In a quest to try to make my home more comfortable, I have been looking at a replacing an original furnace and AC unit for my two story home with a finished basement (build date 1994). The home including basement is just shy of 4000 sq. ft. The AC is about on it's last leg... so it's about time. I think the existing furnace is 125K btu and the AC is 4 ton.

My issue with comfort is that the 4 bedrooms upstairs remain hot in the summer, and cool in the winter. Each bedroom has a single 4 x 10 register with the master having two 4 x 10 registers.

As I educate myself more, I am starting to realize that it may be the ducting in my home that's contributing to the house not being comfortable.

I have had a bid on a new system, and the contractor was trying to claim that a modulating furnace with variable fan would remedy the situation, but I'm not sure I'm convinced it's this simple.

Specifically the return air capability in my home has me suspicious. The only return upstairs is a 6 * 14 outside of all of the bedrooms in a vaulted foyer. The remainder of all the return capability is on the main floor. There is one 6 by 14 and two 6 by 24 return grills on the main floor. All of which are pretty close to each other and the thermostat.

My question to the group is am I on the right track thinking that ducting here is holding back comfort? And if so, what are some approaches to overcome this? Would a new modulating furnace with variable fan resolve my issues here? Do I need something else like a zoning system?

I don't see an easy way to retrofit & add additional return capacity upstairs. The basement is finished, and there ins't a clear path up to 2nd floor. There is an attic above the 2nd floor. One idea I've had in my head is to add ceiling returns and connect via the attic into the 6 by 14 outside of the bedrooms. Not sure this would help though as it's really not adding additional return capacity, but at least would pull some return air from the bedrooms.

Please let me know if I can provide any other info to give me your all's thoughts on what I should be looking to do here.