Error when using Tempering Valve

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OpenStudio is tested in the configuration described in the EnergyPlus documentation. In this configuration the tempering valve is alone on a branch and in parallel to some other heating component. The idea being that the tempering valve will draw some amount of unheated water around the heating component to temper the final outlet condition.

Your situation is a little different since you have the tempering value in series to the hot water tank. I don't entirely understand what it is trying to accomplish, but I am surprised it causes your simulation to fail. The inputs to the tempering valve look right to me. In particular your node inputs. Note that if you leave these blank OpenStudio will take a best guess at what they should be when it creates the idf file. This defaulting is however based on the configuration described in the E+ documentation, and if you specifically set particular nodes as you have, then OpenStudio will respect them.

With respect to the actual error that E+ is giving you about the tempering valve on an active branch, that is surprising to me given that OpenStudio is known to work when the tempering valve is alone on the branch. The OpenStudio Model does not expose the branch object or its active/passive input field, because generally this input is confusing to users, and because OpenStudio can typically interpret the correct choice when making the idf file. For testing purposes you could modify the idf file so that the tempering valve branch is set to active. If that works then report back and we can codify this into OpenStudio and in the mean time you can use an EnergyPlus Measure in your workflow. I would be willing to help you write the Measure if you need some guidance.