Here are pictures of the unit. The supply running out of the top is 20" x 16" and it runs into an 8" x 16" and an 8" x 12" duct. Both have a damper on them. The return is 28" x 8.5".

The installer also insisted on opening some of the return duct work in the basement so alleviate some pressure. The grill you see here hides one of those holes. It's about 8" x 8". There's another, much larger hole at the end of the return. It's probably a good 12" x 12".

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