Oversized Roof-top Unit 2

[Norm01]

I am dealing with a retrofit project where an existing training room (approx.1025 sf) was modified to 5 office spaces (approx. 950 st). The existing roof-top unit (RTU) is a 7-ton constant volume unit (DX cooling). It originally supplied approx. 2,400 cfm to the training room. The system has been modified with VAV boxes to control space temperature in the offices and a by-pass box added to the RTU to maintain a constant duct static pressure. The bypass box is discharging in the ceiling plenum ducted toward the return side of the RTU. The problem is that the offices have significantly less loads than the training room and a total design airflow of only 700 cfm.
I am wondering if there going to be any issues when cold supply air discharges through the by-pass box back into the unit in regards to the DX coil?
Would it make a difference if the by-pass discharges into the plenum away from the RTU return air opening? Any other thoughts/comments?
Thanks

 

[ChasBean1]

Yes, the coil can ice up with the lower inlet air temperature. Oversized DX is a nightmare also from a dehumidification standpoint. DX will kick on, freeze the place out in 30 seconds, than have a whole bunch of off-cycle time to bring in nice humid outdoor air. You should replace this thing with a nice little two-tonner - maybe if it's in good shape you can find an after market for it.

 

[atlas06]

Will second chasbean1 on this.

We have seen engineers sued by client over this issue.
RTU sized for a national guard training facility plus office space. But the guard is there only 2/3 days per month, the rest of the month, the thing is so oversized that it freezes the place in 10 minutes.

You could try a field installed hot gas by-pass kit that may be able to help the situation if it does not already have one. You will fool the compressor with a fake load.

The retrofit approach you describe (VVT system) is a good one, given the circumstances.

But The best solution is what Chasbean1 suggested to you.