Replacing 1-Stage Furnace & A/C for Comfort & Efficiency

Question

Need opinions on imminent HVAC work in a Massachusetts home. House and HVAC system descriptions below:

• House: ~45yrs, 1850 sqft, 2 floors + unfinished basement (furnace located in basement). Each floor (including basement) ~7'5" high.
• Furnace: Gas, 80% efficiency, 1-stage, output 92k BTU
• A/C: 3-ton
• Distribution system: 1x 18"x20" main supply trunk, 1x 18"x20" main return trunk. Floor vents in 1st floor & baseboard vents in 2nd floor. 7x supply vents & 4x return vents on 1st floor, 5x supply vents & 2x return vents on 2nd floor. All 7x lines run from main trunk in basement, through 1st floor walls, along joists between 1st & 2nd floor and then into baseboard.
• Control: 1x thermostat on 1st floor

We've observed the following over the years:

• Temp difference between 1st & 2nd floors (set point is reached on 1st floor, then system shuts off). So we increase set point in order to reach desired 2nd floor temp, thereby heating up unoccupied 1st floor. Some 1st floor vents are also closed to force air to the 2nd floor.
• Temp difference between bedrooms on 2nd floor. Only 2 returns: 1 bedroom + hallway. Bedroom doors are left open to try to balance out air flow.

The system is 19-22 yrs old so it's a good time to upgrade. I am open to ductwork changes in the basement but no ductwork changes in walls or furnace/ductwork addition to attic/ceilings. I've received several proposals:

1. Upgrade to modulating furnace. Advice was that other than 2 separate systems, any zoning implementation will create static pressure issues & reduce life of system. Also that because it is modulating, this gives enough time for 2nd floor to reach temp (compared to 1-stage gas valve with 1-speed fan).
2. Upgrade to 2-stage furnace & add dampers to each supply line (12 total) as long as they are 6" diameter. My concern is multiple electronics that can fail. Also, I saw that some lines are 4"-5", so committing to a job that is unlikely the produce the desired result.
3. Upgrade to 2-stage furnace, add a new main supply trunk (so 2 dampers) & relocate 2nd floor lines to new trunk. In addition, add return vent on opposite wall of 1 bedroom that has a return (utilizing the same duct inside the bay). More labor but fewer electronics and sounds like most feasible.

Option 3 sounds attractive. Any thoughts/suggestions? Our desired outcomes, in order of priority are:

1. Comfort - temperature balance between bedrooms
2. Efficiency - warm only occupied floor

Answer 1

I went through this about 12 years ago. I selected a heat pump (yes I am in Michigan and it works most of the time) but used a gas furnace instead of the recommend electric heating elements for supplement heat. The additional cost was just a few hundred dollars at the time. This allows me to switch to emg heat (supplemental) which is gas when the cost of electricity goes over the top. It has variable speed blowers on the condenser and furnace fan. I also put a variable speed blower on the hard to heat/cool trunk and cycle it with the main blower. This will starve the other area but the thermostat senses this and ups the heat output. The stats are normally indicating within one degree of setpoint. I used an electrostatic filter, for better airflow. Another thing is the main blower runs 24/7, it is amazing how this equalizes the temperature. When your upper floor runs cool in the winter close the return air upstairs and open them downstairs if there is an open stairs down. In the summer use the upper returns and close the lower ones. This only works if you have returns on the upper level. Cold air sinks pushing the hot air up.