can be lowered below a dew point temperature of the combustion products thus causing unwanted condensation in primary heat exchanger 2 although secondary heat exchanger 5 can be designed for corrosion resistance primary heat exchanger 2 is normally not designed for corrosion resistance and thus often cannot handle extended wetting times and thus can suffer serious corrosion if operating under conditions promoting surface corrosion as used throughout this specification and in the claims the term condensate corrosive surface is intended to relate to and be interchangeable with a description of any surface within the furnace system of this invention that can be damaged and or corroded by exposure to condensate particularly unwanted or undesirable condensate including but not limited to external or exposed outer surfaces of the heat exchanger sections of primary heat exchanger 2 and or secondary heat exchanger 5 one possible solution is to limit the amount of outdoor air allowed to enter the unit during cold weather operation this operating mode could effectively protect from or prevent unwanted condensation formation in at and or on primary heat exchanger 2 but this solution might not always be practical from a performance standpoint or as a requirement of the furnace because in some embodiments this invention is integrated with a modulating burner control which.