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External thermostat for Rinnai wall furnace

Rinnai sells a kit to connect an external thermostat to an EnergySaver wall furnace and override the internal thermostat, but it is only compatible with models produced in the last few years (with a "T" in the model number).

I have an EX22C (i.e. not EX22CT) circa 2015 and thus don't have the two molex connectors for the circuit board included with the thermostat kit.

Has anybody ever managed to connect a standard wall thermostat to an older Rinnai wall furnace? This unit is primarily tasked with keeping an infrequently used cottage at 50F and it would be ideal to raise the temperature in advance of arrival.