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It sounds like you're trying to model a boiler that provides hot water to a storage tank, is this correct? If the water heater is providing the energy to heat the water, not a boiler, it should be on the supply side of the plant loop, not the demand side.

If you're trying to model a passive storage tank, there is a way to do this but it's not fully supported by OpenStudio 1.5.0. A work around would be to set the capacity of the water heater to zero to ensure all of the energy comes from the boiler or heat exchanger.

The two inputs I use most often to control DHW energy usage are the "Use Flow Rate Fraction schedule" and " Peak Use Flow Rate".

This video is a good resource on creating DHW systems in openstudio.