Mini split roof mounted condenser noise issues
Hello,
I am working on a project where the homeowner has just finished having a Fujitsu mini split system installed with the condenser mounted on top of a flat roof. The property is located in the Bay Area, climate zone 3C. He is now asking me for a solution to control the noise that can be heard inside the house. Currently the condenser sits on top of a simple 2×4 redwood frame (the 2x4s are on their side for a height of 3 ½”) that the HVAC installer built to lift it off of the roofing surface.
The roofing assembly is composed of rolled mineral surface, underlayment, 1″ of large fiber insulation (I don’t know what type it is – it’s black and a rigid sheet, looks a bit like dried ramen), and 2×6 T&G Douglass Fir decking which is finished and exposed to create the interior ceiling.
My question is what methods would be best to try and isolate the sound and vibration transmission on the outside of the building that can hold up to the elements and UV exposure? All of my preliminary research points to interior solutions which is not possible in this case due to the finished ceiling being the underside of the roof deck.
Thanks in advance for suggestions,
James
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Try a vibration pad. Preferably thick, heavy and covering the entire area under the compressor.
If you know the weight of the unit and the frequencies you need to control, you can get over the counter vibration isolators that will help a lot. Google the terms, and find something that will work. Basically, you mount the isolator to the 2x4 dunnage, and then mount the condenser unit to the isolators. Many even have threaded studs that can be fastened directly to the equipment.
James,
First step is try to get the unit away from bedrooms and close to the supporting wall.
I use an app for my phone to get a reading on the frequency you need to isolate for. For AC this would be between 6o to 100hz. This is important as isolators are only effective down a to a minimum frequency. Good start for parts:
https://www.lord.com/products-and-solutions/vibration-and-motion-control/industrial/isolators
To get better performance, it is better to install the supporting structure over something heavy like ballast pavers. Make sure your roof can handle the extra weight though.
Dealing with the noise from the inside is next to impossible at is carried by the structure. You want to prevent the noise getting into the structure in the first place.
These harmonics are normal, and may be exaggerated by the way the building materials were constructed or the materials that were selected.
For mid roof installs we use what is coined as a spider stand. Big stand with four legs with flat metal under the legs to prevent cutting the roof (and for mounting to the roof surface).
Most of these stands come with rubber isolation bushings for when the unit is screwed to the stand. This may or may not help however installing the unit on a solid surface as it has been insatlled, one that doesnt have mass (like concrete) causes the issue your witnessing.
Keep in mind, most stands prevent the unit from turning over in high wind conditions due to the wide span of the legs on the base. Split units tend to turn over when not secured.
Moe H
I have five LG - 12k Cooling + Heating - 25.5 SEER, condensers mounted on stainless wall mounts that weigh about 94 lbs, but most of the weight is biased on the compressor verses the fan. I used 27+ lbs rated gel mounts under the compressor vibration mount # V10Z61MTHTW ($80 each) and I used 18+ lb rated gel mounts for the fan side of the condenser vibration mount # V10Z61MMN10 ($61 each!), each unit will need four vibration mounts (~$300 per condenser!!!), but it works and the wife stopped complaining!!! If I did it again, I would just get all V10Z61MTHTW's with the higher weight rating..
It cost me some money, but no more vibration or sound, just the quiet whisper of the inside unit.
http://vibrationmounts.dev.inquestmarketing.com/V110/PDF/1_4_Double-Studded%20Silicone%20Gel%20Vibration%20Mounts.pdf
http://www.vibrationmounts.com
Phone: 516.328.3662
Fax: 516.328.3365
Spam