My furnace lists the filter requirment as 20 x 20, but the filter is 14 x 20. I'm planning on having someone come in, but I'd like to become a little better educated beforehand.

The return & exhaust trunks both start at 8 x 24 and gradually taper down in two smaller stages as they run along the floor joists away from the furnace. The system is both air & heat and for the most part the heat works great. The air on the other hand if I let the temperature rise during the mid-day and try to bring it down later in the afternoon by a couple of dergrees, the unit will run for hours (8-9 hrs) before it finally catches up. I'm in Lake Ontario region so we get to see as much snow as sun.

The question I have is why would the trunk be tappered down, which I've seen in several homes? If the oil furnace is oversized for the ducting, what would I gain by having the ductsize increased? It would seem easier to fix the ducting than hauling out the furnace as I have easy access from the basement. I can't see any advantage to increase the filter size without upsizing the trunk yes/no?

Thanks in advance.