Hello everyone, I'm a novice when it comes to a wood furnace. My elderly father in law recently passed away. At his home, he owns a Woodchuck 3100 wood furnace. On the front of it, there are 3 handles. What is the top handle/lever for? We pulled it out, and it just looks like a 2 foot long or so straight rod. Correct me if I'm wrong, but I believe it is called a Heat Exchanger Control Rod. I do notice that on the back side of the furnace at the same height, the exhaust duct is there. Anyone know it's proper function?