Direct supply (without VAV box) in a space served by VAV system
Direct supply (without VAV box) in a space served by VAV system
(OP)
Is it correct to have direct supply (without VAV box) in a space that is served by VAV system.
For example : A building is having 10 different spaces and all are provided with VAV boxes. There are some internal spaces like corridors, Toilets etc. which actually doesn’t need VAV control. Constant flow will be okay for these spaces as there will be no frequent change in the heat gain/loss. Can these spaces have direct supply (without VAV box) from the same AHU which is also serving other spaces with VAV boxes.
For example : A building is having 10 different spaces and all are provided with VAV boxes. There are some internal spaces like corridors, Toilets etc. which actually doesn’t need VAV control. Constant flow will be okay for these spaces as there will be no frequent change in the heat gain/loss. Can these spaces have direct supply (without VAV box) from the same AHU which is also serving other spaces with VAV boxes.





RE: Direct supply (without VAV box) in a space served by VAV system
RE: Direct supply (without VAV box) in a space served by VAV system
RE: Direct supply (without VAV box) in a space served by VAV system
As an extreme example if all of the boxes downstream of your toilets were 100% open and all the boxes upstream of your toilets were at minimum then you will have a lower static pressure if the situation was reversed.
As loads shift, pressures will change.
A CAV box will modulate to maintain a constant flow with a variable static pressure differential.
*it won't be constant
RE: Direct supply (without VAV box) in a space served by VAV system
RE: Direct supply (without VAV box) in a space served by VAV system
So will your hybrid VAV system work? I would say yes, but it is not a good engineering practice. Also, I will not know how much excess air will come out from those outlets without VAV boxes. The whole principle behind the VAV system is to have a better control, yet save energy.
RE: Direct supply (without VAV box) in a space served by VAV system
RE: Direct supply (without VAV box) in a space served by VAV system
RE: Direct supply (without VAV box) in a space served by VAV system
1) Pressure Dependent VAV box:
VAV systems are dynamic. The resulting flow at any given moment through the VAV boxes is very dependent upon the system conditions at that moment. When cooling load in Room A decreases, VAV boxes start to close. Air flow reduces. The result will be an increase in the system static pressure which will increase the air flow to Room B. Room B will start to over-cool until the thermostat sensed it. Then, Room B will start to modulate the VAV boxes to reduce the air flow.
2) Pressure Independent VAV box:
Room B's VAV boxes will quickly sense the increased air flow and start to modulate the VAV boxes to maintain the required air flow.
RE: Direct supply (without VAV box) in a space served by VAV system
you also can just not condition those areas if they don't have much load. for example corridor between conditioned offices will approximate the temperature of the offices unless the corridor has perimeter wall, or large roof. Determine the loads.