Knowledgebase: Error and Warnings
Maximum iterations exceeded
Posted by Michael J. Witte, Last modified by Michael J. Witte on 07 August 2008 03:10 PM
Maximum iterations exceeded warnings can occur for a variety of reasons. EnergyPlus iterates for each HVAC timestep until the mass flows and temperatures in all HVAC loops converge to certain tolerances. This warning means that the simulation was not able to converge to a solution in the specified number of iterations, and has moved on to the next time step with the current result.

If there are a small number of these errors in an annual simulation, then you can probably ignore them.

If there are large numbers of these errors, then the results may be invalid. How to diagnose?

1. Are the zone temperatures being controlled to setpoint? If not, then the max iteration warnings may indicate that the building is getting further and further off setpoint with each iteration, and it will never be able to stabilize. Causes for this could be inadequate equipment capacity or poor controls.

2. Is there simultaneous operation of heating and cooling coils that is not expected? For example, if inconsistent setpoints are placed on the central heating and cooling coils in a single-duct system, the coils may fight over what the system supply temperature should be.

3. Is the system a variable-volume system with floating supply air temperature? A VAV system with many zones and a supply air temperature that is floating at the mixed air condition, or using SET POINT MANAGER:WARMEST to reset the supply temp, could have difficulty converging.

4. Are there other errors in the output, such as node connection errors or mass flow errors? If so, these must be fixed first.

5. If you activate Diagnostics, DisplayExtraWarnings; You can see what tolerance is not being met. This can help narrow down if the problem in an air loop or plant loop, if it is a temperature problem or mass flow problem, etc.

This is not an exhaustive list, but should help one get started on fixing this problem.
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