Pulsating AC Units

[JohnWedell]

Just got a new North Point and sitting here with both airconditioners running i’m getting a pulsating sound on the roof from both of them. When i shut them off the sound goes away. Very strange, anyone have or heard of this.

[us71na]

It is probably a sympathetic vibration of the roof panel due to the vibrations coming from the A/C units. The pulsing is because the units aren't running at exactly the same speed.

[stateboy]

Going along the lines of what us71na said, you probably won't get the same effect when running only one of the two units. If possible, try that to see if it's the lack of synchronization of two units as mentioned above or, however unlikely, just one that may appear to have an out-of-balance motor spinning.

[Marcm157]

I'd say an out of balance motor would be the cause. Try isolating each AC unit and begin by running fan only in each one.

[T&M_Texas]

Originally Posted by JohnWedell

Just got a new North Point and sitting here with both airconditioners running i’m getting a pulsating sound on the roof from both of them. When i shut them off the sound goes away. Very strange, anyone have or heard of this.

I've noticed a pulse from either one or both our units on a 377RLBH. I can even feel the air flow from the vent pulse with the slight pulse noise. Hasn't caused any issues "YET" but I've noticed it.

[hoppers4]

Originally Posted by us71na

It is probably a sympathetic vibration of the roof panel due to the vibrations coming from the A/C units. The pulsing is because the units aren't running at exactly the same speed.

X3

[RedHorse1]

Originally Posted by JohnWedell

Just got a new North Point and sitting here with both airconditioners running i’m getting a pulsating sound on the roof from both of them. When i shut them off the sound goes away. Very strange, anyone have or heard of this.

John, so the pulsating only happens when both units are running?
If that’s the case I agree with the “sympathetic vibration” argument. You can narrow down the contributors by setting the fan to “on” instead of “auto”. If the condition persists with just the fan motors running (not the compressors) then the vibration argument seems even more plausible.

I had a similar problem but only one AC involved. The noise would get louder and fade to almost normal over 20 or so seconds. Turns out the blower motor bearings were wearing and allowing a vibration which would come and go (maybe in harmonics with the compressor). About $125 donation to Amazon and an hour on the roof solved the problem (replaced the motor).

[hoppers4]

Originally Posted by RedHorse1

If the condition persists with just the fan motors running (not the compressors) then the vibration argument seems even more plausible.

I would be more inclined to think it's the unsynced compressors causing the problem.