I have a Carrier furnace that is controlled by a two zone motherboard. When I turn my downstairs heat on only the damper for the upstairs opens slowly until it opens all the way when the fan turns on. I put a multimeter on the open contact on the motherboard and it is sending 24volts to the upstairs damper to open about every 10 seconds until it is open all the way. I switched the damper wires for the upstairs and downstairs around on the motherboard and it does the same thing to the upstairs damper so i don't think its the damper unless both dampers are bad. What would make the dampers open when they are supposed to be closed and held close with that 1 volt rather than being sent 24volts to open? The furnace Model # is 58STA/STX and there wasn't a label for model number on the motherboard but i guess the model number is ZoneCC3ZAC01. Thanks for any help.
Usually there is a temperature sensor installed in the plenum. If it senses too much heat it will open the other zones in increments to keep the furnace from overheating and locking out on a high limit fault. If you don't have a dump zone or a bypass you will probably require on in order for the system to function properly.
Thanks for your reply. Do you think that sensor is bad? If the sensor isn't bad how do i do a bypass for the heat when the downstairs is on. I don't have a relief zone. There is a set of contacts for a third zone on the motherboard but there isn't anything connected to it.