Re: [EnergyPlus_Support] questions about dx vav system

["Michael J. Witte" <mjwitte@xxxxxxxx>]

1.  The VAV with no reheat terminal unit is not intended to provide heating 
control.   The airflow rate is actually fluctuating between the min flow and 
max flow.  When the supply air temperature switches over to heating operation, 
it provides 35C air at the min flow rate.  If the heating load is small, this 
will overheat the zone and the zone will request cooling.  The VAV box then 
goes to full open trying to cool the space, but the supply air temperature is 
high, so the space just gets hotter.  This system will not work unless you vary 
the heating supply air temperature using SET POINT MANAGER:WARMEST in which 
case the supply air flow rate should remain constant at the min flow and the 
supply air temperature will vary to meet the load.

2.  There is a tiny discontinuity in the multispeed DX coil model when going 
from just above low speed to just below low speed (top end of cycling region). 
This discontinuity can occasionally cause the iterative solution to fail.  This 
has been fixed for the next release (CR 6355).  To get around the problem in 
this version, change the low speed capacity slightly.

Mike

On 24 Jun 2004, at 0:51, zhaicq98 wrote:

> I have two questions here.
> 
> First, I am simulating a VAV system with no terminal reheat. I 
> specify the supply air temperature to be 12.8oC in cooling season and 
> 35oC in heating season. According to results I got, it works well in 
> cooling season, trying to keep the indoor temperature under 23oC by 
> varing the supply air flow rate. While in winter, it keeps heating 
> the space to a high temperature. In heating season, the supply air 
> flow rate is always at its maximum value. I can not understand why 
> the supply air flow rate in heating season does not vary.
> 
> Second question is about the iteration. I am using a multispeed DX 
> coil. The program terminates when simulating the system in May. There 
> is a fatal error which says the iteration limit is exceeded when 
> calculating the speed of the compressor. I have no idea how to 
> resolve the problem.
> 
> I have uploaded the file named BOX4_VAV.idf to the files area.
> 
> Thanks.
> 
> 
> 
> 
> The primary EnergyPlus web site is found at:
> http://www.energyplus.gov
> 
> The group web site is:
> http://groups.yahoo.com/group/EnergyPlus_Support/
> 
> Attachments are not allowed -- please post any files to the appropriate folder
> in the Files area of the Support Web Site.
> 
> Yahoo! Groups Links
> 
> 
> 
> 
> 
> 



========================================================
Michael J. Witte, GARD Analytics, Inc.
EnergyPlus Testing and Support      
EnergyPlus-Support@xxxxxxxx



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