[Equest-users] PSZ Very High Unmet Hours

[Dakota Kelley]

The system as Adam described it appears to be (mostly) constant-volume, variable-temperature.  If there are no VAV boxes and the supply fan VFD is controlled by building pressure, then it sounds like it is supposed to be a safety back-up for the relief system.  If the relief system is properly designed and balanced, I don’t see the supply volume fluctuating much.  I also don’t see it fluctuating in the model if the Min Flow Ratio is left at 1.0.  

 

The unmet hours are out of control because of the factors listed below.

 

·         The issue John and Demba raised of an 80 F heating supply temperature.

·         On top of a low heating supply temperature and no reheat, there are no baseboards or other forms of zone-level heat for RTU-4, 3, and 2.  However, RTU-1 has them…?

·         It is extremely difficult for a single-zone system to satisfy 19 separate zones (RTU-4).  It is impossible for a single-zone system to do so if there is no zone-level heating.

·         Using an interior control zone that appears to be a 175 sqft storage room (RTU-1).  This control zone is adjacent to “walking cooler”, which has a connected internal load of 57 kW (34.2 kW peak demand), 1.0 sensible heat ratio, and is 170 sqft.  A walk-in cooler rejects heat outside the building, so the sensible heat ratio should be zero.  The control zone is also adjacent to the kitchen with 21.13 kW of connected electric power and 862 kBTH of connected gas load, both of which have sensible ratios of 1.0.  This heat is having a huge influence on the control zone, which is why it is constantly in cooling mode while the rest of the zones are in heating mode.   

·         Not having a morning warm-up schedule

·         Allowing eQUEST to allocate supply air to the zones – entering design flow rates (supply, exhaust, and OA) at the zone level ensures each zone is getting the airflow it was designed for.

   

I changed the system type for the four RTUs to be PVVT with the heating supply capped at 80 F, and I was at least able to get the control zones satisfied.  I would recommend discussing the design with the mechanical engineer to ensure it is properly represented in the model.  The diversity of space needs and building orientation does not appear to be well-suited for single-zone control.

 

Thanks,

 

 

  
                                

DAKOTA KELLEY 

Project Designer | Energy Analyst

                          

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From: vidya gowda [mailto:vidyacg at vt.edu] 
Sent: Thursday, July 29, 2010 4:32 PM
To: John Aulbach
Cc: equest-users at lists.onebuilding.org; Vidya Gowda; bldg-sim at lists.onebuilding.org; Adam Gonthier
Subject: Re: [Equest-users] PSZ Very High Unmet Hours

 

Dear John, 

 

Have posted the proposed and the baseline case model on behalf of Adam, also responding to your query about VFD, about varying the flows, do we need to model a vav system?.. or I am not very familiar with this part as the specs don’t show VAV terminal units, they have provided the VFD schedules. It would really help if you could let us know how to to rectify the error. I also went ahead and changed the hot deck temperature to default as suggested by you and Demba. But it doesn’t seem to help much  

 

Thanks for responding, 

 

Kind Regards, 

 

Vidya

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