Timpani drum noise from rooftop make up air unit

Redhead865

A replacement rooftop make up air unit was installed in our condo in Spring of 2021. This is the Chicago area. Soon after a noise that sounded like a recurring timpani drum strike began. It only affected the two units directly under the installation. The duration of the noise was 2-5 seconds followed by a quiet interval of 10-20 seconds. This lasted for several weeks until the weather stayed consistently warm. In the Fall the same cycle repeated itself stopping when the weather stayed consistently below 50 degrees. Recently we had a 65 f degree day in mid December. The noised reoccurred for one day and stopped even tough there were a couple of days in the 50’s. The installer and manufacturer say nothing is wrong, but it always takes them several weeks to come check out the noise and by then it is gone. Any ideas of what may be causing this? Is there an ambient temperature control on MAUss that both heat and cool outside air?

Matt_67

A common way to modulate cooling systems is to “unload” a compressor. Basically the motor keeps running but it no longer compresses. The noise from the system changes when this happens. So if the compressor is at 40% capacity it would run loaded for 8 seconds and unloaded for 12. It’s normal operation.

EBEBRATT-Ed

@Redhead865

are these units heating and ventilation only or do they cool as well?

Redhead865

These units cool as well. We have two buildings. New rooftop units on both. The other building does not have this problem. Temperature seems to be a factor. The biggest thing is it is over our bedrooms and we can't sleep with the recurring noise. Is there a fix?

Redhead865

Does anyone know if vibration dampers would help and are they difficult to install?

Jamie Hall

Vibration dampers and sound suppressors can help, but if they aren't part of the original install it is very difficult to retrofit them, and often requires modifications to the rooftop unit itself (heavier, stiffer base) as well as to the roof structure. as well as a different mounting system for the rooftip unit.

Redhead865

What would be the best type of expert to consult about whether noise dampers would help?

Jamie Hall

Oh boy. That is a very specialised field. Some architects might have an expert on staff, or know one. A few mechanical engineers might.

You might also contact the manufacturer of the unit for assistance.

wmgeorge

Are the MUA units in the Cooling mode when this occurs? If so the comment about compressor unloading and loading is valid. If you can safely access the roof, you could make a recording at the unit with your smartphone. Otherwise my guess its a rattle where something is touching intermittently.

HVACNUT

How many thermostats on this RTU? 
Is it ducted with VRV boxes?
Is there an economizer with motorized dampers?
Were any modifications done to the roof curb to make the new unit fit?
Is the other building and unit identical?

Daveinscranton

Sounds like a resonance problem.  Frequency of the unit is matching the resonance in the underpinnings.  Like the roof.  Which may be temperature dependent.  If I owned the property and the problem, I might experiment a bit.  Doesn’t sound like you are in that position.  Maybe the best you can do is document the noise.  And hope that someone sympathetic will call the right person.  Could be easy or complicated to resolve.  

Best wishes 

wmgeorge

Since this RTU is also a MUA unit and its drawing in lots of outside air at times, could be a Relief Damper bouncing if its just based on static pressure (with a spring) and not powered with a damper motor. That recording would help, I am hoping you have a inside roof hatch or stairway to the roof,... if not do not attempt!

Redhead865

Thank you all for your input. It will give me alot I can discuss with the technician who will come out the next time the noise begins. That probably won't happen until the temp hits 65.