What you are explaining about warm air and cold air makes no sense. Warm air and cold air don't make rain. In extreme conditions it may make wind. Rain from air masses is totally different and has to do with saturation and depressurization. Dude, were talking about air going through a tube at a pretty good clip to it's final destination in the burn pot. Let's just say for example that the air coming in was 50and the exhaust going out was 300
Temperature and pressure are related, as temperature goes up, then the pressure goes up given the same volume. That is just laws of physics. The case of the OP can be caused when there is not enough air or dirty stove. However many people on this forum do like Selkirk DT and I recommend it too. Don't forget, that vacuum cleanout is the coolest feature!
Anyway to make a pellet stove work better and easier to keep clean is always a good thing!
