My recently-bought house has a Rheem Criterion II gas furnace (model RGPH-15EARJR). Here is the manual: https://www.questargas.com/ForEmployees/qgcOperationsTraining/Furnaces/Ruud_GPH_GPJ_GLH_GLJ.pdf
The previous owner left me with some disposable filters, but I don't know where to install such a filter. When I removed the cover of the main operating box, I found a permanent filter inside. So I took the filter out, washed it, and put it back. I also noticed a disposable filter inside the duct that is about 2 feet away from the operating box: i.e. the permanent filter and the disposable one are about 2 feet away and parallel: one is inside the box and one is inside the duct. I guess I have to somehow disconnect the duct in order to access and change the disposable filter.
My question is the following: Since my furnace already comes with permanent filter, is it still necessary to put a disposable one? i.e. Is the disposable required in the duct so that two filters work together? Or it is just the idea of the previous owner?
I will call a furnace tune-up service to come to help me replace the disposable filter and see how a professional will do that. If it turns out it is easy to access the filter, then I may keep that. Otherwise, I may only use the permanent one.