Interestingly enough I'm also in a similar scenario. 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. Hence, this comment may be relavent. 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 do 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.



Quote Originally Posted by duckman06 View Post
Agree! Is the current furnace Loud? Just makes you wonder how the furnace has lasted this long if the wrong return was used?

I would get a pro out to overlook the system and get a load cal. Done to size the furnace as needed not go by what is their now. Too many times we all see oversized furnaces with return air grill undersized which makes for loud unit, decreased performance and shorter life span of the unit.

Is it a heat only system or do you have a a/c unit also for cooling? Cooling often requires more cfm/return to work propely compaired to heating system only.