Why don’t you use a software thermostat? Home Assistant has it’s own native Generic Thermostat (Generic Thermostat - Home Assistant). If you do not like that, you could use a PID thermostat. There is a thread here: Thermostat with PID controller - #324 by anonverburg. I do not know what type of heater system you have. If it’s a floor heater, our ControlBox might be interesting for you: ControlBox | Crowd Supply