Hi,
1) That is indeed an issue with the example code which went unobserved for a very long time, someone noticed it and mentioned it a couple of posts back in the thread. Unfortunately the forum doesn't allow me to fix it in the original post. Anyway, you are 100% right, if you're using the HPWM command then you should set the output clamp to 255. With that said the standard CCP module creating the PWM output can provide resolution of up to 10bits depending on frequency, you just can't "access" it using the HPWM command.

2) Sounds like a good start. Getting the tuning values right can be tricky, especially so on slow systems since it takes so damn long to see if you got it right or not. I expect this particular "plant" to be extra tricky due to the somewhat unrepeatable/unpredictable "performance" of a charcoal bed - I imagine.

One thing you might consider is to run the heater/fan/furnace whatever "open loop" until the temperature starts to approach the setpoint and only then switch on the PID. A more elegant but also a bit more complicated way is to create a ramping action in a way that the setpoint, instead of being bumped from ambient to "working temp" is ramped up at a rate that the "plant" can actually follow.

If you're concerned that a dutycycle of 0% won't provide enough oxygen to keep the fire alive then simply don't allow it to go below a certain value to keep the fan running at all times.

/Henrik.