I just switched my upstairs and downstair thermostats for smart thermostats. The C wire was not connected so I opened the furnace and connected it. Both units deliver 27v to the thermostat. I read where 20-30v is ok. The upstairs thermostat functions just fine with both A/C and Heat. The downstairs thermostat is fine with A/C, but when on heat the unit repeatedly blanks out losing power and the clock time. I swapped the upstairs and downstairs thermostats to see if I had a bad thermostat, but the problem stays with the downstairs unit.
I suspect that when the Bryant gas furnace kicks on that the power must be unstable therefore the thermostat doesn't have sufficient power. The Heat stays on and runs and the unit will get power again while its running, but the clock gets reset every time. Therefore its only able to work in hold mode and not with a schedule.
I did verify the cables all have secure connections.
Voltages measured at the furnace.
28.1v when it first kicks on
24v blip when the blower kicked on
25v for a bit
I don't know all the stages it goes through (stages is probably not the right word, it's not a multi-stage unit), but something kicks in a few moments after everything is running to drop the voltage.
Replaced transformer, but no change.
Discovered that when the voltage drops to 14v and the thermostat goes out, that the control board is also blinking error code 33. Tested the blower capacitor and it was below the 6% threshold. Replaced capacitor, but no change.