×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Oversized Roof-top Unit
2

Oversized Roof-top Unit

Oversized Roof-top Unit

(OP)
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  

RE: Oversized Roof-top Unit

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.

RE: Oversized Roof-top Unit

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources