[EnergyPlus_Support] Re: ** Warning ** SimHVAC: Maximum iterations (40) exceeded

["JV Dirkes II" <jvd2pe@xxxxxxx>]

Dear Brent,

That's helpful!
I think I'm in the "when things are changing quickly" situation, but
there are a LOT of "recovery from setback" situations during a year;
almost every morning in my case.  When there are more than a dozen or
so of these max iteration conditions, E+ just condenses the rest of
them into a message saying that you had "XX" max iteration warnings. 
This, of course, means that you don't really know where they're all
coming from.  My hope is that the first few are representative; if
not, I'll be forced to guess a lot.

I was hoping to figure out a process that helped identify the source
of most of the iteration errors.  In my current model, there are
relatively few unmet load hours, which I take as a good sign. 
Nonetheless, there are 2800 maximum iteration warnings and that seems
like a lot...

Any other hints?

--- In EnergyPlus_Support@xxxxxxxxxxxxxxx, "Griffith, Brent"
<brent_griffith@...> wrote:
>
> Good question, difficult to answer.  Here are my 2 cents, 
> 
>  
> 
> It is something to be concerned about, but in many cases there does not
> seem to be a way to completely eliminate them and they aren't
> necessarily a cause for alarm.
> 
>  
> 
>  
> 
> 1.	The total count is a difficult measure to use because it varies
> with number of zones, number and type of air systems, and length of run
> period.  A 1,000 might not be a problem for a big model with an annual
> run, but it could be way too many for a single zone design day run.  The
> errors are more common with VAV than CV. The frequency is key though.  I
> look at the timing of the errors. If they happen every time step during
> some period, then it usually means there is something wrong with HVAC.
> If they happen only sometimes, and those times are when things are
> changing quickly (like recovery from setback), then I don't worry much.
> 
> 2.	I depends if the system is succeeding at controlling the zone
> conditions.  If the systems are controlling well, and the errors are
> intermittent, then the results are probably not affected significantly.
> If the systems are not controlling zone conditions, then the errors are
> probably very significant.  Check the comfort conditions and zone air
> temperatures to see.  
> 3.	When the errors are significant, they usually indicate something
> is wrong with HVAC input that EnergyPlus isn't able to trap in some
> other way.  Possibilities include all sorts of things that can go wrong
> such as:  systems connected wrong (node connections usually), sized
> wrong (mixing hard and auto sizes), controlled wrong (check operation of
> set point managers by reporting node set point values).  
> 
>  
> 
>  
> 
>  
> 
> ________________________________
> 
> From: EnergyPlus_Support@xxxxxxxxxxxxxxx
> [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx] On Behalf Of JV Dirkes II
> Sent: Wednesday, July 23, 2008 1:45 PM
> To: EnergyPlus_Support@xxxxxxxxxxxxxxx
> Subject: [EnergyPlus_Support] ** Warning ** SimHVAC: Maximum iterations
> (40) exceeded
> 
>  
> 
> Dear Forum,
> 
> I am wondering whether this warning ("Maximum iterations ... exceeded)
> is something to be concerned about. 
> 
> I get the warning often in varying quantities. Most of my E+ use is
> for LEED projects and I don't want to look (too) silly by submitting a
> project to USGBC with a lot of these warnings without understanding it
> better.
> 
> 1) If it is a concern, at what frequency? (e.g., "whenever it occurs
> more than 100? 500? 1000? times in a full year run.")
> 2) Roughly how much does it affect accuracy of the simulation? (A lot
> or a little? proportional to the number of occurrences?)
> 3) Any tips about how to avoid it?
> 
> Thanks!
>



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