I recently moved into a new home. This home has two stories and each story has its own AC system. Both units were replaced about 2 years ago. Both units are 3 ton Carrier Units. Everything is identical for both units (blower, furnace, condenser...).

As part of any new home purchase we had an inspection done. We also had the company that installed the AC perform an AC specific check up as part of the home purchase. Both units seemed to be running fine. The house stays cool and dry during hot humid days.

The problem.... I recently noticed that when the downstairs AC comes online to cool the house, the blower "stutters" once (the blower comes on then right away slows down/turns off then back on). This only happens one time as the unit starts to cool. It happens very quickly, but I can definitely tell the blower slows or stops for fraction of a second. The upstairs unit (identical) does not do this.

Both blower units are 58STA070-16. Since both of the thermostats are identical (aprilaire 8400 series), I just switched the thermostat faceplates between upstairs and downstairs. That did not make any difference. Upstairs continued to work normally and the downstairs continued to stutter.

I turned on just the fan without setting the temperature low enough for the compressor to come online. When I did this, the fan came online normally without a stutter. Then I adjusted the thermostat temperature down so that the compressor comes online. When the compressor came online, the fan did a single stutter and continued to work normally after that.

So is this normal? Will this shorten the life of my blower? What could be causing this?

Thank you for your thoughts,


1 Answer 1


That's the sound I would associate with the Starting capacitor of the blower motor having trouble. It could be a number of things; the capacitor is failing, the switch that CONTROLS the capacitor is failing, the blower electrical circuit has a loose connection somewhere, or, and what I suspect is likely, by the time this blower turns on, everything else is ALREADY turned on and is causing a voltage drop, which is making the motor on this blower lose torque right as the centrifugal switch begins to function, then it slows down and causes that "stutter" effect until the blower gets to full speed. because heat rises, your upstairs unit will always come on first, so the total load is at its highest when the downstairs blower is called for, which would be AFTER the compressor is already running as well, meaning the blower is the LAST big load in line and has to contend with a voltage drop issue.

  • Thank you for your thoughts on this. I thought it might be a voltge drop issue also. However, I ran the downstairs unit without upstairs running and the same issue persisted. Also, when the compressor comes online, the lights do not dim at home.
    – Anario
    Commented Jun 28, 2018 at 12:19
  • I'd be looking for a loose or corroded connection somewhere then. That's easier to deal with than the bad capacitor or worse yet, a bad centrifugal switch in the motor.
    – JRaef
    Commented Jun 29, 2018 at 1:17

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