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# SimHVAC Maximum iterations warnings

My model whihc runs ok for design day simulations, when ran for weather file days give many Iteration warnings for VAV with REHAT box's. I have 5 VAV boxes and I am getting around 8 to 10 warnings - few of which are as below:-

   ** Warning ** SimHVAC: Maximum iterations (25) exceeded for all HVAC loops, at RUN PERIOD 1, 03/28 16:40 - 16:50
**   ~~~   ** The solution for one or more of the Air Loop HVAC systems did not appear to converge
**   ~~~   ** Air System Named = VAV WITH REHEAT 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: 4.427527E-002,4.367028E-002,4.334135E-002,4.394626E-002,4.427539E-002,4.367029E-002,4.334158E-002,4.394676E-002,4.427527E-002,4.367028E-002,
**   ~~~   ** 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 WITH REHEAT did not converge for humidity ratio
**   ~~~   ** Check values should be zero. Most Recent values listed first.
**   ~~~   ** Demand-to-Supply interface humidity ratio check value iteration history trace: 9.052790E-008,9.673507E-008,9.731781E-008,9.111049E-008,9.052739E-008,9.673596E-008,9.731832E-008,9.110991E-008,9.052790E-008,9.673507E-008,
**   ~~~   ** Supply-to-demand interface deck 1 humidity ratio check value iteration history trace: 1.237439E-004,1.236712E-004,1.235623E-004,1.236351E-004,1.237441E-004,1.236711E-004,1.235622E-004,1.236349E-004,1.237439E-004,1.236712E-004,
**   ~~~   ** Air System Named = VAV WITH REHEAT 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: 5.214427E-003,5.149490E-003,5.110221E-003,5.175149E-003,5.214442E-003,5.149493E-003,5.110250E-003,5.175208E-003,5.214427E-003,5.149490E-003,
**   ~~~   ** Supply-to-demand interface deck 1 temperature check value iteration history trace: 0.236625,0.236953,0.235742,0.235414,0.236626,0.236954,0.235743,0.235416,0.236625,0.236953,
**   ~~~   ** Air System Named = VAV WITH REHEAT 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: 1067.752024,1053.156244,1045.222737,1059.816714,1067.755021,1053.156433,1045.228238,1059.828629,1067.752024,1053.156244,
**   ~~~   ** Supply-to-demand interface deck 1 energy check value iteration history trace: 326.375391,-337.225776,325.235144,-335.177758,326.375846,-337.226459,325.236864,-335.180489,326.375391,-337.225776,
**   ~~~   ** Node named VAV1 BOX OUTLET NODE shows oscillating humidity ratio across iterations with a repeated value of 1.438061E-002
**   ~~~   ** Node named VAV1 BOX OUTLET NODE humidity ratio [kg-water/kg-dryair] iteration history trace (most recent first): 1.438061E-002,1.438068E-002,1.438061E-002,1.438068E-002,1.438061E-002,1.438068E-002,1.438061E-002,1.438068E-002,1.438061E-002,1.438068E-002,
**   ~~~   ** Node named VAV1 BOX OUTLET NODE shows oscillating mass flow rate across iterations with a repeated value of 0.180034
**   ~~~   ** Node named VAV1 BOX OUTLET NODE mass flow rate [kg/s] iteration history trace (most recent first): 0 ...
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1. VAVTemplate-Maximum iterations exceeded

2. exceeding maximum loop iterations

3. DOAS input Problem

If you don't find any of those answers helpful, you might want to refine your question to be more specific.

( 2016-05-17 14:33:24 -0500 )edit

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These warnings are very hard to diagnose. The key is the change in temperature from one iteration to the next. Each of these "differences" between the demand and supply side nodes show a slight oscillation. Not huge, but exceeding the tolerance expected (tol = 0 would be perfect). From iteration to iteration the difference between the demand side outlet node and the supply side inlet node of this air loop is about 0.0005 C

**   ~~~   ** Demand-to-Supply interface mass flow rate check value iteration history trace: 4.427527E-002,4.367028E-002,4.334135E-002,4.394626E-002,4.427539E-002,4.367029E-002,4.334158E-002,4.394676E-002,4.427527E-002,4.367028E-002,


This is not a big problem. What is a problem is why this happens. Usually a warning like this means there is an OA system or HX in the air loop that slightly changes the mixed air temperature entering the air loop. This slight change in air loop inlet temperature means the air loop components will react just a little bit differently than the previous iteration and occasionally leads to max iteration warnings.

The only advise I have is to review the air loop control and try to provide a stable supply air temperature so that the "SA system air flow" remains constant from one iteration to the next.

more

I implemented the control as in the 5ZoneVAV-ChilledWaterStorage-Mixed.idf, with annual simulation even on that file gives SimHVAC iteration error. @rraustad or can anyone help me figure this out with the idf. I have only one AHU with Variable-speed Fan for the AHU and SPM is - IDF,SetpointManager:OutdoorAirReset,AHU1 Outlet Setpoint,Temperature,16.5,15.5,13.8,21,Air Outlet Node1,,,,,; I dont know other ways tohave the Airloop provide stable supply air?

( 2016-05-20 09:09:27 -0500 )edit

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Last updated: May 19 '16