Over the years there have been many causes for furnace heat exchanger failures, but we don’t see this happening as often because of the addition of more safety devices - such as high-temperature switches - and the elimination of standing pilot lights. This has greatly reduced the number of cracked heat exchangers that result from overheating and the moisture contamination of standing pilots. However, there still continue to be corrosion failures, which can come from several sources.
Back in the middle 1980s, after Lennox had set the furnace world on its ear by introducing and advertising their “Pulse Furnace,” many equipment manufacturers scrambled to come up with their own high-efficiency offerings. And at the time, I put on a class for contractors that was developed by Carrier Corp., about the many problems they would encounter when trying to do this. Carrier had developed their own high-efficiency furnace some 70 years earlier, and they knew what to expect. So it wasn’t surprising to find that many manufacturers had product recalls due to heat-exchanger failures in the late 1980s.
The problem that the folks at Carrier knew about was the amount of contamination in standard household air, which would turn into corrosive acids when run through a flame. Yes, all those household cleaning chemicals, the hair and body sprays and the bug killers can greatly shorten the life of primary and secondary heat exchangers, so the condensed water coming from 90+ AFUE furnaces can itself be quite acidic and corrosive.
Speaking of chemical corrosion, I remember a job that I went out to look at in a funeral home several years ago, where two furnace heat exchangers had almost dissolved. The problem? The combustion air intake was in a closet in the embalming room!
Of course, in recent years most furnace manufacturers have come up with protective coatings for their heat exchangers, which can resist much of the corrosion created by chemicals being burned in the combustion air. But in especially contaminated environments - places where chemicals are heavily used, as in beauty parlors and the like - heat exchanger coating still may not prove enough to stop corrosion. So where possible, it’s best to find a clean source for furnace combustion air, as from an outdoor source (which itself isn’t always reliable).
However, remember that sources of corrosion come not only from inside heat exchangers, but also from their outsides. And with upflow furnaces that have air-conditioning evaporator coils mounted on top, dripping coils and backed-up condensate drains can be a problem. Remember that slab-type coils are far more prone to drip than A-style coils.
I have also been on some engineered jobs where gas furnace return-air intakes were located downstream from cool, conditioned air, which is a bad idea not only because there are significant cooling losses, but the cool internal heat-exchanger surfaces sweat, resulting in early corrosion and failure.