Hi everyone,

I'm new to this site and I have a question that nobody seems to be able to answer. I have a dual zone heat/ac system in my 3100 sqft 2 story home.
Recently my heater has been short cycling, but only when the heat is turned on downstairs. (Upstairs has been working great).

Two story home, approx. 1600 sq. ft. upstairs, and 1500 sq. ft downstairs.

2 zone control.

Carrier Model 58 RAV
115K BTU

Honeywell EMM-3 Minizone Controller
White-Rogers 1F78 thermostat

2 16" supply ducts into 2 J-boxes
1 18" return duct
11 in-ceiling registers upstairs various sizes
9 in-ceiling registers downstairs various sizes

PROBLEM: heater short cycles about every 7 min. (downstairs only)

When heating downstairs only, error 33 high limit switch trips open. I measured the heat at the sensor, and it trips at 210F as designed, so I know my furnance is overheating.

I can run the upstairs indefinitely, and the heater doesn't cycle.

I believe I ruled out any electronic issues with the unit, and I believe that I may have airflow issues with the downstairs ducting.

I have adjusted the upstairs damper to stay open about 30 degrees when I operate the downstairs control only. This has lengthen my cycle time to over 30 minutes ( I only tested to this time, as it may run longer before cycling)

What else should I check? Would it be safe to operate given that I have adjusted the upstairs damper to get better airflow?