Am I screwed?

[ballsonwalls]

I have been working on a renovation project in which there is a 20,000 cfm VAV air handler with 19,000 cfm connected capacity. I sized my return fan at 17,000 cfm at 2.5". The total exhaust through toilet/laundry rooms is 1200 cfm, leaving 17,800 cfm to be returned. There are VAV two air handlers with return air plenums serving the total space. Is this extra 800 cfm excessive? How would this manifest itself if I did screw up?

Thanks

 

[quark]

That results in overpressurization of the building. Generally, design system resistance is higher than the actual resistance and you may have some relief when the fan operates right to the design condition. Just check whether the fan motor can take care of this without being overloaded.

If that doesn't work check to reduce the supply fan cfm(the drop will be 4.5%). If nothing works, go for a fan in parallel with main return fan.

 

[lilliput1]

I think you are close enough to be ok. The Testing & Balancing Contractor will adjust fan speed as required to obtain the air flows indicated at the terminala +/- 10%.

You should indicate your minimum Outdoor Air CFM which should be equal to the maximum of ether 20 CFM per person or sum of the exhaust air CF< + 0.05 CFM/SF x SF floor area of the building.

The 0.05 CFM/SF is pressurization to offset infiltration.

The 2.5" wg for the return fan seems adequate.

 

[simsd]

Crack a window open