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Carrier heat pump 25VNA Fan spinning up only intermittently

6 years ago

Hi all,

I'm at a lost of what is happening to my 3 ton 3-year-old carrier green speed 25vna heat pump.
First noticed that the heatpump was running but that the top fan was not spinning, the system was heating and not in defrost. The compressor was going crazy even though it was only 40 F outside. The only way to get the fan spinning again was to flip the breaker and restart the system.

It continued happening, so I called out a tech. Obviously issue did not occur when he was here. He called with Carrier to troubleshoot, the only thing they could find was one single error 56 Outdoor ambient thermistor/ outdoor coil thermistor out of range, only one single error even though the problem had happened more than once already. He measured the resistance of the OCT but I think not properly enough.
He decided that the OCT was the problem and left.

Even though the parts are under warranty, the labor is not, so I told them to hold off on the repair until I could convince myself that indeed the OCT was the problem.
Every time the problem occurs, which is intermittently, I measure the following:

OAT is always around 27kOhm which is correct for the 40F it's been around here.
If the system is off, bot the OCT and OST measure the same resistance as the OAT.
If the fan is not spinning, but the compressor is working hard, the OCT and OST go up to 70 kOhm, but always in tandem and the pipes inside look very cold with crystals.
So to me if the two thermistors (OCT and OST) measure the same resistance, they are measuring indeed the pipes that are getting very cold since the fan is not running. So the OCT is not the problem. Also no other error 56 has occurred since.

Next step was to measure the voltage given to the Fan from the control board. Measuring in heating mode across PWM1 and PWM2, I find 8 volts, whether the fan is spinning correctly or is faulting. Measuring the duty cycle gives me 50% and both these values are correct according to the service manual. So the control board is sending the correct spin command to the ECM according to me.

Am I correct to assume the ECM Fan motor has failed, even though it sometimes spins up? Sometimes it tries, but is unable to spin up. It does not make any weird sounds when it spins, or when it faults. Is there anything else I should test to ensure 100% of the failing part? I don't want the technician to come out again and not know what is wrong or replace the incorrect part.

Sorry for the wall of text.


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