Ok. So it seems you are trying to put in an anti-freeze system on your heater.
I think using the thermostat you have would work, but it would come on at whatever the minimum is. That's probably 55°F or so, which
means most likely, your heater is going to be running quite a lot when you're not there.
I did a similar thing with my aquahot. I used a thermocouple activated industrial heat tape controller. My father-in-law had
one from a jobsite he had worked on in the 70's. It's heavy duty, but it was free. I attached the thermocouple to the distribution manifold
copper pipe. I then wired it to a 28v relay which controls the electric element in my aquahot, and also wired it to activate
the heating circuit which turns on the circulation pump and the fan in the water bay. The thermostat turns on at about 35°F. I found last
week that my electic element can't keep up when temps drop to about 0°, but there's no reason I couldn't have it activate the diesel burner
I realize you are trying to use stuff you already have, but maybe you can find an affordable heat tape controller somewhere.
Another idea is to find a normally open snap-disc thermostat switch that closes at around 35° and opens around 40° and attach that to your
pipes and connect to a relay coil that controls the heater.