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To "show my work" so to speak, using the 6,032 BTU/hr loop mentioned
above, if I design it with a 180 deg entering temp, and 20 deg nominal
temp drop, it would need to flow 0.62 gal/min. That is actually below
the Takagi's turn on threshold of 0.75 gal/min. If I go with 1.0
gal/min I get a resulting drop of 12.4 degrees. When the water returns
to the heater, if it is getting 17,000 BTU minimum (I'm assuming that
rating is in BTU/hr) put back into it, the resulting temp increase
would be 34 degrees, thus an exit temp of about 201 degrees. If this
cycle simply continues there would be thermal runaway.


I think your work needs work. Why would there be a thermal runaway? Does
the Takagi not have an aquastat to cycle it off if heat is not needed? If
not, it is the wrong appliance to use for a heater.