Furnace filter

[birchbark]

Has anyone found where the furnace filter is located?
I looked from the outside, in the basement and thru the grill in the steps none found. We stay where the furnace or fireplace runs a few days a week. Always dusting and have already cleam the fireplace of dust to keep it running.

Just put a cut-to-fit filter in the steps. Now concerned about airflow. The manual indicates the return air s/b 80 sq inches. Steps provide twixe this.

How you you dealt with this issue?