Return fan of a large AHU

[Engg. Hafiz]

Dear Members!

We recently conducted an audit of an old hospital building project awarded to my company in Saudi Arabia. During my site visit for inspection, I noticed something unusual: several Carrier AHUs, installed around 35 years ago during the original construction, are still functioning perfectly. However, what stood out was that each AHU has a separate exhaust fan installed, functioning as a return fan for the AHUs.

I'm curious to know if this was an older design approach, using separate exhaust fans for AHUs, or if this setup is still common today. In my 6 years of experience, I’ve never come across AHUs with separate exhaust fans—usually, the same fan serves as both the supply and return fan.

if anyone has any idea about this, please provide me with the information.

Thank you in advance.

 

[SAK9]

Please check the design to see if there are any exhaust dampers on the system. I have seen hospitals being designed with a purge system that allows the air handling system to function on a 100% fresh air mode. Usually the purge cycle is done in the morning when the temperature is favourable and helps clear any contamination in the air that has accumulated during operation the previous day.If that is not the case you may want to try shutting down the return fans and see if there is any impact on internal conditions.

 

[JDEngConsulting]

Dear Members!

We recently conducted an audit of an old hospital building project awarded to my company in Saudi Arabia. During my site visit for inspection, I noticed something unusual: several Carrier AHUs, installed around 35 years ago during the original construction, are still functioning perfectly. However, what stood out was that each AHU has a separate exhaust fan installed, functioning as a return fan for the AHUs.

I'm curious to know if this was an older design approach, using separate exhaust fans for AHUs, or if this setup is still common today. In my 6 years of experience, I’ve never come across AHUs with separate exhaust fans—usually, the same fan serves as both the supply and return fan.

if anyone has any idea about this, please provide me with the information.

Thank you in advance.

Along with the purge design this could also for static purposes as well. If they want to have tighter control on their balancing per AHU they may each have a return fan(s) for the return duct static. This could be as simple trying to overcome space constraints that limit ductsize or unit size.

 

[HVAC-Novice]

Dear Members!

We recently conducted an audit of an old hospital building project awarded to my company in Saudi Arabia. During my site visit for inspection, I noticed something unusual: several Carrier AHUs, installed around 35 years ago during the original construction, are still functioning perfectly. However, what stood out was that each AHU has a separate exhaust fan installed, functioning as a return fan for the AHUs.

I'm curious to know if this was an older design approach, using separate exhaust fans for AHUs, or if this setup is still common today. In my 6 years of experience, I’ve never come across AHUs with separate exhaust fans—usually, the same fan serves as both the supply and return fan.

if anyone has any idea about this, please provide me with the information.

Thank you in advance.

Do you have a schematic? A fan is agnostic to what it does - it moves air and creates pressures. There is no exhaust or return fan. A fan can be used for both and what it does is based on location in the system.

Some designs combine return and exhaust and control with dampers. Some use separate exhaust paths (actually required in case of code-required exhaust). smaller AHU often combine supply and return function into a single sup