Weird noise and lights flickering with AC running

June 26, 2021 7:13 AM

I noticed last night that our lights were flickering faintly every minute or so. I believe it is related to one of our AC units, which is also emitting a regular noise, that sounds similar to the "AC startup sound," just a short pulse. It happens the whole time the AC is running, seemingly pretty regularly.

Can you help me gauge whether this is just annoying, or dangerous? Something we need to get an air conditioner repair tech out right away for, or can we ignore it? Or is there anything I can adjust/ restart as a totally ignorant first-time home owner?

posted by jschu to Home & Garden (10 answers total)

Are the two events correlated? Is this a window-box AC unit plugged into typical household socket, or a central air system with its own switch in the circuit breaker? Does air continue to blow from the vents when the startup sound happens?

(My half-assed, uninformed guess as someone who has never done any professional work on HVAC stuff is that either the blower motor or the compressor is on its last legs and regularly hitting either a current limit or overheating and causing the system to shut down. If that's true, the most likely danger is that it will fail entirely and you'll have to wait a while for a replacement. But, I don't know if that's true. Weigh better informed opinions above mine.)
posted by eotvos at 7:32 AM on June 26, 2021 [1 favorite]

Response by poster: Central AC.
No interruption in cold air, as far as I can tell.
Yes, the lights flickering and the sound are correlated/simultaneous.

Just talked to an AC tech and he said it sounded more like an electrical issue.

Other data point -- this began the same day we got a Peloton. It doesn't seem like that would be a huge burden on the electrical system, but who knows.
posted by jschu at 7:44 AM on June 26, 2021

It sounds like something in your ac might be pulling high current. Traditionally, motors will do that when they're starting up.

How old is your AC? Does the flickering happen when the AC is off at all?
posted by drezdn at 7:56 AM on June 26, 2021 [1 favorite]

(It happening the same day you got the Peloton is probably just a coincidence)
posted by drezdn at 7:57 AM on June 26, 2021

If the AC is making that noise every few minutes, it sounds to me (having no expertise but watching a lot of hvacr commercial hvac repair videos on YouTube) like you've got an AC fault and the AC compressor is turning on, faulting and turning off, repeat. Just turning the compressor on is liable to cause your lights to flicker, but if there's a fault, that may be causing it to draw more power
posted by wotsac at 8:17 AM on June 26, 2021

It sounds like something in your A/C might be pulling high current. Traditionally, motors will do that when they're starting up.

Sounds like a failing start capacitor. That's a part designed to bank some extra electricity to help with starting the unit and not pull the line voltage down so hard that everything else in the house dims.

It's something you can replace if you have some DIY skills, but it's probably worth a service call to make sure nothing else is wrong.
posted by JoeZydeco at 8:20 AM on June 26, 2021 [7 favorites]

It's something you can replace if you have some DIY skills, but it's probably worth a service call to make sure nothing else is wrong.

Note that one of those DIY skills should include knowing how to safely discharge a capacitor before touching it. Big capacitors can hurt you.
posted by TrialByMedia at 11:05 AM on June 26, 2021

Those starting capacitors aren't meant to store energy to help the motor start, but with motors having a start capacitor they create a shift in the current through two windings so that the motor can start turning, and turn the right way (in devices with a long lifespan and often long running hours, like the fan in an air conditioner, they nearly always use this type of motor as there are no brushes to wear out). A busted start capacitor can indeed cause high starting currents like what appears to be happening here, as well as possibly making the motor run backwards. If it's driving a fan, that would suck. Literally.

Starting capacitors don't store a lot of energy, but it's always prudent to switch off and fully disconnect the device you're going to be working on, then go for a cup of coffee or tea before continuing work.
posted by Stoneshop at 12:59 PM on June 26, 2021

Really sounds like your AC is running on the start windings, which might mean the start circuit is OK, but then when it switches to the run windings via a centrifugal switch on the shaft of the motor, the motor does not maintain sufficient RPMs to keep the centrifugal switch closed, so it drops back into start mode which spins it up again and the whole cycle starts over.

I think that could be a voltage problem, but not one likely to be caused by the Peloton. I note that you live in Houston, and given the news stories about people waking up in Houston and discovering that the power company has turned up their thermostats against their will in the middle of the night, I'd be surprised if the delivered voltage from your power company isn't considerably below specs right now.

But I'd also unplug the Peloton out of an abundance of caution.
posted by jamjam at 2:17 PM on June 26, 2021

We had a similar issue and the electrician told us to call the power company. We did and they found that squirrels had nearly chewed through the ground wire that was part of the connection from the pole to the house. Replacing the ground wire got us back to normal.
posted by kingless at 3:45 PM on June 26, 2021