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.
I have heard that cleaning with electro-sol dish washing detergent will get rid of smell. You might try a UV light once it is gone. This can be a very hard problem to correct in some situations. Good luck!
Thanks for the tip. Yes, from what I've read about this on other HVAC forums, the cause, not to mention solution, is often a deep mystery. Sometimes it spontaneously goes away on its own. Sometimes it never returns. Sometimes cleaning the coils works. Sometimes replacing the coils works. Though why replacing the coils would not always work is bizarre. Another weird thing is in other situations the syndrome affects heating mode instead of or in addition to cooling mode. Which makes no sense if the problem is in fact bacteria on the coils that get activated when the coils are wet. Most cases, however, seem to occur in shoulder season, which was the case of us. Some mystery in life is a good thing, though certainly not satisfying in something like this that should be fully explainable in physical terms. Getting back to scientific thinking, I did notice a periodic faint musty smell in the mechanical room that started about two weeks before the recent HVAC incident. At the time, I thought it was the sump pit. Our sump pump almost never runs and I just figured trace water in the pit was getting stagnant. I flushed out the pit. But maybe the smell was actually from the coils or condensate pan. In either case, the mech room smell is definitely gone now, whether solved by flushing the pit or cleaning the condensate pan. I'll try mixing some Electro-Sol in a gallon pump sprayer and spraying the exterior of the coil assembly next spring if the smell returns. After flushing out the condensate pan with bleach and water this weekend, I've not actually run the unit in cooling mode since the smell first arose last week. Maybe the pan disinfection alone will fix it. I'll cross my fingers. Can't hurt, as that seems about as rational as the problem itself in many cases.
Do not use bleach to clean the drain pan. Bleach does not play well with the E-Coating on the coils. simply use soap and water to clean, then place condensate pan tablets in the drain pan to hinder further growth of drain nasties. Bergy
Thanks, Bergy. I'll note this for future cleaning. When I flushed the pan this weekend, it was a diluted bleach and water mix (1:5). I didn't spray it on the coils themselves and I washed the pan out with all water afterward. So hopefully I didn't corrode anything too badly. I see you are in my neck of the woods (Iowa City). We're satisfied overall with our installer, but it's good to know about another trusted geo pro in the area.
