with Navien tankless (combi) issues - new possible solution
Recently had a Navien NCB210E combi boiler installed with my existing solar DHW system and it doesn't seem to play well with my existing solar system.
With incoming water at anything above (or over) a delta 20 deg F of the set point, the Navien heats the water well over comfort range and short cycles, e.g., 120 F incoming, 1.2 gal/min, output 138-145 with set point at 125 F.
From what I have learned from Navien tech support is that the DHW will kick on to the min turn down when it senses flow, even if the incoming water is above the DHW supply set point, up until it gets too hot, then it shuts off and restarts.
I have read all of the other solutions posted suggesting TMVs, and/or a motorized 3-way valve, but I have another possible simpler solution that I am looking for feedback on. How about tricking the flow sensor to not sense flow until the Solar gets to below a point, then un-trick it. I am thinking an aquastat on the Solar storage outlet to interrupt the pulse signal to the flow sensor. Another option would be a controller of some sort with a temp sensor.
A couple of problems that might be an issue:
1. the unit will throw an error code if it senses an open in the signal circuit. I asked Navien if it runs a self-test on the signal circuit and the guy I talked to said it doesn't, but my contractor thinks it does. Should be easy enough to test I guess, I just don't want to fry my PC board. Even if it does self-test, a resistor connecting the normally open contacts on the aquastat to the input wire on the flow sensor to complete the circuit might keep it from throwing an error.
2. At the point when the aquastat completes the circuit after the Solar gets too cool, there would likely be a slug of cold water to the end user.
3. Running all the water through the tankless needlessly will wear it out. Although all that water would be flowing through it anyway if I didn't have solar DHW.
Any input is appreciated.