Furnace trouble

Akj08 · 02-26-2010, 09:52 AM

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.

SKIP4661 · 02-26-2010, 04:46 PM

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.

Akj08 · 02-26-2010, 05:13 PM

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.