I'm trying to understand the issue of condensation in gas furnaces (furnai?). My understanding is represented by this simplified image and description:
Non-Condensing Furnace: Cold air from intake vent is fed to the blue burner which produces hot gases that travel through heat exchanger tubes and exit at high temperature from the exhaust vent. Although not clearly shown, there is no place for the acidic condensation to drain and thus it is imperative to minimize/eliminate it. Condensation is minimized due to higher temperatures throughout the path. The heat exchanger shape/pitch is such that should droplets of condensation form inside the tube, they would slide back towards the hotter burner region to be evaporated.
Condensing Furnace: Cold air from intake vent is fed to the blue burner which produces hot gases that travel through heat exchanger tubes and exit at warm temperature from the exhaust vent. There is a drain for condensation, and condensation is always expected in the final segments. The heat exchanger(s) shape/pitch is such that droplets of condensation slide away from the burner and towards the condensate drain.
I don't know if any non-condensing furnaces have "primary" and "secondary" heat exchangers, but I've noticed that condensing furnaces do (with the hotter early segments comprising the primary, and the final segments comprising the often physically different, larger surface area secondary).
Based on this it is much easier to imagine a condensation problem in low-firing non-condensing furnaces. In the condensing furnace, should condensation form it has a path through the "absolutely must be condensation resistant" secondary and out through the drain. The only potential points of concern, that I can think of, would be a) improperly shaped or pitched heat exchanger tubes, b) bad heat exchanger tube junctions which allow the condensation to collect, and c) surface condensation which isn't sufficient to form droplets/streams that move towards the drain. I don't know that any of those actually pose a real world problem though (?).
One thing that came to me as I was preparing this: IF I have a reasonably correct grasp of this subject (I certainly may not), then at least until the furnace condensate drain trap fills, there is a path for hazardous exhaust gases to go right out the condensate drain line. I have seen several instances where the furnace condensate drain line and evaporator coil drain line were tied together (directly, or indirectly through a condensate pump). Thus it appears to me that in certain, perhaps code-breaking, installs there is a [temporary] threat of furnace exhaust making its way up the evaporator coil drain line and out the open evaporator coil pan into the air stream blowing into the home. If there is something I'm overlooking... something besides a potentially dry trap... that would prevent said exhaust from taking that route I'd be interested to know what it is.
Comments on the above welcome and much appreciated!