AHU Outside Air getting sucked in

[Hoons]

Hello,

While the OA damper is closed, outside air is getting sucked into the AHU

Return fan is controlled by CFM offset method and the return air damper is controlled inversely to OA and EA dampers.
There is a building static pressure sensor that controls the EA damper.
I'm not sure what else to check so any help will be appreciated.


Thanks in advance,
Hoons

 

[Drazen]

what about supply fan? outside air is not sucked by return fan...

i truly dislike to list number of possible scenarios until you give us more data.

 

[Hoons]

Supply fan is modulated to maintain initial setpt of 1.5" wc. It is reset between 1.3-1.8" depending on cooling demand.

 

[cdxx139]

How do you know OA is getting sucked in? Do you have air fow measuring stations?

Is your unit leaking and the supply fan pulling from the unit cracks?

Has the AHU dampers been commissioned? Just becasue your drawings say the dampers should be controlled that way, doesnt meen they are.

knowledge is power

 

[Hoons]

We have airflow stations and it is sucking air in through the blades.
System has not been commissioned yet.

 

[HVAC68]

Where is the AHU sucking outside air from, if the outside air damper is closed ?
Is the return air ducted to the AHU or is it free return into the AHU room ?


HVAC68

 

[317069]

Does the return damper control or he damper itself work properly?
What the problem if you get more fresh air?

 

[Drazen]

let's say synchronization is not done properly and when supply air fan is at highest volume extract fan is on lowest. than, it could be imagined that some of air should be sucked via exhaust air opening. however, if there is tight non-return damper on exhaust air duct, than air could possibly be sucked through closed fresh air damper.

the easiest way to verify it, rather than making complex measurements and interpretations, would be to remove exhaust air non-return damper. than, if you notice that air is sucked backwards through exhaust air duct, you know that task is to be delegated to control guy.

 

[EnergyProfessional]

Return fan should be controlled to maintain 0.01" static in return plenum (the one downstream of that fan). That way you have enough pressure to move return air to the supply fan. EA damper is controlled by building static.

Dampers leak a bit if they are not Class 1A, it may also not be closed properly.

 

[urgross]

Have you physcially inspected the linkage and OA damper position? If OA damper is showing end switch closed, why is the SA fan running (unless in warm up period)?

 

[dynamo78]

Me thinks your problem cant be solved on the drawing board, at least the way you presented. Did you ever physically inspected your system? where is the outside air coming from? or are you relying on your controls only? obviously,its from the open OA damper. the controller says its closed but actually its open, right? maybe you need to expound more on your problem.
JMHO Bro.