OK, here's what's happening; If you have a good hot fire at tending time, no problems. Your stack temp may, or may not rise. If it does rise, it wouldn't be much (Example: 25* - 50*) ,then settles back down. Now, if your fire is low at tending time, the first thing you need to do is shut the fan off. You don't want the fan taking any heat off the mass of the stove. As you loose heat off the mass of the stove, the back flapper door will open wider, and wider. Once the fire gets going, it has to heat the entire mass of the stove back up, till the flapper door closes again. By that time, you have a roaring fire, and way higher stack temps than you should. The flapper door acts like a mini ash pan door being left open. The stove runs away on you. Stack temps surge. You loose control (not good). I wouldn't except these high spikes in stack temps as "Normal". The secret is not to loose the stoves heat mass. Also, should the fire be low; it should be built up a little to a time (as if starting a fresh fire), till you have a good hot bed of coals to start with. Because the fan is off, the mass of the stove is heated back up. Or at least rapidly heading in that direction. Only then, the hopper should be topped off. And yes, you're correct about many people not seeing this happen, as off to work they go. All the while, never knowing how close they are to having a house fire Every Day.