Non Ducted Return Air to Air Handling Units 5

[solidspaces]

Achieving the Desired Return Air quantities from occupied spaces.
The HVAC System consists of roof mounted Air Handling units for a single storey building that has many rooms. The system is designed for 20% fresh air and 80% return air that is achieved in the mixing box of the air handler.
The return air from the ceiling diffusers back to the Air Handler is not ducted. A short piece of duct with a bell mouth is extended from the AHU on the roof to the space above the acoustic tile false ceiling to draw air from all the rooms and corridors in the building.
The Supply air in each room is 400 CFM, but the return air quantity measured from the return air diffusers is coming to only 100 CFM.
We could not detect any infiltration of air from the light fixtures or the cracks and crevices in the false ceiling. The measurements from such light fixtures and edges of the false ceiling panels is registering zero on the measuring Hood.
The total return air measured at the bell mouthed duct to the AHU or at the straight piece of duct at the roof before its connection to the machine is as per design quantity. So the AHU is drawing air from somewhere. While we are in the process of checking the point of infiltration, the questions are: 1) What is the acceptable tolerance of return air quantities from the rooms whose type of occupancy is offices? ; 2) Is this achievable in non ducted return air systems?

 

[berkshire]

The total return air measured at the bell mouthed duct to the AHU or at the straight piece of duct at the roof before its connection to the machine is as per design quantity.

So the disconnect if you can say that, is that the space above the ceiling is providing air from somewhere other than the rooms to the air handler.
Has anybody looked at the point where the deck meets the exterior walls, to see if foam seals are installed?
B.E.

 

[DRWeig]

Best to you,

Goober Dave

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[DRWeig]

I just need to know if supply is 400 CFM and return is coming at only 100 CFM, can the subcontractor be paid?

There is no control for return air in a non-ducted system. The contractor cannot make it any different than it is right now. If they balanced the supply air to 400 CFM, they are done. Mission accomplished. The balancing firm did its job.

If you are supplying 400 to the space, sucking 80 of it out the toilet exhaust, measuring design return air flow at the unit intake, and design outdoor air at the unit intake, you have done all that you can do. There is no more that can be done. I would not bother measuring return air in individual rooms. It cannot be balanced in any way, shape, or form.

Sorry for the dead horse above, feel free to red flag it.


Best to you,

Goober Dave

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[SAK9]

You have not told us whether the AHU inlet conditions were close to design.If it is high there may be outside air leaking into the system.As a matter of due diligence you should highlight it to your consultant.

 

[solidspaces]

SAK9: The supply, return and fresh air quantities measured at or near the machine all add up. The only problem is that the return air quantities are a fraction of what they should be when measured by the hood at the ceiling return diffusers.

 

[SAK9]

Then it looks like most of the return is through light fittings(vented?)and through the gap between lay in tiles and the frame.The hood may not register any air flow when held against the light fitting as it will be well below its measuring range.Try a smoke test if you need to put your mind at rest.

 

[berkshire]

SAK9,
This thing is getting circular, I believe DRWeig suggested this in his second reply.
And at the risk of him beating me, with the stick instead of the horse. Suspended ceiling tile are notorious for this behavior.
B.E.

 

[willard3]

This is just recursive and somebody should kill this thread.....it's going nowhere.....

 

[DRWeig]

You're right SAK9 and berkshire, no beatings in the offing.

As for the last post by solidspaces,

The only problem is that the return air quantities are a fraction of what they should be...

I will again, but more gently, state that there "what they should be" is what they are, unless you have ducted return with balancing dampers. Your return quantities measured at the return diffuser in each room are EXACTLY what they should be.

Best to you,

Goober Dave

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