Non Ducted Return Air to Air Handling Units 5

solidspaces · May 18, 2013

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?

willard3 · May 19, 2013

You will never balance a plenum air return properly. If you need to balance accurately, you need return ducts.

solidspaces · May 19, 2013

I agree willard3, but there must be the practice of the trade or Ashrae/Smacna guidelines about acceptable performance. Is 10% deviation Ok or is even 20% excusable. There has to be a minimum acceptable performance in such cases. Getting only 25% return air compared to supply air appears way too far.

DRWeig · May 19, 2013

As long as the plenum is at a negative pressure relative to the space, there should be no problem? Is there a comfort or odor problem you are trying to solve?

You can't specify a tolerance for return air flow if there is no means to adjust it, so there won't be any sort of standard in my opinion. I'm agreeing with Willard3 here, but going a bit farther.

Best to you,

Goober Dave

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solidspaces · May 19, 2013

The building is not occupied yet. With so little return, an odor problem would most likely arise. If the plenum is at a negative pressure compared to the space but the return air quantity that is showing up on the hood is very little, that might mean that the rooms will very soon face heavy air. Is it being implied that we need not do air balancing of the return air?
Secondly if there is no problem, then we can always omit a ducted return (if we can afford a little cooling load increase).

Drazen · May 20, 2013

why would you not open return diffusers to the maximum and measure quantity, as a first step.

open the one closest to ahu, measure, than close it and open the farthest one.

DRWeig · May 20, 2013

I was assuming that you do not have adjustable return diffusers. I have never seen adjustable return diffusers in a plenum-return system. Aren't they something like eggcrate panels? If so, you cannot possibly balance return flows. How can you balance the airflow through several holes that don't have dampers?

I don't understand your comment, "...that might mean that the rooms will very soon face heavy air." If you have 400 CFM going into a room, you also have 400 CFM leaving the room. If it's not going up the return grille, it's going into the hallway. Many places have no return grille at all in the rooms, but a single common return in a corridor that is ducted back to the rooftop unit.

Close all the office doors. Light up a cigarette (or spend some money on a smoke candle). Stand in a room farthest away from the rooftop unit. With 400 CFM supply and 100 CFM return Follow your smoke until it makes it to a return grille. It will probably be in the few offices very close to the rooftop unit.

You mentioned 20 percent outside air. How does the relief air exit the building?

Best to you,

Goober Dave

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solidspaces · May 20, 2013

DRWeig:
Since 400 CFM are coming in, the same quantity is definitely leaving but not from the return grille and this is the issue. If the door is closed this might even pressurize the room forcing the supply air to go to adjacent rooms. I feel this will somehow destabilize the system and would cause odors in rooms that are not having the same CFM leaving the room. It would also cause a nuisance when the room occupied by a smoker is not getting the smoke pulled out from the ceiling diffuser and it is instead going to the hallway.
The 20% make up of fresh air is due to the 20% being extracted by exhaust fans from the toilets.

DRWeig · May 20, 2013

That's great, solidspaces. You are right, odors will go from office to corridor to the other offices. However, this will happen whether the door is open or closed, unless the door is gasketed and sealed well at the floor.

The point is -- you cannot change it unless you install return dampers in each room and balance them.

Try my experiment. Close all the doors. Then start at the last office on the end of the supply duct run. Make some smoke and follow it. See where it goes.

Best to you,

Goober Dave

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EnergyProfessional · May 20, 2013

not sure I fully understand, but did you account for all the air that goes "through" the acoustical ceiling and not through the return grill? An acoustical ceiling with laid-in tiles , fixtures etc. is like a sieve and the air goes path of least resistance.

In addition at such low flow rates your measuring hood isn't accurate. Just measuring adds resistance, diverting more air "through" the ceiling.

Obviously as much air as comes in will leave the room, same for the AHU. If the plenum doesn't have leaks to the outside or restrictions, it probably will be fine unless you experience actual problems.

solidspaces · May 20, 2013

I am grateful to all who responded. The net result of this discussion is that matching the return air quantities with supply is not as critical as I thought

willard3 · May 21, 2013

You will find out, after the building occupants complain about humidity, odors and drafts, that there should be return air ducts and the air volumes should be balanced.

solidspaces · May 21, 2013

Willard3:
I thought the consensus was that it is OK even if the quantities of supply and return do not equal. Different arguments and explanations poured in about this disparity from the sensitivity of measuring hood to the cracks and crevices of the false ceiling panels to the positive and negative pressures to the argument that the air that is coming in must be exiting also, etc.
Your eye opener response goes against all the above consensus and has revived my concerns. So more responses are requested about the final action that is needed in this matter.

DRWeig · May 21, 2013

Willard3 is not going against the consensus, he is adding information to it.

I just finished working 15 years in a building with plenum return. Complaints were few and far between. There is no way to tell until 10 occupants all get together for a bean burrito lunch on the same day that another occupant burns popcorn in the microwave and another occupant heats up chicken tikka masala on a hot plate in his office. That happened in our office maybe twice a year.

Ducted return would swoop that stuff back to the AHU to be mixed with OSA before returning to the spaces. It's a chance you take.

Try my suggestion. Let some smoke loose in a room farthest from the RTU. Follow it. When it leaves the hallway and starts entering other offices, imagine that it is "bioeffluent gas" instead of just smoke.

The point is: It's OK to have supply not equal return with a plenum system because you can't do anything about it. It occurs in all systems in which the owner doesn't want to spring for ducted return.

There is nothing you can do to balance the return air, so ignore it. OR, duct the return.

Best to you,

Goober Dave

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317069 · May 22, 2013

design conditions: 400 cfm/room, 320cfm of return. 80 cfm of fresh air.
actual conditions: 400 cfm/room, 100 cfm of return, 300 cfm of fresh air.
did you check supply air condition?
what if you close the fresh air inlet then check return flow again?

solidspaces · May 23, 2013

317069: Yes we did check the supply air quantities. They are as per design. We could do the experiment that you have suggested of blanking off the supply and check out the return quantities. I am curious about your expectations or predictions about the effect of doing so!

SAK9 · May 23, 2013

Plenum return is fine if you have air tight ceiling void as in office towers with concrete floors. Plenum return is not to be used in buildings with metal deck or similar roof structure that are unlikely to be air tight. By measuring room conditions and the return conditions at AHU inlet, you can establish if there is outside air leaking into the void. An easy way to check if return is all right is to see if the doors are easy to open.

solidspaces · May 24, 2013

SAK9
All that you say makes sense. I am receiving the Air Balancing work from a subcontractor.
I just need to know if supply is 400 CFM and return is coming at only 100 CFM, can the subcontractor be paid? Will these numbers be approved by the consultant? Or will the company have to do it all over again? No objection was raised about the non ducted return at the time of accepting the Air Balancing Job, so can anyone back track and start raising hue and cry now, that it is not ducted?

317069 · May 25, 2013

I didn't mean air quantities, (300 cfm fresh air+100 cfm return air)is a different than (300 cfm return air +100 cfm fresh air) for the same AHU or RTU, may be you have to hire an air balance contractor to balance your system.

317069 · May 25, 2013

ooops you already have a contractor.

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Non Ducted Return Air to Air Handling Units 5

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