This is exactly why I am convinced I don't have a thermostat to operate my secondary air inlet (or maybe my secondary has never worked). The only way my cat stays running is if there is fuel in the box and my primary air lever is open more that 60%. If I choke down the Primary to 50% the stove will just glow very very dull and stays about 300degress on the STT until the wood is gone.This is where @arnermd theory of secondary air infiltrating into the primary combustion comes into play with our bigger Defiants. To me it seems a little bit dangerous. If there is no way to really ‘choke’ down the fire in a run away situation, because we have no control of the secondary air, how could we be liable for an over fire situation? Honestly, if I wasn’t in this forum, I defiantly would have over fired this stove countless times by now. I’m sure my cat would be destoryed.