Question-and-Answer Resource for the Building Energy Modeling Community
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The level of detail you need will depend on the equipment type and design decisions made from a BEM model. If you are modeling individual unit gas or electric hot water heaters and don't care much about heat losses, a centralized loop with no distribution losses is appropriate.
If you want to model heat pump hot water heaters, which can significantly influence zone temperature and humidity, or you want to compare against central hot water options with distribution losses, then I recommend using a higher fidelity approach with water heating equipment in each unit.
For ASHRAE's upcoming Zero Energy Mixed Use Multifamily design guide, there was some modeling done to integrate realistic residential draw profiles with a central system, including being served by a waste water heat pump option.
The process is described here: https://github.com/DavidGoldwasser/PAT_projects/blob/master/sddc_multifamily_openstudio.pdf
In particular, you want to use the ResidentialHotWaterFixtures and ResidentialHotWaterDistribution measures from ResStock. https://github.com/DavidGoldwasser/PAT_projects/tree/master/sddc_osw_multifamily/ze_multifamily/measures To use these, you'll need to tag units with additional properties including number of bedrooms/occupants to inform the draw profile generator.
If you have access to the ASHRAE / IBPSA BPACS 2020 presentations from last year, the presentation "Integrating Residential and Commercial Modeling for Zero Energy Mixed-Use Multifamily Building Design" gives a comparison of multifamily hot water options.
