Open Studio Multifamily Hot Water

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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/PA...

In particular, you want to use the ResidentialHotWaterFixtures and ResidentialHotWaterDistribution measures from ResStock. https://github.com/DavidGoldwasser/PA... 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.

I agree that a single central plant isn't the best option if there are multiple individual DHW systems in the multifamily building. I would recommend modeling each of the building's DHW systems as their own service hot water plant in OpenStudio. You don't need to model a separate service hot water loop for each apartment unless that's how the individual DHW systems in the building are configured. If that is the case, this post discusses using a measure to automate the process of assigning each water draw to its own service hot water loop and water heater, in case you want to investigate that approach or use that as a starting point for a measure that does something different.

The documentation isn't totally clear for how (or even IF) multipliers for a single zone or group of zones in EnergyPlus & OpenStudio affect service hot water draws. There are some closed issues on EnergyPlus Github from years ago here and here on this topic, so you may want to do some testing on the impact of zone multipliers on:

• the flow rate of hot water delivered
• heating gains for the zone(s) where the hot water draws are assigned
• heating gains for the zone(s) that contain the hot water heater

If the results from testing make sense, then I would recommend leveraging multipliers to speed up your workflow and represent identical zones. This means:

• zone internal gains (lighting, occupants, etc.) are identical (schedules and peak gain values)
• zone hot water draws are identical (schedules, peak flow rates, target temperatures)
• those hot water draws are on the demand side of a service hot water plant with identical supply equipment (i.e. hot water heaters). In other words, if one zone is served by a heat pump water heater and another zone served by a natural gas water heater, those zones should not be combined together with a multiplier of 2.

You can see how zone multipliers are used in the hospital commercial prototype.