Non Ducted Return Air to Air Handling Units
Non Ducted Return Air to Air Handling Units
(OP)
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?
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?





RE: Non Ducted Return Air to Air Handling Units
RE: Non Ducted Return Air to Air Handling Units
RE: Non Ducted Return Air to Air Handling Units
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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RE: Non Ducted Return Air to Air Handling Units
Secondly if there is no problem, then we can always omit a ducted return (if we can afford a little cooling load increase).
RE: Non Ducted Return Air to Air Handling Units
open the one closest to ahu, measure, than close it and open the farthest one.
RE: Non Ducted Return Air to Air Handling Units
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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RE: Non Ducted Return Air to Air Handling Units
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.
RE: Non Ducted Return Air to Air Handling Units
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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RE: Non Ducted Return Air to Air Handling Units
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.
RE: Non Ducted Return Air to Air Handling Units
RE: Non Ducted Return Air to Air Handling Units
RE: Non Ducted Return Air to Air Handling Units
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.
RE: Non Ducted Return Air to Air Handling Units
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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RE: Non Ducted Return Air to Air Handling Units
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?
RE: Non Ducted Return Air to Air Handling Units
RE: Non Ducted Return Air to Air Handling Units
RE: Non Ducted Return Air to Air Handling Units
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?
RE: Non Ducted Return Air to Air Handling Units
RE: Non Ducted Return Air to Air Handling Units
RE: Non Ducted Return Air to Air Handling Units
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.
RE: Non Ducted Return Air to Air Handling Units
Best to you,
Goober Dave
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RE: Non Ducted Return Air to Air Handling Units
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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RE: Non Ducted Return Air to Air Handling Units
RE: Non Ducted Return Air to Air Handling Units
RE: Non Ducted Return Air to Air Handling Units
RE: Non Ducted Return Air to Air Handling Units
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.
RE: Non Ducted Return Air to Air Handling Units
RE: Non Ducted Return Air to Air Handling Units
As for the last post by solidspaces,
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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