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Ducted vs. ductless minisplit and silence

Thorsten Von Eicken | Posted in Mechanicals on

I’m trying to get to the most silent A/C in a small 1200 sq ft house (old construction) and am zeroing in on a mini-split system (prob 1x 15k-18kBTU and 1x 9k). I have a 2007 vintage Mitsubishi MSZ09UN wall-mount in a studio and while it’s quiet it’s not very quiet, e.g., when sitting reading or working silently it gets to our nerves after a while. That unit is rated 22dBA on low. I’m looking for quieter…

Looking at the engineering manual for the MItsubishi slim duct units I see that at first glance they seem to be louder, for example, rated at 23 and 30dBA for the 12k vs. 15k units (SEZ-KD12NA4R1.TH and 15 variant). But then I look at the engineering manual, and I see that for wall mounted units the sound measurement is taken approx 1m down and 1m in front of the unit, i.e., in the direct air flow, while for the ducted units it’s taken 1.5m straight under the unit, i.e., away from the airflow. That makes it a bit of an apples vs oranges comparison, plus presumably the ducted unit sits on top of a soffit or ceiling.

Does anyone have experience with the noise level that can be achieved by the ducted vs ductless units?

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Replies

  1. Trevor Lambert | | #1

    In an ideal world, keep the unit as audibly far away as possible, either by distance or by separating it in a different room, sound insulated closet, etc. Then have ducts as big as possible. The smaller the duct, the noisier it will be. If you're limited in either of those parameters, then it's possible a ductless might be quieter.

    The different sound test methodology you describe may not make as much difference as you think. They are virtually the same distance (1.4m vs 1.5m). The one at 1.4m is diagonal to the air flow direction while the 1.5m is in line with it. While the sound is going to be directional, the effect will get less and less as you move away from the source. I'm not convinced you'd be able to hear a difference between 1.5m at 0deg and 1.4m at 45deg.

  2. GBA Editor
    Martin Holladay | | #2

    Trevor,
    Thanks for providing a helpful answer.

  3. Thorsten Von Eicken | | #3

    Thanks for the helpful answer! I disagree about "The different sound test methodology you describe may not make as much difference as you think", however. My description wasn't clear enough... See the attached diagram from the engineering manual which shows the location of the sound measurement for the ducted model. Note that the air intake is at the far right of the diagram and the air outlet at the far left. So the distance to the unit is approx the same compared to the wall mount but there would be the soffit or ceiling in-between, and the distance and direction to air intake and outlet are very different.

  4. Expert Member
    Dana Dorsett | | #4

    Connecting the longer duct runs to the plenum with a short section of flex, and the duct boots to the hard-piped duct with short sections of flex also helps reduce the lower frequency noise.

    Air sealing every joint & seam with duct mastic reduces the higher frequency hiss that can arise from air leaks.

    Using only radiused ells & tees (not sharp throated) also cuts down on low frequency noise from induced turbulence.

    There are also commercially available "duct silencers" to help isolate noise, if the result isn't satisfactory on the first attempt.

  5. Thorsten Von Eicken | | #5

    Dana, thanks for the tips! When you say 'short sections of flex' how short is 'short' for it to make any difference? Do 3ft or 6ft make a difference?

    Do any of you have a recommendation for good quiet air intake and exhaust grille manufacturers or model series?

  6. Expert Member
    Dana Dorsett | | #6

    There isn't much to be gained by going to 6' of flex, but doesn't hurt. Even 2' would be plenty for mechanically un-coupling the rigid plenum from the rigid duct across a range of frequencies.

  7. Thorsten Von Eicken | | #7

    Ah, it's about mechanically uncoupling, not about dampening the sound. Thanks!

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