Return Ducts

[CuriouslyGeorge]

This may be a stupid question, but it bugs me: Are return ducts ALWAYS under negative pressure? I must have been taught some simplistic thinking that air moving towards the fan is negative; hence, it is under negative pressure. And yet, trying to picture a return duct under positive pressure just doesn't sit with me right.

I know...it's my upbringing!
- Curiously George

 

[quark]

Yes, actually it is more negative near the fan and less negative at the start of the return duct. Just puncture a hole in the return duct and see how outside air flows in.

You can also have positive return duct by hooking an inline fan to it. When Bag in Bag out HEPAs are used to tackle contamination issues in Pharmaceuticals, the return air fan is a must and the return duct of main AHU after inline fan becomes positive.

Regards,

 

[imok2]

If you take atmosphric pressure as the reference point then, Yes the return duct is a negative pressure