Sorry this is so long. I wanted to give all the info I thought might be useful, and also outline the various options I've thought of, although I'm inexperienced at HVAC. I'll probably call in a pro, but I want to have an idea of what to expect....
I own a 3 year old, one and a half story house that has had problems heating and cooling the second story since I bought the house a year ago. Since it's currently summer here in Kansas City, MO (current temps in low 90s), I'm currently focusing on improving the cooling aspect. The second floor stays about 10 degrees warmer than the first floor, which I keep about 78 degrees.
Since the house is one and a half stories, the second floor basically sits in the attic. The house is a basic rectangle. The stairs to the second floor are in the front middle of the house. At the top of the stairs is a bathroom at the back middle of the house. There is a bedroom to the left (east) and another bedroom to the right. Each bedroom is approx 15'x17'. The door to each bedroom is near the back of the room. Each bedroom has a single supply register at the front of the room. The bathroom also has a single supply register at the back. There are no return registers.
The gas furnace/blower is in a utility closet in the middle of the first floor, under the second floor bathroom. It has a common return on the first floor. It is a downflow furnace to a trunk in the basement that supplies the first floor. There is an 8"x16" vertical trunk that runs up next to the furnace to supply the second floor. This vertical trunk supplies what I think is about an 8"x16" trunk under the second story. This trunk runs about 12' both directions from the intersection. At each end is 14' of 6" flex to the supply register in each bedroom. The bathroom register is also supplied with about 7' of 6" flex from this trunk. All 3 registers are 4"x10".
I'm experimenting with one of the bedrooms before working on the other, since they're basically the same. As a first step, I tried a register booster fan from Home Depot. It sits on top of the supply register and turns on when it detects cold air flow. It hasn't helped at all.
The next step I'm thinking of trying gets a little unusual: Behind the bedroom kneewall, I found an unused 8" flex duct that runs from the back of each bedroom (opposite of the supply register) to a vent in the ceiling of the utility closet, where the air can enter the common return. They never cut a hole in the bedroom floor to put in the register for the duct, though. I'm wondering if completing this return register in each bedroom could improve airflow on the second floor, even though each bedroom would still have only one supply register. Currently, the return air has to go through the bedroom to the door, down the stairs (3 ft wide and not open on the sides), do a 180 degree turn at the bottom, go 13' through the living room to a large open door to the dining room, and then about 5' to the common return. I'm not sure if this is too far for a good return. On the other hand, I could also turn the unused flex into another supply register for each bedroom. I could run a duct off the 8"x16" vertical duct in the utility room that already supplies the second floor. This new duct would go about 8' across the utility room ceiling to where the unused 8" flex is that goes to the back of each bedroom. In addition, I could put an 8" duct booster fan in each of the currently unused flex ducts to help supply or return air. Would any of these options be very helpful? Is one better than the other?
My prefered solution would be to zone my current system. The second story would be a zone, another zone would be the first floor west duct from the furnace (master bedroom, master bath, guest bath), and the third zone would be the east duct (dining, kitchen, living rooms). However, I'm afraid the current duct sizing won't handle only a single zone being active. I could upsize the first floor ducts since they're in the basement. The second floor ducts are mostly inaccessible though. Even if I add two more supply registers, making a total of five second floor supply registers, would the 8"x16" supply duct be too small to handle the full airflow from the blower? I could probably increase the vertical duct size to 8"x22" and then go back to 8x16 after branching off to the duct to the two new supply registers...would that be enough? If it's close, could put bypass dampers in the basement to compensate?
The furnace is a Lennox Merrit Series, model G40DF-36B-090. This is a 3 ton unit with 88000btu heating. Per the engineering specs from Lennox: at high speed, the blower does 1295cfm at 0.50 in.w.g. With a zoned system, I could reduce the blower speed to compensate for the smaller duct capacity of a single zone. This is done on the circuit board and can reduce the blower to 1200cfm, 1055cfm, or 900cfm (all at 0.50 in.w.g). I suspect this would really hurt efficiency, though, and could be a problem if 2 or 3 of the zones are open. Thoughts?