VAVTemplate-Maximum iterations exceeded

asked 2015-08-13 01:09:40 -0600

Eva
151 ●6

updated 2015-08-13 08:47:29 -0600

MatthewSteen

10147 ●1 ●19 ●26 https://www.nrel.gov/b...

Hello,

I got 2645615 Warnings which are same as link text when using HVAC template - ZoneVAVwatercooled for a large building .

My question is that: Even if there are so many warnings, the system doesn't converge for only 07/27 09:30 - 09:40 and 08/18 11:20 - 11:30 and my timestep is 10 min. Could I ignore these warnings? If not, how can i fix this problem?

Thanks=)

More details:

* Warning * SimHVAC: Maximum iterations (20) exceeded for all HVAC loops, at RUN PERIOD 1, 07/27 09:30 - 09:40 * ~~~ * The solution for one or more of the Air Loop HVAC systems did not appear to converge * ~~~ * Air System Named = VAV SYS1 did not converge for mass flow rate * ~~~ * Check values should be zero. Most Recent values listed first. * ~~~ * Demand-to-Supply interface mass flow rate check value iteration history trace: 65.536106,83.211257,23.244115,58.052126,90.984201,27.363113,65.536130,83.211285,23.244115,58.052126, * ~~~ * Supply-to-demand interface deck 1 mass flow rate check value iteration history trace: 0.000000,0.000000,0.000000,0.000000,0.000000,0.000000,0.000000,0.000000,0.000000,0.000000, * ~~~ * Air System Named = VAV SYS1 did not converge for temperature * ~~~ * Check values should be zero. Most Recent values listed first. * ~~~ * Demand-to-Supply interface temperature check value iteration history trace: 5.555677E-002,7.269640E-002,1.850951E-002,5.245884E-002,7.519209E-002,2.136337E-002,5.555679E-002,7.269643E-002,1.850951E-002,5.245884E-002, * ~~~ * Supply-to-demand interface deck 1 temperature check value iteration history trace: 1.241118,1.675333,0.419872,1.213315,1.669355,0.470383,1.241119,1.675334,0.419872,1.213315, * ~~~ * Air System Named = VAV SYS1 did not converge for energy * ~~~ * Check values should be zero. Most Recent values listed first. * ~~~ * Demand-to-Supply interface energy check value iteration history trace: 1713822.876912,2173451.995929,606978.653270,1516346.701414,2378253.912276,714557.723354,1713823.524251,2173452.732933,606978.656808,1516346.706538, * ~~~ * Supply-to-demand interface deck 1 energy check value iteration history trace: 666322.964022,-1039523.438037,250718.781313,653731.204601,-1052064.940010,283512.232175,666323.237488,-1039523.823601,250718.781425,653731.203318,

* Warning * SimHVAC: Maximum iterations (20) exceeded for all HVAC loops, at RUN PERIOD 1, 08/18 11:20 - 11:30 * ~~~ * The solution for one or more of the Air Loop HVAC systems did not appear to converge * ~~~ * Air System Named = VAV SYS1 did not converge for mass flow rate * ~~~ * Check values should be zero. Most Recent values listed first. * ~~~ * Demand-to-Supply interface mass flow rate check value iteration history trace: 34.666404,34.205766,36.931524,37.392130,34.666596,34.205498,36.934014,37.384023,34.722262,34.125857, * ~~~ * Supply-to-demand interface deck 1 mass flow rate check value iteration history trace: 0.000000,0.000000,0.000000,0.000000,0.000000,0.000000,0.000000,0.000000,0.000000,0.000000, * ~~~ * Air System Named = VAV SYS1 did not converge for temperature * ~~~ * Check values should be zero. Most Recent values listed first. * ~~~ * Demand-to-Supply interface temperature check value iteration history trace: 2 ...

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answered 2015-08-14 20:14:41 -0600

rraustad

13834 ●79 ●12 http://www.fsec.ucf.edu/

updated 2015-08-19 14:13:26 -0600

There are over 2 million warnings and only 2 maximum iteration warnings? I would check the bottom of the err file to see if there are recurring warnings listed there.

Maximum iteration warnings can be troubling. What they mean are that the simulation was unable to find a solution, and in most cases, the solution bounces from one result to another. Usually a different temperature or mass flow rate, or both. This means that the answer found during the iteration process is whatever the last iteration found. This answer may not necessarily be the correct result.

Over the years I have found two recurring reasons for maximum iteration warnings. There are certainly other reasons for these warnings. These are:

1) HX used in the outdoor air system WITHOUT tight controls on the air loop supply air temperature
2) Multi-speed air loop systems that cannot decide how to meet a small load while bringing in outdoor air. The simulation begins by calculating an outdoor air quantity and mixed air temperature based on the last time step results. Then the multi-speed system is simulated and decides a different speed or mode (i.e., cooling or heating) is needed to meet the load. This changes the outdoor air mass flow rate and can result in oscillations or maximum iterations. This typically happens at low load conditions.
3) ??

Other users/developers feel free to edit this list as needed....

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Comments

thanks rraustad, I will try these two. Just one question, how can we set the cooling and heating supply air temperature? Is that in object "Sizing:System", Central Cooling/Heating Design Supply Air Temperature? Thanks again=)

Eva ( 2015-08-17 01:49:02 -0600 )edit

If your system is controlled based on coil outlet node set point temperatures, then use SetpointManger:* to place a set point temperature at the outlet node of the coils. SP based system control will likely be the type of control used when seeing maximum iteration warnings (but not always). If maximum iterations warnings occur, try setting these set points to be constant to see if that helps eliminate the warnings.

rraustad ( 2015-08-19 14:09:51 -0600 )edit

Thanks, i just did it =) All warnings were gone.

Eva ( 2015-08-19 21:40:47 -0600 )edit

Please comment on how you eliminated these warnings This will help other users.

rraustad ( 2015-08-20 21:17:10 -0600 )edit

yes. In my case, i used the EnergyPlus Expand IDF getting by HVACtemplate:VAV as my new idf. Firstly, i changed from Pump:ConstantSpeed to Pump:VariableSpeed, and Boiler Flow Mode in OBJECT Boiler:HotWater from ConstantFlow to LeavingSetpointModulated. Then setting the Central Cooling Design Supply Air Temperature and the one of heating the same. The warning were gone=)

Eva ( 2015-08-24 02:36:05 -0600 )edit

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answered 2015-08-19 13:45:46 -0600

OS-user-AT

2284 ●3 ●14 https://mak-analytics.com

updated 2015-08-19 13:47:01 -0600

I have recently found another cause for maximum iterations exceeded: fluctuations in airloop mass flow due to two different fans in the airloop.

The example is as follows:

AirLoop constructed with OpenStudio containing UnitarySystem consisting of a Fan (On/Off), a WAHP cooling coil, heating coil etc.; i.e. OpenStudio creates an air loop with a VariableFlow-Fan and an OnOff-Fan in sequence. Whenever the OnOff-Fan switches On or Off, there can be significant deviations in mass flow between successive air loop nodes, which apparently is numerically difficult for the EnergyPlus solver.

My solution has been to eliminate the VariableFlow-Fan, so that the air loop contains only one fan, i.e. the Fan of the UnitarySystem. This way the convergence problems were eliminated.