VAV System Not Converging

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The usual reason that a larger size air handler will tend to show more of the dreaded SimHVAC: Maximum Iterations warnings is because with the larger system the terminal units spend more time running at the minimum stop on the VAV damper. A common situation for getting these warnings is during times of low load when loads are just starting to pick up and the dampers begin to ramp up from the minimum stop. If you can diagnose that these errors are occuring during such times, by studying detailed frequency time series results, then I believe it is safe to ignore them. What usually happens is that when the damper varies, both the flow and supply air temperature are changing at the same time and the air system solver doesn't settle into one solution. The supply deck temperature changes because of economizer action as the ratio of outdoor to return air changes with variations in overall flow. This has been a long standing problem with no easy solution.

LEED asks you how many warnings you have, and you'll have to provide a supplemental narrative describing the warning messages and why they do not adversely affect the results of the energy model.

Without looking at your idf file, I'm not sure what's causing this specific error. Nevertheless, a few things to mention:

• I wouldn't necessarily be worried if it doesn't converge for only one timestep.
• I would try running the simulation with a smaller time step, say 6 per hour instead of 1. If your model is complex and you're afraid of computing time, try for just January

• It seems that while the supply inlet temperatures are indeed oscillating, they are not wild. Here's one extract:

  * ~~~ * Node named VAV2-37 SUPPLY INLET shows oscillating temperatures across iterations with a repeated value of 15.419107

  * ~~~ * Node named VAV2-37 SUPPLY INLET temperature [C] iteration history trace (most recent first): 15.419107,15.416305,15.426937,15.419107,15.416305,15.426937,15.419107,15.416305,15.426937,15.419107,

The recent history shows that it stays very close.

That being said, did you mean to have some apparently in cooling mode and others in heating mode? (You have 24 VAVs delivering air below 13°C/55°F, 21 VAVs delivering over 24/75°F, and 6 VAVs around 16°F/61°F)

• Something useful is to request an output at "Detailed" (= HVAC variable timesteps) reporting frequency of the supply inlet temperatures, and then plot it to see what's going on (I suggest using SQLite output and using the ResultsViewer from Openstudio)

• Check what convergence tolerances you're using for your VAVs (I think default is 0.001°C)

• If after all of the above you're not satisfied, try the helpdesk.