not sure what, lit it off from cold start at 10am, slowly choked off the air (all the way, that's never happened before), stove was up to 600 in 1/2 hr, secondaries lighting off on all 4 tubes (that's never happened before either). Now the family is running out of the basement living room because it is too hot !! Only 29 outside and very little breeze. Must be the dry wood !!