I recently moved in to a new home that is setup with a single stage Goodman furnace that is divided in to two zones (upper/main). When we first moved in to the home, the upstairs was always cold. I have since had the furnace serviced, and installed new thermostats and upstairs now gets to temperature. I am currently working out of an unfinished basement and I have been noticing the furnace is running a lot (and the NESTs are now telling me the furnace is running ~16 hours a day).
After some digging, I am seeing that the furnace is regularly erroring with a "primary limit circuit open" LED error code. I switched out to a low resistance (almost no resistance) air filter to confirm the filter was not restricting air flow too much. I recently had the wiring reviewed as well since I needed to add a common wire for the NEST thermostats. After talking with the technician again, they have indicated that single stage furnaces do not work well with multi-zone and I should disable the zone control and second thermostat and run everything as a "regular setup". The internet seems to confirm that single stage furnace + multiple zones is indeed "bad design".
I am the type of person who likes to fix problems rather than work around them. I plan on adding AC to the setup in the spring and I was planning on getting a two stage AC. Would I be crazy to replace the furnace in order to have a two stage furnace and keep the zone control? or should I really just disable the zone control as suggested?
The zone control is handled by an Aprilaire temperature zone controller connected to 4 electronic dampers. Image below showing one side of the setup.