[onewhippedpuppy]

Both units in my home are original, 1989. Upstairs unit started blowing hot air with the A/C on. Low side line is much hotter than high side, compressor unit outside is operating but the air being blown out feels cool. Heater burner is not operating on the furnace. Not sure of the model number, but it's a Coleman with natural gas heater, installed in an attic. I had the A/C system charged (slightly low) about one month ago, and have a record of the a-coil being replaced in 1999.

So, suggestions? I've replaced a-coils and compressors before, but don't have enough experience to properly diagnose the actual problem. Upstairs unit hasn't ever blown particularly cold, hence the charging about one month ago.