Just a homeowner here, but it sounds like this is the bypass that houses your furnace's humidifier. In heating season, the damper would be open allowing some warm air to return to the return side picking up moisture from the humidifier on the way.

During cooling season this damper would be shut.

Some of the older homes in my part of the country have very old metal duct systems that were originally sized for heating only. When they finally install a full central system, they find they have too large of a cooling system... so they install these bypass trunks to dump the extra air back into the return. I dont have my ductulator handy but i imagine that 10" will be around 300-350cfm of air. Close the damper and see if your vents become noisy (cooling mode).

Hope that helps

I understand that bypasses are sometimes used on zoned systems.

Thanks all for all the theories. There was an old humidifier connected, but it was on the supply plenum, on the right side of the picture, near where you see the exhaust vent. I took that out awhile ago. Perhaps the system was originally sized for heating only. In that case, would I leave the damper open or closed in the winter? I have it closed now, and the vents are a little noisier, but nothing to complain about. I am not familiar with a zoned system, but my ductwork does have a lot of manual dampers around. Do people ever replace or redo their old ductwork? Mine seems poorly pieced together, dirty, and disorganized. My home also has an addition that receives weak supply so it's supplemented with electric baseboard heaters -- not to mention returns that use the basement ceiling joists instead enclosed metal ductwork. Seems redoing the ductwork may result in a more efficient, cleaner, and better designed system. Sounds expensive and not sure if it would add to the resale value. Thoughts?

Your duct work sounds very inefficient to a point where you have to supplement it with electric baseboard heating. You are wasting money with your present set up. Fixing the duct work will save money in the long term.

New duct work does not add to the resale value of the house. A buyer does expect the duct work to be in good condition.

We had an HVAC company install a 3.5 ton unit in our 1,248 sq.ft. ranch style, no basement house. They chose that unit not us. It was not cooling off the house enough, then this company said we needed larger vents for another $3000, on top of the $9800 we spent on the unit. We would have chosen a smaller unit if we had known this! We no longer trust that company so hired a different company to replace our vents. They replaced our 6" rigid vents with 8" and 10" flex duct. We have good air flow in our kitchen and living room, but nearly no flow in 2 bedrooms. They split my bedroom with our laundry room, and split my daughter's bedroom with the living room. Both of these rooms are hot. We cannot sleep, and its hard to even spend time in there for any reason. Our plenum has condensation. They ran 6 lines directly off the plenum, for 10 vents. Does each vent need its own line directly to the plenum???

I am not impressed with their handiwork, looks like a mess to me...?

I thought it related to strange ductwork. That's why I shared pics.

A 3.5 ton unit seems over sized for a 1,248 sq. foot ranch. You have enough cooling capacity to keep the house very cool. If you are not getting air flow to bedrooms then it is a duct supply problem. I see in the photo flexible vents joined together with ties constricting the diameters. Kinks and sharp bends reduce air flow. Call the installation company back and have them straighten everything out. A direct connection back to plenum is always best, but with some care and proper connection and sizing one duct can be be split into two.