How do I locate the filter in the Day & Night Furnace Model 100D-19?

The filter could be located anywhere in the return air duct. Sometimes it is inside the blower door on the furnace.

-Pull the top (larger) door panel up and off the furnace.
-Open the right door (little key-like knob) and pull the panel away
-With a flashlight, look in and up (way up) and you will see one of the two filters leaning towards the middle. Remove the right filter, then remove the left filter through the right opening.
-Replace both filters with a 16x25x1 filter media. The two filters will make a "V" shape, with the top of the filters leaning on the sheet metal cold air return.
-Replace the inner panel, then the outer (larger) door panel.

Early Day & Night Model M100D furnaces do not have a safety switch to detect if the inner doors are secure. Be sure to check that both inner door panels are securely latched, so no air leaks in there.