Re: [EnergyPlus_Support] Exceeding max iterations!

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

The max iteration errors indicate that the simulation is unable to converge on 
a set of flow rates and temperatures that meet the current load conditions.  
Changes to equipment and internal loads can change the behavior, but will not 
solve such a problem.  And yes, if the max iteration errors increase, the run 
time will increase, because it means the simulation is iterating more often.

Here are some general comments regarding max iteration errors:

Maximum iteration errors are usually related to mass flow or temperature 
problems around the loop.  This occurs when the air loop side is doing 
something contrary to the zone equipment.  Examples of situations which may 
cause this include:  sum of peak terminal unit airflows does not equal the peak 
supply fan air flow, node connection errors, bad control nodes, incorrect use 
of active and passive branches, inconsistent equipment schedules, and 
inconsistent control setpoints.  Loose controller convergence tolerances can 
also cause max iteration errors.  Changing to smaller values for tolerances can 
sometimes eliminate the problem.  In version 1.2.1, the ability to autosize 
controller tolerances was added, so autosize these wherever possible.  The most 
unstable systems are VAV system with large air flows where the supply air 
temperature is floating.  Check that the coil and plant equipment (if 
applicable) capacities are sufficient to meet the supply air setpoint.

Mike


On 6 Feb 2005, at 3:45, beyangj wrote:

> 
> 
> Dear,
> I built a model with water-cooled chiller, there are some warnings 
> about exceeding max iterations as follow in the ERR file
>  ** Warning ** SimAirLoops: Maximum iterations exceeded for VAV SYS 
> 1, at HONG KONG CHN IWEC DATA WMO#=450070, 01/03 13:25 - 13:26
>    ** Warning ** SimAirLoops: Maximum iterations exceeded for VAV 
> SYS 1, at HONG KONG CHN IWEC DATA WMO#=450070, 01/03 13:26 - 13:27
> ---reduced for brevity ---
> ** Warning ** SimHVAC: Maximum iterations exceeded for all HVAC 
> loops, at HONG KONG CHN IWEC DATA WMO#=450070, 03/17 15:26 - 15:27
>    ** Warning ** SimAirLoops: Exceeding max iterations continues...50
>    ************* EnergyPlus Completed Successfully-- 29 Warning; 0 
> Severe Errors; Elapsed Time=00hr 19min 25.00sec
> 
> When I only changed the water-cooled chiller with air-cooled chiller 
> in that model,and I didn't specify the condenser(water) loop in that 
> model, there are much more warnings about max iterations in the ERR 
> file as follow:
> ** Warning ** SimHVAC: Maximum iterations exceeded for all HVAC 
> loops, at HONG KONG WINTER, 01/21 08:15 - 08:30
>    ** Warning ** SimHVAC: Maximum iterations exceeded for all HVAC 
> loops, at HONG KONG WINTER, 01/21 08:15 - 08:16
> ---reduced for brevity ---
> ** Warning ** SimAirLoops: Exceeding max iterations continues...200
>    ** Warning ** SimHVAC: Exceeding max iterations continues...2650
>    ** Warning ** SimHVAC: Exceeding max iterations continues...2700
>    ** Warning ** SimHVAC: Exceeding max iterations continues...2750
>    ** Warning ** SimHVAC: Exceeding max iterations continues...2800
>    ************* EnergyPlus Completed Successfully-- 88 Warning; 0 
> Severe Errors; Elapsed Time=00hr 28min 30.00sec
> There were more warnings and it took more time in the air-cooled 
> chiller model than in the water-cooled chiller model, why do this 
> happen?
> In addition, when I changed the lighting intensity as half of the 
> original in the air-cooled chiller model, there are much more 
> warnings as follow:
> ** Warning ** SimHVAC: Exceeding max iterations continues...6650
>    ** Warning ** SimHVAC: Exceeding max iterations continues...6700
>    ************* EnergyPlus Completed Successfully-- 162 Warning; 0 
> Severe Errors; Elapsed Time=00hr 43min  3.00sec
> It took much more time than the model mentioned before. I wonder why 
> this happened? Could you help me?
> 
> Sincerely,
> 
> Stewart
> 
> 
> 
> 
> 
> 
> 
> 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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