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[EnergyPlus_Support] Re: Exceeded Max Iterations Trouble Shooting

I've experienced this when my model has very thick contructions, like a slab, combined with where zone temperatures vary greatly due to large infiltration swings or setpoint changes. This is understandable. Small timesteps help. Controlling window airflows like an occupant does also helps. Now it is also possible to use different heat balance algorithms on specific surfaces, like a thick slab, which allows greater control on the numerical solver constraints for that part. 

--- In EnergyPlus_Support@xxxxxxxxxxxxxxx, Jim Dirkes <jim@...> wrote:
>
> I hope someone knows the answer, because it's been bothering me every so often, too.
> 
> p.s., Great explanation and great effort to research answers before posting!  I'm inclined to think that those warnings for specific days are OK, but would like to have more confidence.  I'm guessing that they occur at startup of the HVAC system when things are less stable.
> 
> James V Dirkes II, PE, BEMP, LEED AP
> www.buildingperformanceteam.com<http://www.buildingperformanceteam.com/>
> Energy Analysis, Commissioning & Training Services
> 1631 Acacia Drive, Grand Rapids, MI 49504 USA
> 616 450 8653
> 
> From: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx] On Behalf Of ntraughpee
> Sent: Tuesday, October 23, 2012 5:18 PM
> To: EnergyPlus_Support@xxxxxxxxxxxxxxx
> Subject: [EnergyPlus_Support] Exceeded Max Iterations Trouble Shooting
> 
> 
> 
> So this is a problem I seem to have on a regular basis and I am attempting to trouble shoot it.
> 
> I do a lot of retrofit modeling and one of the big sources of potential savings we have is from upgrading older pneumatic systems with OA resets (if they even operate) to newer electronic demand control systems. However, one problem I always seem to run into is it saying that the maximum number of iterations have been exceeded (or the average) where E+ has an issue converging the time steps into smooth iterations. That is to say that the difference in values from one iteration to the next is larger than E+'s tolerance which causes E+ to do more iterations until it can "converge" the values.
> 
> http://energyplus.helpserve.com/Knowledgebase/Article/View/24/33/maximum-iterations-exceeded
> 
> The above article goes over what some of the problems are involving this particular warning but it never explicitly says that this warning means your results are invalid. For instance, in the model I am running now which gives me the exceed max iterations warning, when I view the results in ResultsViewer I see that the issues only occur on specific days during specific time intervals (ie I'll have ~500 errors in one day) but the energy consumption, the HVAC system, and the zone temperatures all appear to be within reason. So if everything looks fine would this model be considered valid or not?
> 
> Furthermore, this E+ article explicitly states that E+ has an issue attempting to converge values using the higher order SetpointManager functions which more accurately detail demand control HVAC systems.
> 
> Now I have been attempting to increase the amount of time steps per hour which hasn't fixed the error and I have read that you can use the ConvergenceLimits object to fix this but I don't really know how it would be fixed.
> 
> If everything looks good in trouble shooting is it something to worry about? Is there a way to up the max iterations, increase the modeling run time and fix the problem?
>




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