Hi,
I don't have any hands-on experience with radiant heating of living spaces etc. I can't see any reason for it not working with incPID (or any other PID routine) but I think it's going to be a bit tricky (time consuming) to tune considering the slow response of such a large space.
You could go with a phase angle control scheme or a low frequency PWM, depending on the power of the heater and the size and losses of the room a PWM period of one to several minutes shouldn't be a problem.
With that said, a simple on/off thermostat (perhaps with a second sensor outside to provide some feedforward) should work properly if you don't have extremely high demands on a stable temperature.
/Henrik.
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