I was watching a Youtube video yesterday where a HVAC repairman got a call for a central ac not cooling right (no link, I have no idea what video it was). He found the condenser fan was running backwards. The capacitor measured good but when it was replaced the fan ran the right way. ???

Edit  - Found video -

I had this happen a couple times, potentially it was the thermostat (nest) sending a little half signal mid running that would trip mine. We would hear a bang, then wahhhhhhhhhhhhhhhhhhhh. It toasted a couple capacitors before we realized it was the thermostat. changed to a Ecoobee and everything is good. I had the compressor running backward.

That makes NO sense at all. All the 24v side does is close a contactor at the condenser which allows 240v to power up the fan and compressor. Past that, the 24v side doesn't do shit to the actual condenser components.

Homeowner tried to replace capacitor but got condenser fan wires backwards. Tech either finds it and purposely replaced the capacity to get a little more money out of the job or, most likely, he accidentally figured it out while replacing the capacitor.

Annnnnd this.

He probably has a heat pump and had the reversing valve backwards. Possible if you don't wire stats up correctly or enable heat pump mode in the stat

It does something, b/c I replaced it with a temporary cheap thermostat, hanging from the wall. If you bumped it while the unit was running, hitting the the analog thermometer it would trip the compressor in the backyard into going backwards. Whatever the signal that tells the capacitor to turn on the compressor is sent again while running and then it would take a shit, I would have to throw the breaker. After a few times of this happening the capacitor would be toast.

True. A regular A/C unit, there are just the two 24v wires from the board going out to the contactor.

Now I HAVE seen several units where both the A/C and Furnace would get powered up at the same time. Sometimes a wiring issue, sometimes a Stat issue.

People always lie though and claim they ''didn't do anything'' when it's a jacked up wiring issue. [and you can tell they replaced something like the stat]

He's right, it makes no sense. Was it a heat pump?  If so your stat was turning the reversing valve on and off, which would toast a compressor.  The reversing valve doesn't cause the compressor to reverse direction but it does reverse the refrigeration circuit

The issue with any single-phase motor is its lack of ability to start rotating/spinning. Without any internal or external help, the rotor will just ossolate quickly back/forth but won't start and continue to spin.

Split-phase motors have a smaller gauge start winding that is wired offset of the run windings. The gauge difference causes currents (and its subsequent magnetic field) to flow ahead of the run winding which creates an additional set of poles to interact with the stator windings. Once up to speed, a centrifugal switch opens & removes the start windings from the circuit. They'd overheat and burn up if left energized. Low torque starting method

Capacitor-start and capacitor-start-and-run motors use a capacitor to create a phase shift. Cap-start motors typically use electrolytic caps that aren't designed for continuous use, so a centrifugal switch drops them from the circuit too. A cap-start-and-run motor uses an oil-filled cap that stays in the circuit the whole time. High torque starting methods, don't recall if one is more than the other. Probably is

OP's condenser and fan motors are (assumingely) cap-start-and-run motors that share an oil-filled cap with two separate farad values/ratings within the same enclosure. Never heard of a bad cap causing an opposite phase shift at startup though. Unsure if that is possible or not...



...another single-phase motor is a shaded-pole motor. These are the gutless turds that remove gut turd stink from the bathroom. No torque whatsoever, your finger can stop them from spinning. FYI, reversing polarity of the AC doesn't reverse is rotation. You gotta physically mount it the other way around.

Ceiling fans are outer rotor motors. Taken one apart before, odd things. Low torque as well

I'm no expert by any means when it comes to AC. My AC guy was scratching his head pretty hard, couldn't find anything wrong. Changed to a EccoBee and is hasn't happened is over a year. It was happening pretty often on all the same equipment, no changes. Spoke with him recently about my shop AC and he mentioned he and other around here have also been having similar problems with Nest thermostats.  ?

FWIW, I had a stinker of a job that took a couple of days to figure out. Smoked 24v transformer in the furnace but the board was smoked also, the ecobee 3 thermostat wasn't working properly, and the A/C didn't work either. Ended up replacing the transformer twice, [smoked a seconds one] the contactor, the thermostat, and the board. Reason why all that stuff got smoked? The original installer used zip strips on the thermostat wiring and tightened the damn things down so tightly that they were cutting into the outer sheath of the stat wiring, he zipped it to the duct hangers and on on of the bends, there was a sharp edge that, due to expansion and contraction over the years, cut thru the outer sheath and then thru the yellow [A/C] wire coating and then dead shorted to the ductwork smoking everything.

Zip strips are nice, use them myself but I do NOT tighten them down so tight they can't move or are against something that can cut thru the sheathing just from simple expansion and contraction of metal.

And quit doing the same damn thing to condensate drain lines, you have to be a special type of retard to zip tie a hose so tight you cut off the flow thru it.

No a capacitor can make the fan spin backwards if it dies.


Happens all the time.


The run cap on PSC motor “orients the phases” giving you a faux 3 phase.


If the capacitor fails it can spin backwards like a incorrectly wired 3 phase motor.

ELI the ICE man.

Nope. He checked all the wiring first and that's what he thought. I think he said he'd seen that capacitor problem before, though.

@fnxtime

If I remember right, you do this for a living. I know how motors and capacitors work, what I can't figure is how a bad capacitor would make a motor run in reverse.

Found video