I did some work quite a few years ago on what is called "cold end" corrosion which was corrosion caused by flue gases from combustion causing corrosion in large paper mill steam boilers in the economizer section. The economizer is a heat exchanger within the flue gas exhaust stack that takes heat from the +600 degree exiting flue gases to heat the incoming boiler feedwater water, usually from 240F up to 360-280F or so, basically a way to extract more energy from fuel. What the problem always is, is the dew point of sulphuric acid an carbonic acid, with sulphuric being worse.
The solution has always been one of two things. Metallurgy, using better and more expensive metallurgy to handle the acid condensing, or raising the temperature of the entire system to push away from the known dew point. Everyone goes with the latter, as acid resistant metallurgy is pricey.
In this case here, it seems that under normal conditions when the furnace is running in a warmed house and maintaining it at say 70F, the heat exchanger must warm up enough and for long enough to keep the sulfuric acid/carbonic acid from reaching it's dew point, at least in the exchanger portion where this normally happens. When running at cooler temperatures, this dew point will be reached easily and sooner and not be overcome with hotter flue gases, even if it does, the skin temperature of the outside heat exchanger in a cold house will be cool enough to condense all gases. I also wonder in these furnaces, is the cold-end part of the exchanger (the final section before gas exit) already is acid-resistant materials? Sulfuric acid dew points out at 267F, much much hotter than the exit gases of a high efficiency furnace that exits gases at below 150F. Depending of configuration of exchanger and how the condensable gas-condensates drain from it, did the engineer use acid resistant metals in the area of the cold end, skimping on the rest of the metallurgy to save money, but when the cold end temperatures move down in the heat exchenger...is the non-acid resistant metallurgy now in acid condensate?
Could this be what is going on? HVAC guys chime in? Sorry for my wanderings...it's a slow day.