Rooftop curb noise and vibration

[chris989]

Hello,

I recently had two new trane RTUs installed. I went with curbs and down draft install instead of the original design of stands with side draft attached to a 90 elbow.
I now have a constant vibration that i can hear through the entire house. 90 feet away in the laundry room i can still hear the unit over the bedrooms.
The contractor has been back to look at it an has no solution. I feel the new curb is touching the existing metal from the original rooftop flashing and making the sound??

Does anyone have advise or personal experience with curbs vs stands?
Should I have felt or some membrane between the curb and roof?
Should i have sound deadening material in the partition of the curb that is unused.
I do not have much faith in the contractor, They had all of the right answers before the install and now only point the finger to someone or something else.

All help would be greatly appreciated.

Chris

[luskys a/c]

Chris - It sounds like its not secured to the curb or metal to metal vibrations. There should be some type of vibration dampeners for your unit.

[Chris_Worthington]

2nd opinion needed.

Could be as simple as a blower over sized/sped up to the point of cavitation causing the noise?

Cant see via the web :whistle:

[chris989]

I do not feel it is the air flow, the unit is already lowered 50 cfm per ton by use of an enhanced start feature. The unit make the noise when the fan is running at 50% in the start up cycle.
My hunch is that it is the metal of the existing roof flange touching the new curb or the curb in general magnifies the vibration like a tin can held to a wall to listen.

[Chris_Worthington]

Curb installs are way to many for all to request a stand mount due to noise or any other issue.

Generally speaking there is nothing more them 1/4 foam tape installed between the 2 surfaces.

Somebody is missing something obvious.

Just my 2 cents :D

[genduct]

A second opinion from the manufacturer or their local distributor might be a good way to go.

[chris989]

I did speak to the manufacturer rep and I was told the curbs can be louder because of the flange directly on the felt paper and not on top of multiple layers of shingles. Also that the downshot can add to some noise. A second contractor has looked at the situation and also confirms that the downshot and short return and supply ducts can enhance noise. When i spoke to the county inspector he also said curbs are often louder than stands.
It is more the acoustics than overall noise. I can hear it better 50 feet away than under it. The fan noise lessens as i move away but the vibration hum is constant. I think my entire attic is now a speaker?
I Think I will pay to reinstall back to a stand??

Thank you,
Chris

[Hunter844]

Probably need to look at decoupling the noise in the duct work. Just a quick googling...

http://www.acousticalsurfaces.com/hvac/duct_silen.htm?d=26

[ACFIXR]

What to look for

1. Is the bottom of the unit touching the edge of the curb? You should be able to have your finger tips fit into the space between the unit and curb, if not then the vibration of the unit can transfer to the curb.

2. Is there a gasket between the bottom of the curb and unit?

3. Is there a flex connector between the unit duct opening and house duct work?

4. Did your installers use hard duct or flex between the unit and house duct? Flex is a better sound and vibration absorber. A flex with a 90 degree turn in it will eliminate most noise's.

5. Are the electrical and gas connections too the unit hard or flexible?

6. Is your duct work small and the high static making the blower work too hard?

just a few things to look at.

[acwizard]

Chris, What do you mean by the new curb touching original roof flashing. Do you have a picture of installation. Curbs sometimes cannot span the roofing rafters correctly to distribute the weight. Sounds likes this is a slope roof deck. A carpenter may be needed to solidify structure from beneath if possible. This will deaden sound transmission. Noise transmission thru ductwork is very common as well. Insulating the interior of the curb also is a very effective way to reduce sound.

[chris989]

I do not have a picture, the units are in place and covering this flange.
The original install was a side shot into a flex connector and then a 90 elbow. The installer removed the 90 and placed the curb over the existing supply and return that was protruding through the roof. The flashing of the curb is sitting on the original flashing from this duct. The installers then used duct board and mastic to seal the two together prior to setting the unit. I asked about sound deadening material for the portion of the curb that is not in use and I was told it was unnecessary. I guess they were wrong because i feel that is were the noise emanates from.

Next week i will lift the unit, inspect and reinstall either the curb with better sound absorption or use a stand.

Thank you to everyone for the help,
Chris

[sammy37]

Seems like everyone nowadays is quick to jump on curbs and downshot for residential.
If the elbow would have been left up there, then all the contractor would have had to do is change the stand and maybe have a small transition to attach the new unit to the existing elbow.
Now when these units need to be replaced, chances are the new unit will not adapt to that curb and it will be a nightmare changing that whole mess out.
Chances are, the contractor used one of those cheap adjustable roof curbs and set the unit directly on top of it without anything to absorb the vibration.

[acwizard]

Chris, Was this changeout permitted, and did it pass inspection.

[chris989]

the change out was permitted but they went to the city and not the county so it was voided. I am still waiting for the permit and inspection from the county. I inspected it myself and already told the company what would not pass. (incorrect amp for breakers, knock out hole taped over) They sent a man out to correct this but he fixed only one unit. I still have a 3 ton rated for 40 amp using a 60 amp breaker and they did not replace the 25 year old liquidtight from the roof to the disconnect which is cracked and exposed. ( 5 feet max )

This leads me to a second issue, I now have my low voltage thermostat wire spliced together inside a line voltage disconnect. I can not find a rating on this wire. I know it can run in the same chase but now it is stripped and wire nutted to extend its length. I phoned the county and they said no to the splicing because the wire is not rated to the higher voltage once the outer sheathing is removed.

I feel anger and disgusted when i see how sloppy the job is. I hired a licensed contractor and paid a premium in price. I think most of the reviews on the net for the company must be false and written by them.. Another lesson learned.

Chris

[chris989]

It is an adjustable curb, and they used a 3/8 x1? foam tape.
the two adjustable portions of the curb (to correct pitch) are fastened with sheet metal screws. I can tap on the sides and make the two pieces resonate. Did i get the cheapest of cheap? Do companies make or offer the correct pitch curbs if ordered in advance. My roof is a 5/12, nothing crazy. I would have easily paid for and waited a week if it needed to be made if this would eliminate the two separate pieces from producing noise. OR is there some type of rubber skirt that should have been installed between the two pieces??

Thank you,
Chris

[Hunter844]

Yeah there are companies that specialize in pitched roof curbs. Some are cookie cutter adjustable and others are custom built for the exact equipment and pitch requirements often with a 10'' rise or higher. Both tend to work if they are installed right and use the proper gauge of metal.

These same companies also make curb adapters for the very reason mentioned above and track old equipment specs so even if the equipment is 20 years old they'll still have accurate information that can be used to make an adapter.

I have a hard time believing a "cheap curb" is the problem...it's more likely it just wasn't installed as well as it could have been or there are other problems (Structural?) perhaps the new unit is a lot heavier than the old?

[ACFIXR]

It is an adjustable curb, and they used a 3/8 x1? foam tape.
the two adjustable portions of the curb (to correct pitch) are fastened with sheet metal screws. I can tap on the sides and make the two pieces resonate. Did i get the cheapest of cheap? Do companies make or offer the correct pitch curbs if ordered in advance. My roof is a 5/12, nothing crazy. I would have easily paid for and waited a week if it needed to be made if this would eliminate the two separate pieces from producing noise. OR is there some type of rubber skirt that should have been installed between the two pieces??

Thank you,
Chris

No....that is a standard type of curb and the gage of metal is more than likely 16 or 18 gage......5/12 pitch on your roof? That requires a 30" wide platform in front of service areas.

[acwizard]

Chris, I have seen plenty of these style curbs and yes i have used them. I prefer welding our own frames . The curb can be fixed and your sound problems can be solved. You may need to find yourself a more qualified contractor to make these repairs. The most important question about your installation is do you have any roof leaks.

[khc95]

Hey Hunter that duct silencer is cool !!! Have you had experience with these?
Does it affect the static or velocity of the air flow or is there away around the effects.

[joey521]

Hope this is not in violation of rules but was this issue ever resolved? I have the same problem

thanks

[Hunter844]

Hey Hunter that duct silencer is cool !!! Have you had experience with these?
Does it affect the static or velocity of the air flow or is there away around the effects.


No I haven't used them...but they do seem pretty nifty.