GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Fujitsu Heat Pumps cause weird sound in breaker box and exhaust fan

Clay Whitenack | Posted in Green Products and Materials on


Weird issue I’m experiencing and this obviously isn’t an electrical forum but I wondered if there was enough mini split installations here that someone else may have experienced this…

The weather is turning warmer in Zone 4A so I am turning out my Fujitsu mini split heat pump A/C for the first time since our house has been under construction, and I noticed a weird sound coming from my whispergreen bath fan. It is hard to describe, but it is like an electrical hum similar to a flourescent light ballast. The sound gets louder when you actually turn the fan on.

At first I thought it was a problem with the fan itself, but I checked the other whispergreen fans in the other bathrooms and they make the same noises. Then, I went down to the breaker panel and the same kind of sound is down there right where the electric comes into the panel.

The hum only occurs when the A/C is on. When the Fujitsu is running in fan only mode, there is no sound. My electrician is stumped. Could this be some interference of dirty electricity coming from the heat pump?

And just to be clear, this isn’t the hum/sound from the outdoor unit itself. This sound is showing up at the electrical panel and bath fans.

I know this is obviously an issue for a licensed electrician, but I was wondering if anyone here had this experience and if there was a known cause.


GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Anon3 | | #1

    Is the grounding/neutral line done correctly?

  2. Mike M | | #2

    It sounds like that or somehow your variable speed drive on the compressor or head is causing harmonic noises in the system. I would bet Dana would be all over this.

    I would guess they could be induced in the fan circuit by long parallel runs with that bath fan, being tied to the same neutral or other similar connection. I also noticed my Mitsubishi manual talks about breakers for leakage current as well, which baffles me as we would usually call this a GFCI or something similar.

  3. Mike M | | #3

    With that being said, the manual does say the inverter circuit shoudl be on a ground-fault interrupter.

  4. Mike M | | #4

    Another quick thought would be that the inverter is wired into a sub-panel that is not correctly grounded all the way back to the main panel, like ANON3 suggested.

    Sub-panels would normally not have the neutral bus bonded to ground, but require the ground to be ran all the way back to the main panel where the neutral strip is normally bonded to the ground terminal strip. I know around here, not all of the inspectors don't require the ground conductor all the way back to the main panel (against the NEC to my knowledge) and allow it to be grounded to an earthing rod at the panel. My issue with this, as Dana's will probably be as well is that the grounds at these two points may have different resistances and therefore protection maybe different. I generally deal with much larger systems though.

  5. Jon R | | #5

    Harmonics make sense to me. They could be reduced with a load reactor, but maybe it indicates a problem with the heat pump (like a bad input capacitor).

  6. Clay Whitenack | | #6

    Thanks for the replies. I checked the outdoor unit and the noise is strong there. I'm not sure about the wiring and grounding of the sub panel but I will have the electrician start his troubleshooting there.

  7. Charlie Sullivan | | #7

    If I were troubleshooting this in person, I'd first check grounds and neutrals to make sure it's safe to work on. Then I'd check line to line and line to neutral voltages at the unit and at the fans.

    If that all looked good, I'd then I'd look at voltage and current waveforms and/or harmonic content on an oscilloscope. It could be a coincidence of high line impedance at the frequency of a particular harmonic that's being excited, and a filter inductor in the fans that happens to create audible noise with a small excitation at that harmonic frequency. I'm not sure how many electricians these days routinely carry instruments with the capability to display waveforms or harmonic content, or know how to interpret that information. If with the right instrumentation, you then have an interesting challenge to figure out whether the mini-split it out of spec, or whether the coincidence of conditions is making it audible even if nothing is out of spec, in which case you'd need to add some kind of harmonic filter or reactor to knock that out. You aren't supposed to need to get into that kind of detailed analysis for a simple residential mini-split system!

    So maybe the practical advice is that if the electrician finds nothing wrong, tell the mini-split manufacturer's rep about the problem, and see if they want to step up to the plate and send out a technician or engineer who would be willing and able to get into those details, or perhaps they have encountered s similar problem before.

  8. Clay Whitenack | | #8

    Hey all, just to update the situation. My builder completely disconnected the ground from the outdoor unit, wondering if maybe the noise was travelling through the ground. The noise continues, but that doesn't really prove anything, correct? If the trouble is caused by being improperly grounded, all we did was make it even more improperly grounded, right?

    My builder has a theory that maybe the grounding rods outside the panel box are not totally in contact with the earth. They are currently driven into the gravel surrounding the foundation (haven't backfilled with dirt yet), and maybe the grounding rods aren't yet in contact with enough soil.

    We're at the point of backing up and calling an electrician to troubleshoot the issue, but I just want to make sure we have tried all the easy stuff first.

  9. Expert Member
    Dana Dorsett | | #9

    This is almost certainly a wiring problem, possibly a loose or disconnected neutral somewhere, but it could also be an illegitimate connection between neutrals (or neutral to ground) in the wiring.

    Inverter drives use high frequency power switching with weird wave forms unrelated to the 60 hertz harmonics, and any current loops from illegitimate return paths can cause quite a variety of interaction with other equipment. It's common to see syptoms like this with switching power supplies and high efficiency fluorescent ballasts when the power levels are high and the wiring has errors.

    Can't say I've seen it first hand with a mini-split, but it's a big enough load to matter. In a vague sense an inverter drive on a mini-split could be viewed as really big high frequency dimmer.

  10. Clay Whitenack | | #10

    Thanks Dana. If it were a ground/neutral issue somewhere in the house, could I flip the all the breakers until the noise goes away?

  11. Walter Ahlgrim | | #11

    Before you go into the rabbit hole of the ground loops, eddy currents and harmonics.

    Did you try replacing the circuit breakers with a different set?

    I would next try to not have other wires closely paralleling (6 inches) the mini splits wires for any long distance.(3 feet)


  12. Clay Whitenack | | #12

    Thanks Walter. No, I haven't tried that yet, but my builder suggested that as a possibility. I will try that tonight. Would you assume that I need to try the both the breaker at the panel box and the breaker at the exterior sub panel at the outdoor unit.

  13. Charlie Sullivan | | #13

    Clay, you said, "My builder completely disconnected the ground from the outdoor unit, wondering if maybe the noise was travelling through the ground. The noise continues, but that doesn't really prove anything, correct?"

    Actually that does prove something: it proves that your builder is reckless and can't be trusted with electrical systems. Grounds are there for safety, and operating without them, especially when you suspect something might wrong with the wiring, creates a shock hazard.

    Similarly, a ground rod that is only in dry gravel is unlikely to be providing an adequate ground for safety purposes.

  14. Clay Whitenack | | #14

    Thanks Charlie. He did that just temporarily to see if the noise stopped, then connected it back.

  15. Clay Whitenack | | #15

    Just to update this issue.

    Had the HVAC guy and a different electrician check the wiring of the heat pump, and also wire the 2nd outdoor unit (there will be three total). They says the heat pump is wired correctly (for what that's worth). It could be that he is confirming the mistake, thinking that the mistake is the correct way to wire. Regardless, the 2nd heat pump produces the same noise in the fans that the first unit did, so it shouldn't be an issue with the heat pump itself.

    His idea was there was a wiring mistake in the fans. He mentioned something about the control wires showing a current on them when the fan was on. He was not thinking this should be the case, but maybe he doesn't understand the way these fans work. He didn't have time to pull the fans out and check the wiring this weekend, so the builder and I pulled a fan down and checked the wiring this morning. From what we can tell, the fans look to be wired correctly.

    We've pulled the breakers out and tried different ones. We thought maybe the arc fault breaker was causing some troubles so we tried using a regular breaker. We have run new grounding rods at hte breaker panels, making sure they are driven into the soil and not the dry gravel.

    Still can't seem to find the trouble.

  16. Clay Whitenack | | #16

    Hey folks, just to update...

    I took one of the fans down and checked the wiring. Everything looks to be wired correctly. Then, I connected the fan to a plug and plugged it into an outlet. The noise occurs there too. One thing I have been able to verify is that it isn't specific to the heat pumps. Any sort of load on the house causes the noise. For example, when the air compressor pump kicks on, the noise gets louder. I would imagine all sorts of appliances like the oven, water heater, clothes dryer, etc., will have a similar affect. I have attached a link to the sound below, if anyone is interested. Can someone tell me if this is just the way they are supposed to sound?

  17. Joshua Van Tol | | #17

    It seems to me that possibly the building isn't well grounded. Perhaps the ground rod wire is broken or disconnected?

  18. Charlie Sullivan | | #18

    I listened to the sound on your video. It's strange--it sounds like a small engine idling.

    I'm not sure what type of compressor you are talking about causing it, but an easy troubleshooting step to take would be to try it with a simple heater load, like the oven, which draws current with a simple waveform, exactly the same as the line voltage.

    But I come back to my previous suggestion, to get someone who can put a meter or scope on your voltage and see what the waveform and harmonics look like. Where are you located, in case a reader here knows someone near you to recommend you contact?

  19. Clay Whitenack | | #19

    Thanks Charlie. That "idle" ramps up when the fan is turned on, then drops again when the fan is turned off. I recorded another video with that, below. I have a friend of a friend who has an electrical background and has heard about my issue and is curious to find the answer. I believe he will have the equipment that you recommend. I'm just trying to gather as much information at this point so some of the early troubleshooting is already done. Thanks so much for your insight.

  20. John Semmelhack | | #20

    There should be zero sound from the exhaust fan when the exhaust fan is off.

    Is the exhaust fan the only device that is exhibiting this kind of noise? If so, I would suggest the simplest explanation: there's something wrong with the exhaust fan itself. Unplug the exhaust fan and take it to a neighbor's house and plug it in there to test.

  21. Clay Whitenack | | #21

    Thanks John. There are three identical fans at the house and they all emit the same noise. I was assuming that meant that it wasn't an issue with the fans, but I guess I could there could have been a bad batch of fans all with the same faulty part. I will take the fan offsite and try other locations.

  22. Charlie Sullivan | | #22

    Consistent with John's comment, the new sound example makes me think it's a fluke of those fans, rather than a problem with your power. Some power supplies will pulse when there's no load on them. We might be hearing something vibrating with each pulse. It very well could be a manufacturing defect that makes them pulse differently, or make more noise when they pulse.

    You also say there's a sound at the electrical panel, but perhaps that's a more typical electrical hum.

  23. Clay Whitenack | | #23

    Hi all, just another update. I took the fan to our current home and plugged it in. It makes a very slight noise, but you can only hear it when you put your ear right up on the electrical board. The only thing I couldn't test was putting a big load on the house, like a heat pump, since this current house has no central air.

    I called Panasonic Tech support and they echoed Charlie's suggestion of putting a voltage meter on the line and seeing what it does when the heat pumps are turned on.

  24. Clay Whitenack | | #24

    Let me ask you to listen to one more sound. This is a video of the outdoor unit. It is making the same sound, but louder. When the AC is first turned on, the indoor unit blows for a bit, then the outdoor unit fan blows for a bit. During this time, the noise is not heard. However, after a few minutes of the outdoor unit fan spinning, the noise starts. This seems to be the source of the sound, is this what outdoor units should sound like?

  25. Clay Whitenack | | #25

    Sorry to bump, but this is driving me nuts. Can anyone go to the link below (this is a better recording of the sound). You can obviously hear the main fan noise, but if you turn the volume up you can hear the high pitched buzz of that same noise that is coming from the fans. Is this normal heat pump sound? I need to know if I should contact Fujitsu to get their help.

  26. Jon R | | #26

    So you still haven't measured harmonics on the line or tried a load reactor to filter them out?

  27. John Semmelhack | | #27

    Sounds pretty typical for when the compressor is ramping up to high speeds.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |