Heat Pump Cooling Seems To Swing 2-3 Degrees While Running - Motor Bad?

Old 08-07-15, 03:16 PM
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Heat Pump Cooling Seems To Swing 2-3 Degrees While Running - Motor Bad?

A week ago my AC was off. It turned out to be a blown 24v transformer. I reset it and just tried to have the tstat turn on the blower. It tripped again. I checked the blower capacitor and it was about 25% off spec (reading about 15 when spec 20 +/- 6%). I replaced the cap. The 24v transformer still tripped just trying to have stat turn on blower. At this point I suspected the digital thermostat (ecobee Smart) and I hooked the original mercury tstat and everything ran and the house cooled.

Replaced the thermostat, and the new thermostat went bad after 2 days. The tstat worked fine but when it called for cool or just blower nothing happened. Back to the mercury and everything fine. 2nd new electronic tstat now in place last 24 hours and all is running. BUT I'm seeing data from the tstat that has me concerned.

Between noon and 4:30pm the heat pump compressor and the blower seemed to have been running constantly. Outside temps in rising from low to high 80's. The inside house temp was going from 77 (stat set point) up to 80 then down to 78 then up to 80 then down to 77. Each 3 degree swing took about 45 minutes.

I think the last time I experienced something like this the AC guy eventually found the fan motor on the heat pump was failing - not enough to stop but enough of a slow down on occasion to reduce cooling efficiency and then eventually speed up to normal and then cooling efficiency shot back up.

I'm wondering if 2-3 years later that motor is again going bad or if a similar failure is starting on the inside blower motor. Is there any way to test the motors while they're running but the house temperature is rising to see if either is going bad? Or is there something else I should look for?
Old 08-07-15, 03:35 PM
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The thermostat runs a relay that controls the blower fan. The blower motor could be shooting sparks and the thermostat would have no idea. Therefore the condition of the blower doesn't affect the stat.

It is pretty rare for the transformer to trip from an inside blower motor control issue. It is a real possibility that you have a contactor or reversing valve problem out in the condensor. Another possible problem is a short in the wiring between the house and the condensor.

You should be able to visually see or hear the condensor fan slowing down.

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