said by rockotman:
I would be concerned that the extra humidity from the denser outside air would hasten corrosion in the heat exchanger and flue, especially with a furnace that is not designed to typically use outside air - but I am by no means an HVAC expert.
I'm no expert either, but the humidity is already lower during the periods where air is actively being drawn in when the furnace is being used. That's one of the reasons to use outside air as you aren't using conditioned air and pulling new air in lowering humidity levels.
The times when condensation would be an issue is when the outside air is warm and moist, and the heat exchanger is cold as if the AC was running. But that's not an issue as you aren't drawing air through the exchanger when the exchanger is cold enough to condense moisture. And when the burners have just turned on and the exchanger hasn't reached operating temperature, there's far more moisture in the air from the combustion process than what is in the combustion supply air.
Exchangers can rust out, but that happens when the combustion gasses condense, there isn't sufficient draft, the furnace short cycles, etc. But those all happen regardless of where the supply air comes from. Condensing furnaces are designed with corrosion in mind not because they are drawing cold outside air in, but because the exhaust gases become more corrosive when they cool and condense into liquids.