I have had a 120V 80 gallon Rheem ProTerra Performance Platinum heat pump hot water heater sitting uninstalled in my garage for a while now but I finally finished the connection and conducted sound tests on it. I am a noise and vibration consultant, so I have a Type 1 sound level meter. I wanted to report back what I did and what I found.

Test Conditions
I did not run a before and after test to quantify the benefit of my noise reduction solution. The measurement is at 3-ft just off axis from the fan to reduce wind on the microphone. The HW heater is sitting in my two-car garage with an average ceiling height around 10-ft, smaller spaces will see higher sound levels.

Noise Reduction
Originally, I had planned to put this in a closet off a basement bedroom and was going to employ every trick I knew. But upon receiving this very heavy thing and having my installation assistance/father point out we could not do that and put it in the garage I abandoned all but the easiest noise control plans and only added absorption inside the compressor cavity. But for educational purposes here’s what I was going to do.

There are three techniques you can apply to reduce noise:
(1) choose quieter equipment. Rheem offers a quieter fan. Start here. This thing generates noise from the fan and the compressor so that’s 1 out of 2 sources right off the bat. They also provide a “vibration kit” consisting of what appearsto be Mason ND rubber mounts to set the entire unit on. You don’t need this unless you’re installing above grade and if you’re in a wood frame structure these may not help very much. But I’m me so I’d still add them or go for a cheaper rubber waffle pad like a Mason Super W (not as effective but cheaper and easier to source). The biggest thing you can do for vibration is increase structural rigidity. That will get you more mileage out of your isolators. Add bracing in your joists. Create a sub-floor buildup with layers of 3/4″ plywood or 2x4s screwed into multiple joists. Rigid, rigid, rigid. In commercial applications we’ll use spring mounts to account for the floppy floor but this is unapproachable for a residential installation.
(2) attenuate the noise sources: radiated, inlet, and outlet.
This is about managing the energy that is present: block and absorb.  To address the inlet and outlet you have to add ductwork with an absorptive lining (fiberglass) so the sound arriving at the end of the run is reduced. You will also need to purchase Rheem’s duct adapters. I spent time scratching my head on the two seemingly identically described versions of this HW heater offered by Home Depot and discovered the other one has screw holes to direct mount ductwork to it so you don’t have to buy the adapters. You can see them in the pictures. Anyway. My plan was to duct the inlet to one room and the supply in another to split the energy. Since internally lined sheet metal ductwork (the right thing to use) is hard to get a hold of, I planned on using flex duct which both absorbs sound with the fiberglass lining but also dumps sound out into the room as it is mostly acoustically transparent. In my application I was going to seal up the closet air-tight and use that envelope to contain the sound so the breakout from the duct wouldn’t matter as much. The absorption and the dumping of sound into the closet would result in a substantial attenuation by the time the duct reaches the room. There is a limitation that sound in the closet can break back into the flex and head out through the hole in the wall to the occupied room but I was working with what was available.

As for radiated noise, this is where sound blocking comes in. This is the sound primarily coming from the compressor. This is where people start talking about compressor wraps but meh. The availble wraps out there are basically fiberglass blankets which don’t block anything, certainly not the 63Hz compressor tone. We also see these commercially with a proper insulation plus mass loaded vinyl jacket and the benefits are still pretty limited. So for my closet installation I was going to “jacket” the metal casing and line the interior of the compressor cabinet with adhesive mass loaded vinyl (frequently used in cars). This will reflect sound back into the cabinet instead of letting it pass through the sheet metal. On top of the MLV on the inner side you can then add absorption to absorb the sound energy in the cabinet. This reduces the amount of sound – including the inlet side of the fan – that then gets pushed down the ducts.  Managing where the energy goes essentially. Keep it in the cabinet long enough to dissipate it with absorption. Since I installed in the garage absorption is the only thing I did on my unit. Easy and cheap. For absorption I cut neoprene pipe wrap lengthwise to create half cylinders then placed them vertically covering the entire inside of the compressor cabinet. The pipe wrap has adhesive on one side of each cylinder so installation is simple. I also did my best to keep clear of the compressor to avoid any risk of trapping heat, a risk that a compressor jacket presents and I didn’t want to come anywhere near that. Fiberglass is a more typical absorber but at the compressor frequencies (63Hz and 125Hz) neoprene behaves similarly so ultimately, you’re looking for thickness to get down to lower frequencies. The cylinder shape of the pipe wrap creates an air cavity adding thickness. And most importantly: neoprene pipe wrap is very easy to get. 

Results

At 3-ft from the fan my unit measured 50dBA. The three spikes on the left side pf the graph represent the fundamental and harmonic tones generated by the compressor and the blade pass frequency for the fan (probably the 125Hz). 

Conclusions
Noise levels in my garage are comfortable. 50dBA is low for a garage, high for a bedroom but in that application, there would have been some intervening assemblies. The WHO (not the band) recommends 30dBA in a bedroom which is rather low (for other people – not me – I like to sleep in a tomb). Most folks would likely consider 35dBA or 40dBA comfortable and if you’re sleeping with a consumer white noise machine it’s safe to say you’re over 50dBA- not good for your cortisol levels btw. Your apple watch provides a pretty accurate dBA reading FYI.

In my garage the HW heater is right next to a washout sink with the fan pointed at your head. At full speed with only the absorption in the cabinet I can still easily hold a conversation at that sink while getting a nice cool air conditioning, an unexpected benefit – this thing can actually drop the temperature in my garage by 5 degrees when it runs.  My guess is if I had installed this in the closet off the bedroom as planned and installed the noise control as planned, disturbance to the room would be minimal. I also expect that the proposed noise control would have substantial effects on the high frequencies and very little effect on the low. To address low frequency sound you pretty much need to select different equipment and focus on the sound blocking performance of the architectural enclosure. I’m guessing the 125Hz tone is the fan. Changing to Rheem’s low noise fan would probably render a noticeable reduction there. Usually low noise fans shift the tone down and reduce the higher frequency “white noise” component which is the stuff people notice. Adding MLV and absorption to the cabinet may affect those lower spikes from the compressor by a few dB but the cost per dB starts to climb here. On a dollars per dB basis MLV is expensive. Pipe insulation is cheap so I added the absorption and would every time but that’s probably not doing a thing for 63Hz. That stuff leads you back to your room construction. Make sure it’s well sealed (duct the unit out of the room so you can seal it up) and the constructions are dense: e.g., open walls can get insulation and a layer of gypsum closing them up and doors should be solid core and have neoprene/rubber full perimeter seals. In commercial construction we use commercial construction materials to reduce noise. While physics is magical there is no magic in physics. Density, damped air cavities, and air-tight seals. In the construction world that’s gypsum, fiberglass batt, and caulking.

Good luck!