Hi, I had a thermal runaway recently, just as the second layer started, which was annoying as the first layer took half an hour. A retry of the event showed that when the fan started, the nozzle temperature dropped a lot so I stopped the fan and the temperature shot back up again. So the fan is cooling the hot end. I tuned the fan speed until it was not affecting the hot end temperature - 255 down to 155. and all seems well.
I started thinking about this ... The fan is there to cool the recently melted plastic but also cools the plastic that is about to be printed on top of. So if the hot end is cooler than expected and the plastic is cooler than expected, then this might explain the inter layer lack of adhesion problems that I have been experiencing for a long time .. always, actually. If this is a solution to the inter layer problem, I'll let you know. Also about the thermal runaway.