Noisy Aaon Rooftop Burners

[ktr52]

I have a 26 ton Aaon rooftop unit that is 3 years old and has since installation made excessive burner noise when the burners initially come on. The burners come on full for the initial 45 second start up and then modulate down. Nothing has been modified on the unit, the gas pressure, orifices, alignment and blowers are all checked out. The unit sounds like a jet engine on the roof. Aaon says its normal.
Any help would be appreciated.

[2sac]

2sac says it's normal as well

[tech45]

And if you ever find a failed inducer motor on these get a new fan wheel with the motor.

[ktr52]

That would be OK if below the units was a warehouse but its classrooms and they refuse to use the space due to the jet engine noise.
Has anyone had experience in replacing the burners with quieter ones?

[Sicofthis]

I doubt replacing the burners is the way to go. Maybe some changes to the duct work? Where is the sound entering the space?

[jtrammel]

Use some flex with a couple 90 degree bends to absorb the sound if possible. Is the gas pressure too high, I've seen some that light with too much pressure and sound like a jet engine.

[HvAckid82]

This is normal, is it a drop ceiling? If so install batts of insulation

[Jarreau]

This is not an equipment issue as much as it is a building design issue. Architects are using lighter and lighter construction and HVAC designers, in many cases are, not able to place units away from these sensitive areas. This leads to the end result to be exactly what you are complaining about. If it is not burners, then it is compressors or fans, does not matter which brand of equipment is on the roof. To kill sound transmission the structure has to have mass. Unfortunately the perception is that Acoustical treatment costs too much until the owner has a space or building that is uncomfortable or unusable.

J

[ktr52]

Use some flex with a couple 90 degree bends to absorb the sound if possible. Is the gas pressure too high, I've seen some that light with too much pressure and sound like a jet engine.

The gas pressure is right at spec.
thanks

[ktr52]

Thanks for the reply.
Yes, it is a drop ceiling, have you tried to spray foam the botton of the roof decking instead of the batt insulation?

[ktr52]

You maybe right but something needs to be done the space below the unit is unusable. They have a 90 deg duct attenuator to help cancel out the noise but dosen't look like it works too well. Your thinking that the noise is not coming through the ducts but transmitted through the metal roof decking.

[Betabass]

I had some issues with noisey condenser that sat on sleepers that we installed heavy duty vibration springs under the unit to lessen the sound issues. I am not sure if the AAon RTU Curb has options that may include some sort of sound elimination design.

[Jarreau]

You maybe right but something needs to be done the space below the unit is unusable. They have a 90 deg duct attenuator to help cancel out the noise but dosen't look like it works too well. Your thinking that the noise is not coming through the ducts but transmitted through the metal roof decking.

That is what I am thinking. The solutions while a pain in the a$$ may get you a usable space. Lift the unit off the curb and put 2 layers of 5/8" gypsum board wall to wall in the curb with a coating of the Green Glue product in between. Be sure to lap the joints and seal the edges. You do not want a pathway for noise. Secondly add a similar solution underneath the roof deck and add some 6" batts on the top of the gyp panels for sound absorption. Make this as large as possible under the unit hopefully 3 - 5' beyond the unit's footprint. I know easier said than done.

[firecontrol]

Inshot burners = small and basically crude jet engines when it comes to the sound they produce, especially when directed down a round tube.

Tubular heat exchangers made of as light of material as possible to get the best efficiency, but yet heavy enough to not fail within warranty period.

Complaints of cool/cold air blowing out of supplies on start-up. Solution: Fire burners at max. at start-up with blower delayed and/or running slowly to pre-warm air.

Complaints of noise at start-up. Help from anyone at the co. or the engineer that spec'd it with a solution: That's just the way it is. Sucks to be you that you're the lowest person on the food chain and now have to come up with an after-the-fact solution. The manufacturer sold their "best thing since sliced bread" to the engineers that specified the equipment but didn't understand it's non-documented characteristics, but it looked good on paper. And then the famous "you must've installed it incorrectly".

Real world is to know what you're dealing with and how to guarantee a solution. IE: What frequency is the offending noise. How is it being transmitted. What are the documented ways of attenuating this frequency. How to "fit" this now known and documented solution into the situation you're dealing with.

Aaron probably is not going to be much help because they are only responsible for the equipment and nothing beyond the base or duct connections from it. What they MAY be able to supply you, if they're willing to share the information or not, is the frequency that their engineers and designers "know" is being produced by the burner/heat exchanger arrangement. If you can't get this info from them then find someone in the audio world that can come measure the "noise" and tell you what frequency(s) is/are the problem. Make sure the teachers are there at the time so they can go "THAT'S IT!" Each frequency has a way of being attenuated. It might be as simple as lining the duct work with an acoustical liner. It may require the installation of a section of "muffler" duct work with sound trapping built into it.

Fixing things that others have decided is not their problem because they can hide behind the veil of "We only design, build or specify it" is what makes our job not only frustrating, but it also gives us the advantage of being able to brag about skills that the "others" will never appreciate or understand.

Good Luck.

[mazdajoe]

Aaon's are pretty noisy when operating normally but I have experienced the "jet engine" noise you described. Normally this is caused by the baffles in the heat exchanger falling down because of corroded or broken screws. All you need to do is remove the heat exchanger and reinstall the baffles with stainless steel screws. You might need a little bit of fire rated caulk just in case the gaskets are in bad shape. But since you said the unit has been noisy since start-up, I am not positive this is your culprit.