The blower motor in the furnace stops working at night most of the time when the AC should be on. The blower motor (in the furnace) might run for one or two cycles at the beginning of the night but then it stops working the rest of the night. The outside condenser will run all night, draining my bank account, since it's being told to by the thermostat. Because the blower motor stays off most of the night, this causes the AC lines to freeze from the compressor (outside) to the evaporator (inside on furnace). At some point during the morning, the blower motor starts working again somewhere around the time where the thermostat set point changes (programmable thermostat) and everything thaws out and starts working again. During the daytime, there seems to rarely be problems, but they do happen occasionally as verified by my online data gathered by the smart thermostat (Ecobee).

I have tested as follows. At 2:00AM, when AC lines are frozen, the condenser is running (fan and compressor), and the blower motor is called for but not working, I completely shut off the system at the thermostat. I then use the thermostat's installation testing feature to manually turn on the "fan" (aka blower motor). The blower motor does not come on even though I have verified 28VAC is present at the "G" (fan) connection at the furnace when this manual test is activated. I have even disconnected power completely to the furnace, waited 10 minutes, plugged everything back in, and still see the same results most of the time.

When the problem persists for 4 or more hours, the thermostat will detect something is up and display the following on the screen - "Cooling problem... There may be a problem with the Cooling system. For the past 4 hours the thermostat has been calling for cool, but the room temperature has increased by 3.9F"

My AC repairman has taken out the blower motor and tested everything, seeing no issues. Now a new motor control board is on the way but I'm a bit skeptical that this will solve anything after reading some other posts on Stack Exchange with similar issues and no solutions...

Update: The new motor control board was installed yesterday and everything worked during the day. However, the system again froze last night after the furnace blower motor had been non-functional for enough time. The thermostat again displayed the "Cooling problem..." alert as we were sweating in bed. The low pressure line outside was a Popsicle when I checked it at sunrise as the system continued to cycle on/off with no blower motor.

  • Hello, and welcome to Stack Exchange. It would be helpful to know the models of the various components in your system. And, does the problem behavior start and end at a consistent time? – Daniel Griscom Aug 15 '18 at 16:05
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    What happens if you flip your themostat's fan switch to "on"? (As in, always run)? I for one would use whatever means necessary to force the fan to run continuously, simply as a diagnostic to see what happens next. – Harper - Reinstate Monica Aug 15 '18 at 18:51
  • Both the furnace and condenser are Nordyne and about 3 years old. The capacitor in the condenser was replaced, just as a matter of checking that box, even though it looked brand new. The furnace is a Nordyne(Maytag) variable speed upflow model TA-060 with a SmartStart Control Board (not so smart, IMHO). Exact model is PGC2TA060CVA1. @Harper - I did indeed flip the thermostat's fan switch to "on" as described in the 2nd paragraph of the problem statement - no fan came on. – Blazing Saddles Aug 17 '18 at 12:42
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    I'll try replacing the thermostat (maybe its fan relay is going bad.) – DDS Oct 30 '18 at 8:30
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    Is it possible you have the order of things wrong? I think maybe what's happening is that your system has some issue which causes the coils to freeze (low refrigerant maybe), which in turn is detected by the airhandler as a restriction, and the blower motor gets shut off to protect it. – PhilippNagel Jul 18 '19 at 15:51

You may have a run capacitor for the blower fan that is going bad.

  • I cannot disagree with the blower fan run capacitor theory. However, an AC tech came out with special equipment to check it a couple weeks back, basically taking it down to the basics, and ruled out the blower motor assembly as having any issues. – Blazing Saddles Aug 17 '18 at 12:36

Do you have an electric water heater on a timer that turns off the heater elements at night? I can imagine a weird and unsafe cross-connection between the air handler and the water heater. Perhaps the power line to the A/C lacks a neutral wire or some such because a previous A/C system didn't need one, but your newer one does.

To test the theory, pick a time when the A/C is running correctly and blowing cold air, then turn off the water heater's breaker at your electric panel. If the A/C blower stops, you found your problem.

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