When do you actually need a bypass pipe?

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And note that the ice storage units have a built-in bypass to regulate how much flow goes through the ice coils.

@MJWitte I'm automating generation of building models, and one of the automations adds in a tempering valve, which means I can't have a bypass on all my supply side components because I have used up the splitter mixer for the plant side of the loop on the tempering valve.

Are there any objects that are restricted to 0 flow when off? Is energyplus good at generating an error when flow exceeds a components capability?

I can't look at all the automatically generated models and warnings, but a fatal error would let me know there's a problem.

I don't understand the question here. Most (maybe all?) plant components will allow flow to pass through when they are off without any error. If there is a flow imbalance in the plant loop, this will generate a fatal error.

@MJWitte Okay. That answers my question. In general, plant components like chillers, boilers, hot water heaters, etc. allow flow whether or not the component is on or off. And if flow exceeds the maximum flow of the component, then a fatal error is generated, which will be easy to spot in an automated group of simulations.

I would be sure to test that by creating a model you expect to fail.