This may be impossible to answer without a visit but a general answer would be great.
House: 2400 sq ft, two story with unfinished basement. First floor has 9ft ceilings and one vaulted ceiling going to the second floor. Furnace is in the basement. Rheem Critirion model RPGJ-12EARJR. Thermostat is Honywell CT3500/3595 (that's what the manual says).
House has a total of 4 return air grills. All are 29" x 5" with return air going through the empty spaces between studs. 1 return is at floor level of main floor. 2nd is at ceiling level of 1st floor. 3rd is half way up the stairs. 4th is at ceiling level of 2nd floor in the hallway. This last return goes down the hallway wall then across the garage (unheated garage) ceiling and down the garage wall to the basement. I used this return to run internet to the second floor and found the return filled with insulation where it goes horizontal across the garage ceiling.
There are no return air grills in any of the 4 bedrooms.
Is that sufficient return air? I would have thought there would be returns in the bedrooms.
While sitting here this morning I've noticed that the furnace cycles on for ~10 minutes then cycles off for ~1 minute then back on again (not sure if this has anything to do with return air).