We have a 2-stage, 5-ton FHP GSHP with slinky loop. It's 2.5 years old and overall has worked great. Location is Iowa, where it is no longer consistent cooling season. A couple of days ago on a day that was warmer than normal, I noticed a fairly/pungent acrid, vinegar-like smell blowing from the ducts when I came home during a cooling call. As far as I could tell, the smell was uniform from outlet grills in the house. I tested and the smell is not present in fan mode or in heating mode. This seems to suggest the problem is not duct related per se (e.g., mildew in ducts, dead critter, leakage). The air filter (MERV 8) is new, clean, and dry. The smell was new. Our system got a tremendous workout all summer in what was record heat and the smell was not present then. I removed the unit panel and confirmed the drain pan and line are clear. The pan had a small amount of dried, whitish residue in it, but not a lot by any means. I flushed the pan out with water and bleach. From what I've read, it sounds like the problem could be bacterial growth on the cooling coils, which is activated when the coils got wet during the recent cooling call. There is a lot of info online about the so-called "dirty sock" syndrome with regard to air-source heat pumps. but I've not seen it mentioned with GSHP, though it seems it would apply to either. When flushing the drain pan, I inspected the coils. The coil section on our unit is covered with a black, coated wire mesh on both sides (blower panel and return side), so I could not really see the coils themselves. The mesh was not pristine clean and had some spots with similar dried residue. The return side of the coil mesh seemed worse, though I couldn't tell if the residue was mildewy crap or just dust/pet hair/etc. Other than spraying Lysol or a similar cleaner on the coil mesh, is there anything DIY that can be done? I bet that to truly clean the coils, the entire coil assembly would have to be removed. I'm not comfortable messing with that myself. Short of detaching the return side duct, it is also hard to get full access to the entire surface of the coil mesh. With a spray of some kind, I can probably hit most of the mesh on the blower and return sides. Would spraying the mesh to clean it even be worth trying? I'm suspicious only because it seems the spray would get the mesh but hardly penetrate the middle part of the coil assembly. Would running the system on high in Emergency Heat for ten minutes or so be worth trying? The thought being to "cook" off the coils? Not sure how feasible that is since the EH strips are on the blower side and might not heat up the coil enough to matter. I'm willing to call our installer for a service call if that is what it takes. Obviously, having to replace the coils would be a bummer. In either case, I'll probably wait until next spring since it is not a problem in heating mode for whatever reason. It seems odd this problem would crop up only now, especially when the coils were wet and in use all summer. FHP supposedly coats their coils in baked enamel, though perhaps that helps resist only rust, not bacterial growth. Any thoughts? Thank you.