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Community and Q&A

noise in Fujitsu multi zone mini split

skapur | Posted in Mechanicals on

I recently had a multi-zone heat pump installed at our house 2 hours north of Toronto.

Fujitsu ASU24RLF
Fujitsu ASU12RLF1

The 24k indoor head is approximately 30 ft from the 3 ton condenser and the 12k indoor head has a loop in the refrigerant lines for a total length of 18 ft.

The issue I’m having is the 12k unit makes significant noise when in standby or even turned off. The sound varies between a loud hum to a gurgling sound. It happens when the 24k unit and condenser are running. The 24k unit doesn’t exhibit the same issue.

The installer has tried multiple things to address the noise…checked charge, pressure, isolated the line sets with rubber dampers…but the noise persists. The 12k unit is installed in a bedroom and the condenser is located outside behind the indoor unit. The condenser is on a stand and not mounted to the house.

Do you have suggestions? What else can be check or adjusted? Am I correct in expecting the 12k unit to be silent when its not operating?

Thanks for your help

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  1. Expert Member
    Akos | | #1

    Most likely what you are hearing is the refrigerant flow reversal when the unit goes into defrost. When this happens all indoor units will make some noise even if they are off. This happens with all units, there is not much you can do about it short of turning the both heads off.

    Unfortunately a bit too late for you, but this type of issue (and the fact that is way oversized for the load) is why it is not a good idea to install a wall mount in a bedroom.

  2. skapur | | #2

    Thanks...appreciate the feedback. The noise is constant (when the bedroom unit is off or in standby)...not just during defrost. I've seen/heard defrost and it only lasts for a few minutes. Agree the unit may be oversized...would that have an effect on the noise when its off or in standby?

    Also...the noise is only at the bedroom unit. The other unit is silent when not operating....seems strange that one would be quiet and the other makes a racket...

    1. Expert Member
      Akos | | #3

      Most units put some small amount of refrigerant through all the heads even when off. This generally should not be too noticeable but you can hear some water flowing sounds.

      It could also be that some contaminant got into the expansion valve of the smaller zone which is preventing it from closing all the way. You can do a quick check by running the unit on low fan only while the 24k head is delivering heat. The air from the 12k head should not be warm.

      1. skapur | | #4

        Unfortunately Fujitsu multi zone systems don't allow one unit to be in heat mode and the other in fan mode (apparently cool and fan works, but not heat and fan).

        One data point...I had the units off for a few days. When I started the system, I only turned on the larger head...kept the small head off. I then checked the smaller head and it was making the running water noise. I opened the cover on the unit and the fins/coil were warm. As if refrigerant was flowing through the indoor unit. Also, you can hear similar sounds at the outdoor unit where the refrigerant pipes connect to the outdoor unit.

        I don't know if that helps. Is there another way to check operation of the expansion valve?

        1. snugglez | | #5

          I had this exact problem with my mitsubishi multi-head unit.

          I basically suspect that these multihead units do not have any kind of valve on the outdoor branches. If it's pumping refrigerant to one head, it's pumping to all of them... The head being "on" or "off" only changes the louvers and the fans, NOT the fluid being pumped through it.

          That is the only explanation I could come up with for my bedroom units gurgling, buzzing, and heating cooling even when "off." I would turn them off at night, but leave the living room one on. When it had a call for heat, ALL THREE heads had refrigerant circulating through them. Just the LR head had the louvers open and the fan on...

          I'm sure this saves some money, but it sure was infuriating. I'm pretty surprised this is still something that isn't given more attention.

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