I always tried to use the proportion along with a little integral for control. (I only use Derivative when there appears to be no other way) The biggest issue is how much of an impact does an adjustment have on the temperature. (Do you have swing between all the way open to all the way closed, or will the system find a sweet spot and only trim itself a little each way to keep The temperature OK.) I personally base my propotional number on how much of a swing my analog output will be making. A larger propotional gain if the output needs to swing the entire range, and a smaller for apps where we find a sweet spot. An enclosed system where the heat is relatively constant should be pretty forgiving. You may need a start up sequence to wait for your temperature to get close before you turn it over to the PID routine. After that, I get a notebook and document each change I make in the PID loop and record the results. This may take some sit and watch time with the process. It is just difficult to nail a loop without some trial and error.