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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.

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  • What does the manual show for the filter(s)? Is the disposable one a fine filter and the other for flies / spiders etc?
    – Solar Mike
    Oct 26, 2021 at 9:05
  • Generally speaking, one should not have to "somehow disconnect the duct in order to access and change the disposable filter". The filter is usually in a housing that has a door of some sort that is reasonably easy to remove so the disposable filter can simply be pulled out and replaced. Post a picture of your furnace (from all angles that are accessible), including the return air duct leading into it and someone will be able to point out where the filter access is.
    – FreeMan
    Oct 26, 2021 at 12:21
  • @SolarMike The manual (on page 30) shows a mesh filter like the pic in this post: diy.stackexchange.com/questions/193756/… I think it is a filter with a small MERV rating but certainly not just for spiders.
    – Tony B
    Oct 26, 2021 at 14:38
  • @FreeMan Thanks for your point and completely agree with you that the filter should be easy to access. However, right now I can only see the filter from inside the duct through the operating box (2 feet away). To access it from outside the duct, there seems to be a way to open a small slit on the duct and insert the filter but I don't dare to do that myself.
    – Tony B
    Oct 26, 2021 at 14:42
  • @TonyB in all likelihood, that "small slit" is probably the door for replacing the filter. If you'll provide some pictures (a wide angle showing the general area of the duct, and a close up or two showing what the "slit" area looks like), I'm sure someone will be able to confirm that's where you change the filter and how to open it. If you choose to remain in the "don't dare do that myself" category, I'm sure your local HVAC company will be more than happy to charge you $50-$100 to replace a $10 filter.
    – FreeMan
    Oct 26, 2021 at 15:03

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It turns out the disposable filter can be accessed through the whole house humidifier installed along with the furnace: just open the cover of the humidifier (as if you want to change the humidifier pad) and then one can see the disposable filter inside the duct and replace it. I just took it out without replacing it. My old furnace comes with a permanent filter and it does not have a filter box for disposable filter.

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