Re: [EnergyPlus_Support] Convergence problem

[Richard Raustad <RRaustad@xxxxxxxxxxxx>]

This is a result of the simulation performing differently on successive 
warmup days. For example, if the HVAC system is controlling RH and on 
the first warmup day the RH at the end of the day is different than the 
RH at the beginning of that day (which it should be), on the next warmup 
day the HVAC system will respond differently and will therefore have a 
different RH profile . Now on the next warmup day, the HVAC system again 
responds differently and the RH profile changes again. This scenario can 
repeat itself throughout the warmup days resulting in a convergence problem.

I would suggest you use the following report variable. It allows you to 
see your report variables during the warmup period. Look at the zone 
temperatures and see if there is a cyclic response for each successive 
warmup day. Plot the daily temperature profile for each warmup day. You 
might also look at zone relative humidity if a humidistat is used. You 
will probably find that there is not much difference in temperature 
profiles for each warmup day "near the end of the warmup period" (the 
last few days). If so, I would disregard the warning and proceed with 
your simulation. Also, this warning only impacts where the simulation 
starts on the very first day of the simulation. It does not impact how 
the simulation proceeds for the remainder of the run period.

Output:Diagnostics,ReportDuringWarmup;

The warning message shows a Max Temp Difference [C] =2.269 (tolerance = 
0.002C). That means that the maximum zone temperature difference is 
2.269C between warmup days. This might mean that the profiles are 
shifting upward and downward from day to day, but the profile looks very 
similar. If you find significantly different temperature profiles, see 
if there are other control issues that might cause these differences.

Richard Raustad
EnergyPlus Support



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