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What is the WaterHeater:Mixed?

The I/O reference guide is your friend. Start by reading the documentation for the WaterHeater:Mixed object. What you will learn from it:

  • The WaterHeater:Mixed models a storage tank. Meaning an object that holds a volume of water, and that can be connected to up to two loops: a Source Side Loop and a Use Side Loop
  • The WaterHeater:Mixed can also have an internal heating element
  • The field Heater Fuel Type controls the fuel type used by the internal heating element. That can be electricity, or Natural Gas, or whatever
  • If you set the Heater Maximum Capacity to zero, you are modeling a plain storage tank, with no internal heating element.

Loop connections

Typically the WaterHeater:Mixed (WH) is placed on the supply side of the Use Side Loop, and the Use Side Loop has the "load" (eg: WaterUseConnections) on the demand side. This will be your case too here.

In most applications, the WH is placed on the demand side of the Source Side Loop. In the case of solar thermal applications, there is some gray area on whether the solar collector should be on the demand side or the supply side of the source side loop, and the E+ examples seem to indicate that it should be the opposite of the usual, and the loop is controlled by availability managers instead: See https://unmethours.com/question/27837/which-side-of-the-plant-loop-for-solar-collector-in-eopenstudio/

Back up boiler: WaterHeater:Mixed or dedicated Boiler:HotWater?

As far as whether you want to model your boiler back up via the internal heating element of the WaterHeater:Mixed or via a dedicated Boiler:HotWater (and zero-out the internal heating element turning the WH into a plain storage tank), that is up to you and your project. If your boiler in real life is an atmospheric gas boiler, then it's fine to use the WaterHeater:Mixed as the backup. If you need to model part load efficiency (for a forced-draft boiler or a condensing boiler), the Boiler:HotWater will allow you to input curves to characterize the performance better.