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How do you assign instantaneous water heater load to water systems?

asked 2023-08-14 09:23:58 -0500

RCulham's avatar

updated 2023-08-14 12:55:33 -0500

I have set up an instantaneous water heater to a service water load through a heat exchanger in OpenStudio. This load reports to the heating load rather than water systems even though I have identified the End-Use Subcategory as Water Systems.

Is there a way to do this or do I have to delete the heat exchanger and place the boiler directly in the service water loop?

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Since posting this question, the End-Use Subcategory "Water Systems" is reported in the EnergyPlus output report as a separate line item. The Boiler is automatically assigned to the major category "Heating", whereas a storage tank is assigned to the major category "Water Systems". There appears to be no direct way for the boiler load to be assigned to another major category. To have the Water Systems sub-category show in the OpenStudio Results report, it appears that it would require a custom modification to this report code.

RCulham's avatar RCulham  ( 2023-08-15 10:44:57 -0500 )edit

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answered 2023-08-22 16:11:13 -0500

RCulham's avatar

Although physically speaking an instantaneous water heater transfers energy from the boiler to the service water through a heat exchanger, a delve into the EnergyPlus Input/Output Reference guide shows using a WaterHeater:Mixed instead of the heat exchanger shown in the plant loop image in this question. The guide suggests setting the volume of the mixed water heater to 1 gal (0.003785 m3) and set the Heater Control Type to Modulate to model an instantaneous water heater. Heater Fuel Type can be chosen, such as Electricity or NaturalGas. The WaterHeater:Mixed can also be placed in a Boiler Plant Loop, replacing the heat exchanger shown in the image in this question.

When using the WaterHeater:Mixed object, I discovered that the Water Systems energy consumption is identified in the OpenStudio Results Report in the Annual Overview End Use table, as well as the accompanying Pie Chart.

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I have seen many multi-residential buildings as well as large commercial buildings where a boiler is the source to heat service water heating tanks. The OS input screen parameters for a hot water boiler and a mixed storage tank are significantly different. Of particular note is that you can define the efficiency of a boiler based upon the return water temperature. This may be a feature request that goes to the EnergyPlus code, but having a boiler option in the Group - Water Heaters and Thermal Storage would be very useful for these applications.

RCulham's avatar RCulham  ( 2023-12-19 10:34:48 -0500 )edit

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Asked: 2023-08-14 09:23:58 -0500

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Last updated: Aug 22 '23