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Looks like the culprit is an EnergyPlus bug.

I took the CondFD1ZonePurchAirAutoSizeWithPCM.idf example file and made a few changes to it to simplify the model, the most important deleting the MaterialProperty:PhaseChange object in the IDF.

The model showcasing the bug has HeatBalanceAlgorithm = ConductionFiniteDifference. I made a sum of the heat flux into and out of the wall using

Output:Variable,*,Surface Outside Face Conduction Heat Transfer Rate per Area,Hourly;
Output:Variable,*,Surface Inside Face Conduction Heat Transfer Rate per Area,Hourly;

which results in the graph below in a five year simulation. As you can see, there is a trend of accumulation of heat in the wall. Keep in mind that this wall has NO PCM in it.

The bug disappears by changing to HeatBalanceAlgorithm = ConductionTransferFunction, which is the default choice in the IDFEditor. The graph is below and heat storage in the wall is cyclical, as it should be.

Looks like the culprit is an EnergyPlus bug. Unfortunately for @DerricksRoss, MaterialProperty:PhaseChange objects must use the HeatBalanceAlorithm to simulate, which appears to be causing the bug.

Here is a google drive link to the files.

Here is a link to the github issue I posted.

I took the CondFD1ZonePurchAirAutoSizeWithPCM.idf example file and made a few changes to it to simplify the model, the most important deleting the MaterialProperty:PhaseChange object in the IDF.

The model showcasing the bug has HeatBalanceAlgorithm = ConductionFiniteDifference. I made a sum of the heat flux into and out of the wall using

Output:Variable,*,Surface Outside Face Conduction Heat Transfer Rate per Area,Hourly;
Output:Variable,*,Surface Inside Face Conduction Heat Transfer Rate per Area,Hourly;

which results in the graph below in a five year simulation. As you can see, there is a trend of accumulation of heat in the wall. Keep in mind that this wall has NO PCM in it.

The bug disappears by changing to HeatBalanceAlgorithm = ConductionTransferFunction, which is the default choice in the IDFEditor. The graph is below and heat storage in the wall is cyclical, as it should be.

Looks like the culprit is an EnergyPlus bug. Unfortunately for @DerricksRoss, MaterialProperty:PhaseChange objects must use the HeatBalanceAlorithm = ConductionFiniteDifference to simulate, which appears to be causing the bug.

Here is a google drive link to the files.

Here is a link to the github issue I posted.

I took the CondFD1ZonePurchAirAutoSizeWithPCM.idf example file and made a few changes to it to simplify the model, the most important deleting the MaterialProperty:PhaseChange object in the IDF.

The model showcasing the bug has HeatBalanceAlgorithm = ConductionFiniteDifference. I made a sum of the heat flux into and out of the wall using

Output:Variable,*,Surface Outside Face Conduction Heat Transfer Rate per Area,Hourly;
Output:Variable,*,Surface Inside Face Conduction Heat Transfer Rate per Area,Hourly;

which results in the graph below in a five year simulation. As you can see, there is a trend of accumulation of heat in the wall. Keep in mind that this wall has NO PCM in it.

The bug disappears by changing to HeatBalanceAlgorithm = ConductionTransferFunction, which is the default choice in the IDFEditor. The graph is below and heat storage in the wall is cyclical, as it should be.