Forced Air Heating - Temperature Balance and Noise Control - AVS Forum
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post #1 of 2 Old 11-18-2011, 03:41 AM - Thread Starter
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Hi all, I am currently seeking out ideas to control noise through the air ducts of my house.

It's an older two-storey home which was split into two units. The walls and floors are insulated reasonably well, however 90% of the floor spaces are linoleum or laminate. The building was divided after the ductwork for the natural gas furnace was installed, so the heating system operates as a single unit - there are no "zoned" elements. As a result of all of this, the level of sound that is piped between the two apartments is ridiculous! You can't usually make out what's being said due to distortion, but someone talking at a "normal" volume on the 2nd floor sounds like they're right beside your head in the basement - and as the tenants are frequently students, the volume is often above "normal".

I have looked at a few products that could help - the most popular being a piece of ducting with a motorized louvre (aimed more at creating separate heating/cooling zones), and fibrous "bags", which replace a section of ducting to provide vibration decoupling and a bit of sound absorption. The main problem with these items is that they would require removing sections of wallboard, and as the basement is finished, one can only guess where the ducts have been routed.

Other alternatives include installing carpeting (a maintenance nightmare from a landlord's perspective), or generating some white noise by either running the furnace fan continuously or piping a noise generator into the duct. I have also seen designs for boxes that you can mount over the registers, which disrupt the sound waves (and the flow of air...) by obfuscating their paths. While that might be feasible and affordable, the building's heat distribution is not very good as is, and success would depend on the diffusers not being messed with. Lastly, sound absorbing materials like DynaMat might help, but they're usually quite expensive.


When I was looking through some store flyers this week, I came across something called the Vent Miser*, built by Springfield Precision Instruments on sale for $7. I went and checked it out, and it's a motorized register/grill/vent cover which has a removable circuit box... The box is powered by 2 AAA batteries, and has a 5+1 programmable timer in it which can open/close the vents up to 2 times per day. It also has a low battery alarm. I had never heard of this type of device before, so I did a bit of googling and also found the Auto Airflow Register*, which is similar but operates via a thermostat and a 120VAC-24VAC transformer, which they recommend wiring together using telephone cable. *I'll refer to these systems as VM and AAR, respectively. Of course, both systems would have to be hacked a bit to suit my purposes. Here are some currently observed pros and cons for each system:

VM Pros:
-Available locally, known price
-DC motor means an old AC-DC wall-wart can be used to power the system
-The snap-in control module simply contacts the positive and negative terminals for the register's motor - so building a different control system would not be that difficult

VM Cons:
-The build quality is questionable at best (witness the $7 price tag)
-Presumably designed to run 4 times a day, and I would guess it would get actuated 76 times per day (if furnace runs ~2 times per hour = 48 openings + 48 closings per day) - so I would not be surprised if it suffered premature failure.
-The louvres are a typical 2-blade design, and are obviously meant to diminish airflow, not to provide a "seal". This aspect should be easy to improve with the addition of insulating material.
-The control unit blocks roughly 1/5 of the vent opening as compared to a standard register, reducing airflow even when open


AR pros:
-3 year warranty
-Presumably intended for more frequent use (as system will actuate whenever temperature fluctuates)
-Appears to use a cylindrical AC stepper motor centered below the grill opening, should not restrict airflow much
-No batteries to change
-Little modification needed - just mount the thermostat on/inside the duct near the furnace, set the temperature to ~24C, and switch it to "Cool" - whenever the furnace comes on, the thermostat will activate (to let "cool" air in)

AR cons:
-The AAR site does not list any retailers, and none come up in a search
-Wiring all units together is required
-Unknown louvre mechanism (appears that it could be a "sliding grid", where a piece with identical holes moves sideways a minimal distance to block all openings, which might block more sound, but cannot be further insulated and is more restrictive of airflow)
-Unknown availability, build quality, suitability, and price


So, could automated registers be the answer here? By preventing most of the sound from entering the duct in the first place, they would hopefully reduce its propagation quite a bit. As it stands, my mental image of the desired system would be simply to have all registers closing once the furnace shuts off, and all registers open once the furnace turns on. Ironically, this could easily be accomplished with a passive system, using vent registers that spring closed when air isn't flowing through them. Why don't these seem to exist? Any input would be welcomed. Cheers!
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post #2 of 2 Old 11-23-2011, 08:14 PM
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I don't think the dampers are going to do enough for you, I have lived in a few apartments that were in houses when I was younger. They all suffered from the same problem, noise tunneling through ductwork. It sucks when you fart and your neighbor hear's you and smells you lol.

Currently we own a revenue property and I pulled the HVAC system, it was due for replacement however after considerable thought ended up doing electric baseboards. We used Roxul between floors and interior walls, between that and no ductwork you can play music as loud as you want and as long as something is on downstairs you wouldn't know anything was on upstairs.

Take something and cover the vents and see if you get the results you are looking for, if you do take it to the next step you want something hard wired to keep maintenance to a minimum.

Good luck
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