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1. If installer did not put a filter grille in ceiling or wall rather than in the furnace, the only way to be sure of size is to get someone to crawl up there and get size or get model on furnace.No way to tell by the outdoor unit model.(Unit provided is for outdoor unit)
2.If you can not get to filter to get size, you will not be able to get to it to replace it.
Sorry about the errant post, it was meant for another customer. I have edited the original post. -Brian
Please disregard post by Brian.He mistakenly posted on the question . Being that the outdoor unit is a 3.5 ton unit, you most likely have a 90,000 btu Lennox furnace . But again without knowing for sure, we are doing an educated guess based on size of the ac unit.
If so your filter size will be 16 x 25.
If you can have someone get me the furnace model I can tell you for sure.