Yes I have a Day&night up draft furnace which was missing the air filter when I purchased the house and can find no record or a local source for the size of filter or its proper locations. My furnace model number is 394GAW048100.

Can anyone help me determine the right size filter and where to locate it on the Unit?

There is NO specific size of filter except for the original Hog Hair filter that is shipped with the unit. The Hog Hair filters are junk and usually tossed away. Look on the large return air duct on the side of the unit for a slot to put a filter in. Then measure it for the filter size if found.

I have a concordd 80 plus furnace that I need a filter for. I do not know what size filter to use. Model number is EC1P36BM-1
Serial nunber is 6007E43360