Dear Paul:

I downloaded and tried to run your model, and I got the same error that you did. After the simulation fail, you are left with the options to return to the building description or to open the simulation output file. If you open the simulation output file you will see, in the "ATTN" report, that you have the following problem:

***ERROR***********************************************************************
Loop: DHW Plant 1 Loop (1) has a pump, but no return
flow. If a DHW loop, specify the LOOP-RECIRC-FLOW
or the LOOP-MIN-FLOW to create a return flow

I would suggest simply removing the pump from the hot water heater (ignore the warnings, your pump is auto-sized anyway).

Your other option would be to move the pump from the water heater to the DW Loop, and then to give the loop a recirculation flow. Because this is a DHW loop, there is no recirculation flow (all of the flow goes down the drain after it is used). When prompted for the recirculation flow, you can enter a 0.

I would also check the geometry of your building. Your shells labeled EL1, EL2, and EL6 all have Below-Grade floor constructions. From what I can tell, these floors will not be in contact with the earth. A useful trick that I use to make sure that I don't have roofs in the middle of my building, floors and a ceilings overlapping each other, or below grade walls where they do not belowg is the following:

Go to the 3D View
There, you can select to view the constructions and windows by their construction type.

I hope this was useful

Eddie Corwin