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fdahl · Jul 12, 2009, 08:56 AM

Hi. My son's HVAC unit is an older Carrier model and needs replaced, but he is trying to limp through this summer until he can afford to buy a new one.

The problem is the cooling fan will not run in the outside unit. I took a look at it and the contactor is pulling in and the compressor is running. I messed around with a "thermodisc" unit and the fan started running. It ran for several hours but then stopped again.

The thermodisc is a 3-wire unit mounted to the HVAC unit housing. It is unclear to me the purpose of this unit. It is exposed to temperatures near the starting capacitor for the unit, but that is about it. It is not even inside the condenser coil. Can someone please tell me its purpose and how I can check the device with a multimeter? I hate to just replace it not knowing how it is supposed to function. Unfortunately we do not have a schematic for the unit.

Thanks for any help you can provide.

siberianair · Jul 12, 2009, 10:30 AM

That could be the defrost control if it is a heat pump.
Check the capacitor... that is probably your problem. Also the fan could just be tripping out on thermal overload protection.

fdahl · Jul 12, 2009, 02:09 PM

Thanks Siberianair for the quick response.

Well I dug into the unit a bit more and figured out that the thermodisc unit controls the high and low speed of the fan. After pulling the temperature switch out and replacing it, the unit started working. When the switch heats up, the fan goes to high-speed and when it cools down switches to low speed. I tested this with ice cubes and a hair-dryer :). I did clean the "fast-on" spade connectors and now am wondering if it just was getting a bad connection. It has been running for about 4 hours.

Not sure why that switch is even in the circuit. It is in the electrical compartment of the unit; not in the compressor or the condenser area. The only thing in that area is the startup cap and the contactor.

fdahl · Jul 12, 2009, 02:09 PM

Thanks Siberianair for the quick response.

Well I dug into the unit a bit more and figured out that the thermodisc unit controls the high and low speed of the fan. After pulling the temperature switch out and replacing it, the unit started working. When the switch heats up, the fan goes to high-speed and when it cools down switches to low speed. I tested this with ice cubes and a hair-dryer :). I did clean the "fast-on" spade connectors and now am wondering if it just was getting a bad connection. It has been running for about 4 hours.

Not sure why that switch is even in the circuit. It is in the electrical compartment of the unit; not in the compressor or the condenser area. The only thing in that area is the startup cap and the contactor.

fdahl · Jul 12, 2009, 02:09 PM

Thanks Siberianair for the quick response.

Well I dug into the unit a bit more and figured out that the thermodisc unit controls the high and low speed of the fan. After pulling the temperature switch out and replacing it, the unit started working. When the switch heats up, the fan goes to high-speed and when it cools down switches to low speed. I tested this with ice cubes and a hair-dryer :). I did clean the "fast-on" spade connectors and now am wondering if it just was getting a bad connection. It has been running for about 4 hours.

Not sure why that switch is even in the circuit. It is in the electrical compartment of the unit; not in the compressor or the condenser area. The only thing in that area is the startup cap and the contactor.

fdahl · Jul 12, 2009, 02:10 PM

Thanks Siberianair for the quick response.

Well I dug into the unit a bit more and figured out that the thermodisc unit controls the high and low speed of the fan. After pulling the temperature switch out and replacing it, the unit started working. When the switch heats up, the fan goes to high-speed and when it cools down switches to low speed. I tested this with ice cubes and a hair-dryer :). I did clean the "fast-on" spade connectors and now am wondering if it just was getting a bad connection. It has been running for about 4 hours.

Not sure why that switch is even in the circuit. It is in the electrical compartment of the unit; not in the compressor or the condenser area. The only thing in that area is the startup cap and the contactor.