I have a 30-year-old heat pump that's working great and I want to be sure to take good care of it. When I bought the house, a HVAC guy suggested putting a filter behind the cover to the return duct instead of the filter that was in front of the fins. I have two long-haired dogs and he figured that would help keep things cleaner. When I look inside the return duct I see a black liner that is falling apart and obviously has 30 years of stuff stuck to it. Since the liner is after the filter, none of that stuff is getting filtered before I breathe it. I've had serious allergy problems ever since I moved here -- worse in the winter when the heat's on -- and I'm casting a suspicious eye on the heating system. Would it make sense to remove the filter at the return and go back to using one before the fins? I was told I can't have both at the same time. Or do you have any other suggestions? I'm lost and don't mind admitting I barely know what I'm talking about. I did have the ducts cleaned a couple of years ago, but never thought to ask about the liner. I'm just now (at Midnight) noticing it. Thanks!