@b4tn I have done the same as you stated above. I have dual temp probes, 1 in the display and 1 in the sump. I added a 3rd room temp for good measure. I also use a float switch with mine. I did not add any resistors or anyting. just GPIO > Float switch > Ground. it works flawlessly thus far.
Are you controlling you heater from just one input from the temp probe or combining an average?

. We are still happy to have you with us

