AC Fan Knocking - Interesting Sense Signature

Hi everyone. Relatively new Sense user here.

I had a new HVAC unit installed in my home in May of this year. Before installing Sense, I noticed that occasionally in the morning - between 5a and 7a generally - I’d hear a knocking sound coming from the fan unit about once every second or two. This seems to start right after the compressor finishes running, but not necessarily. My AC company is going to come out to look at it, but now that I have Sense I’ve seen a corresponding signature that I’m curious if anyone has seen before.

This image is zoomed in on about a ten-minute segment where this happened this morning. The waveform is practically identical each time it occurs, although the duration varies.

It’s definitely the AC company’s problem to fix, but I wonder if any of the experts on this forum have any thoughts or insights into what may be happening.



This is exactly the type of thing Sense people (@RyanAtSense) will want to trace and will likely request your data to do so.

Sounds like (perhaps literally) you have a physical fan imbalance and perhaps Sense is showing the corresponding load oscillation with the impediment to the fan. At least that would be my first naive assumption.

I had a fridge repair tech come while I had Sense installed and was super excited to show them the waveform but they ended up ignoring my enthusiasm for the added data. Experts! Will be interested to know if your AC tech can actually use the Sense info as well.

Please keep us informed.

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Seems like everyone is having AC issues lately!

Yes, as @ixu wrote, we’d absolutely love to take a look at your data as it will help us to build out HVAC-related models in the future that can actually detect problems before they become catastrophic. Let me know if you’re OK with that, and I’ll pass it along to the team.

Definitely keep us posted on what you learn.

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Hi Ryan. Yes, very happy to help. Let me know if you need anything else from me, but no problem at all with the data access.

One thing to note is that Sense has found my AC, but not the fan. Not sure if that’s related or just a function of the short time Sense has been installed.