Return Fan Required?

[new2hvac]

I have seen several different mechanical room configurations...some with a return fan(s) and some without. It seems that most of the larger systems have a return fan.

These design variations have led me to several questions and I thought I would pass them along to the pros:

Under what conditions does a building require a return fan?

If the supply fan is sized large enough, does that eliminate the requirement of a return fan?

Can exhaust fans be used in lieu of a return fan while the economizer is being used?

thanks!
new2hvac

 

[lilliput1]

For large systems serving multiple rooms that have economizer cycle, use of return/relief fan would be required. The return relief fan would allow directing the return air to either the relief air discharge or to the net return air to the AHU. The modulation of the Normally Closed max OA damper, the Normally Open return air damper and the Normally Closed relief damper can be modulated simultaneously by interlocked damper linkage or control sequence. If the AHU serve only one room, the relief can be by gravity operated roof vents to relieve the excess outdoor air brought in during the economizer mode. Small rooftop systems have a barometric relief or power exhaust option. I do not think the barometric relief located in the rooftop unit can work as well because this type unit does not come with a return damper that is economizer controlled & located between the relief vent & the OA intake. How can there be positive pressure at the relief while negative pressure at the OA intake? The power exhaust feature would work better. Units about 20 tons & smaller indexed the power exhaust fan(s) in sequence as the OA damper opens more & more. Units about 20 tons and larger can have space differential pressure switch that could index be use to sequentially run the power exhaust fans.

 

[LSA]

Very well put Liliput1!

 

[carlosgw]

I would agree with the above except that it has little to do with the size of the system and more to do with the return and esp. exhaust/relief air pressure drop. I have seen small systems with a return fan and very large systems without one. That said, generally you will see them larger systems. You can use exhaust instead. Everything is easier to control with the return fan.