RTU noise in new loft construction

[gt9606a]

Hello all! I'm enjoying reading the posts in this forum.
With all the expertise out there I thought I'd
bring up a problem I'm having and see what happens.

I (my contractor) just completed work on a new 2800 sq.ft
loft residence for me and my family. The contractor
mainly does commecial build-outs and treated this
much like a commercial job.

In order to fit the HVAC system into the space,
a 5 ton Trane RTU unit was selected. The ductwork
runs directly down from the curb and T's towards
the front and back of the living space with take-offs
towards each side as needed.

The system seems to be cooling well enough,
although we have not be through a period of
our extended 90 degree days yet. I'm afraid
it may not ever shut off during these times!

The problem I am having is with noise. The return
air vent is pulling hard enough to bow the filters,
and makes a loud hissing noise that can be heard
pretty well throughout the space.

Also there is a deep low frequency thrumming noise
that can be heard more in some areas and less in others.

Is there any way to silence the air intake?
What could be the causes of the low frequency noise?

Thanks for the advice.

Mark

[bradgreen80]

The noise that can be heard in the space is most likely from how the ductwork is run. The frequency waves carry directly down through the “T” in the supply directly into the space. To verify that see if it is louder under the “T” in the supply. If that is the cause you can lessen that by using a high density material to absorb the low frequency waves.

[joegrind]

it sounds like the ductwork is undersized! It is possible that he used .1 instead of .08 in his design( typical comm) . pull out the filter and see if the noise drops, pull off the r/a grill and see if it drops further.if it does, sounds like undersized return and/or supply duct. improperly sized duct could be amplifying your"low freq noise"

[tab-tech]

Is the supply and return ducts connected solid to the unit or was flex (canvas) used.
Hard ducted units can generate vibration and harmonics, and harmonics can create additional vibrations and harmonics, which can create additional vibrations and harmonics, which can..............get my drift?

How much total air are you moving?
You could try slowing the unit down a bit if you have too much.

[rubobornot]

Have the contractor contact a test and balance co. to check the air flow and total CFM from the unit. A high supply duct static pressure will cause the unit to deliver a lower CFM.To low of a supply duct static pressure could cause the return air to make noise due to a high return static,the negative side of the blower. Also ask to have a copy of the Heat gain and loss calculations and how much CFM should be delivered to each supply, this will help the test and balance co. set the air flow .

[Roscoe]

Originally posted by rubobornot
Have the contractor contact a test and balance co. to check the air flow and total CFM from the unit. A high supply duct static pressure will cause the unit to deliver a lower CFM.To low of a supply duct static pressure could cause the return air to make noise due to a high return static,the negative side of the blower. Also ask to have a copy of the Heat gain and loss calculations and how much CFM should be delivered to each supply, this will help the test and balance co. set the air flow .

I doubt the contractor is going to pay for or agree to a certified balance co.

Some commercial companies can't do residential and vice versa.

jmop

[Islander]

thrumming noise...possible warped belt.
return air grill hissing...ra duct possibly too small.

if it was me i'd look into slowing the fan speed down a little...might help both items.

[gt9606a]

Thank you all for your expertise!
I fear there is more going on here than I expected.
I hope there will be some solutions to make
things better (not perfect) without re-doing
the whole thing.

I'll try to follow up everyone here:

Islander:
The unit is not belt drive, but is it possible
the fan/motor is out of balance?
Would this cause the thrumming noise or is
it more likely to be cavitation(?) in the way
the air is flowing through the ducts?
Can the fan be slowed down?
Could an undersized ra duct cause the thrumming
as well as the hissing?

pjs:
You are probably correct about the contractor.
He always has his guy do the HVAC work and
I never saw any calculations. The contractor
is reputable and would work with me if any
glaring problems were uncovered however.

rubobornot:
Who would normally do the gain and loss calculations?
Is this something that is always done?
Can the static pressures on the supply and return
be adjusted in any way without changing the duct work?

tab-tech:
The duct work is all round rigid. Not spiral duct,
just plain metal. The duct is all running in
the "conditioned" space. The attachments to the
bottom of the curb appear to be direct with some
sort of riveted collar. The supply is a 16-18"(?) T, which
steps down in size as it runs to the front and back of
the space. The return is two 14"(?) ducts which run a short
distance in parallel to the return box which has two
filters and grilles. It makes a lot of hissing!
I'm not sure how much air I'm moving...is there
a way to tell. Islander also suggested slowing
things down. Can this be done on a direct drive unit?

joegrind:
It seems as if things might be leaning to duct
problems. I'm not sure what calculations were done. What are the two factors you gave for?
The hissing definitely goes away with
the return grilles off and filters out.
Is it possible to get a more efficient grille/filter or do I really need larger return ducts? It is now two short 14" (smaller than the supply, but two of them) sections which end in the grille box. I can't tell for sure if the low freqency noise is duct air standing waves or noise from the unit carrying through the ducts.

bradgreen80:
I'm sure the duct work could at least be better
isolated from the unit if not even better sized
or laid out. Is there an easy way to do this
after the fact?

All:

What about fan/motor balancing for the deep vibration?
What about vibration damping curbs?
Or is the noise more likely from air flow/pressure problems?

Thanks again. This is educational!

[joegrind]

It sounds like you have no "canvass connectors" at the ret and sup plenums, that would amplify the vibrations! the factors I gave are for static pressure .1 typical light comm, .08 typical residential(where I come from). basically, the lower static a system is designed = larger ducts which in turns = lower noise

[coastline]

if this is a 5-ton unit /then from the duct sizes you gave supply starts at 18" and 2-14" returns then is was sized at .1 typical for commercial use //the ductwork is exposed in the space therefore in a commercial application it would not be insulated// and you said no flex connectors between unit and ductwork //this is a complete typical commercial install all the above factors contribute to noise level

what size registers where installed and how many// do they tap right into the side of the trunk duct// and are they high flow grills (not stamped) as for as the return you said your using filter grills is there a reason the filters cant be installed inside the unit trane does offer filter racks for thier equipment (typ commercial) if they can you probaly could change to a eggcrate style return grill with more open surface area on the return reducing noise but if the return is straight thru the roofline with less that 10' or so of ductwork it may not make a big differance//

to help eliminate noise the first 10' of supply air duct should have been lined and possibly used an isolation curb or at least flex connectors between unit and ductwork to help reduce mechanical noise tranfer at this point there may not be alot you can do whithout changing some of the ductwork

this is why there are two sides to the bussiness (commercial and residential)

the fan speed in the unit may be able to be slowed depending on its currnt setting most units have a multi-speed motor in them but you need to verify that you are going to have enough airflow across coil(contractor will know this)to prevent other issues.

as far as the calcs the contractor should have done heat load calcs on space prior to install/ whithout those you would guessing at the correct size unit.

[gt9606a]

Once again thank you to everyone.
I think from what I've learned,
the system could be made better without
starting over!

Coastline:
I'm glad to know that the install sounds
like a reasonable commercial job.

As to your questions:

1. There are 3 completely finished rooms
that have 6" round insulated runouts with
what look like standard rectangular registers in the ceiling.
There are 10 other 6" runouts that end near
the perimeter walls (and windows) with round diffusers.
Would the registers/diffusers make any difference?

2. The filter could go in the rtu. A filter
rack is offered. Would it make a difference in sound?
The ra ducts are about six feet long with a 90deg. bend
up into the curb.

3. It would be fairly difficult to rework the supply at this point since it runs over finished space with little
clearance. Possibly a flexible section could be squeezed in. Is the isolation curb something that can be retrofit onto the existing curb?? Or would curb have to be completely replaced? The reason I ask is that a new flat tar/gravel roof was put in after all the roof penetrations were done and I'd hate to screw it up since it doesn't leak.

I'm sure the contractor did some calculations because
he originally wanted to use a larger unit until I told
him we were replacing all the single glazed aluminum commercial windows with the double pane low e clad variety,
with awnings on the west and whatever else we could do.

You are all great. I'm glad this forum exists.

[coastline]

gt9606a

are you sure you have a total of 13 6" round supply air ducts on the entire system// they might be 7" (hope)

a 5-ton unit is designed to provide approx. 2000 cfm of airflow // make sure the supply duct coming of the plenum (unit)is at least 18" round duct .1 static

if you only have 13 6" round supply ducts /that comes out to .2 static at the grills not accounting for any register face restrictions// in other words you would be pushing the airflow out of the registers at approx. 750 feet per minute that would be loud (kinda like having a airport in the distance) typ. design in light commercial (small package units like yours) is approx. .1 or approx. 600-700 FPM based on register design but we use very high flow regsisters with virt. no face restriction /and typ have higher cielings (i dont know what your cieling hight is higher that 8' i hope) not positive but i believe standard res. is factord around 400-500 FPM at register face\

as far as return goes if the 6' of return duct is lined (insulated) moving the filers into the unit may help some but probably will not eliminate return air noise. do you know if the contractor is pulling outside air into the unit and how much// if you have filter grilles (as discussed earlier) try removing the grill face and see if it make a differance in return noise level i would assume that each on is approx. 2'x2'