I have a York heat pump and furnace installed in 1979. In cooling mode, water is dripping on the air filter. I cleaned out the drain pipe and water tray under the edges of the inverted-V-shaped cooling coils. But right after the furnace switches off, I still have water dripping on the filter and I can see water dripping directly from the coils rather than flowing down the angle to the drip tray below. I can see specks of dirt or stuff on the coils which I assume is breaking the capillary action that would normally happen. I assume I need to clean the coils. Any suggestions on a cleaning solution and process?
I had the same problem in my previous house. I am not a HVAC guy but I'll tell you what I did. I bought some coil cleaner and applied it with a small hand pump sprayer. Let the cleaner do its thing and rinse with water with a hand sprayer. Rinse very well. It helped the problem fairly well but didn't totally solve it. I called in a HVAC company and they cleaned it again and it didn't totally fix it either. My coil was in bad shape and probably near its life span, and yours being over 20 years old may be as well. Some dirt and debris just won't come out after 20 years, may have to replace it.
The cleaning method is going to be determined by what is on the coil. Sometimes a simple vacuum or light cleaning is all that is needed. If you are in florida, you will need to have the coil pulled out of the unit and cleaned. Its a mold thing with the environment. If it has to be pulled and cleaned, expect to pay $400-700 depending on circumstances and installation of the air handler.