Modeling FCUs with VAV in EnergyPro 9

Hello! I'm looking to get some thoughts on the right way to model mechanical systems in EnergyPro 9 for a performance compliance model for a commercial laboratory/innovation/research building. The model only includes the tenant improvements scope. The building is connected to a central utility plant that is in the core and shell scope. The CUP includes boilers, chillers, cooling towers, and AHUs. The building has office and lab spaces. Office spaces have fan coil units providing heating and cooling, with VAV fans. Lab spaces have fan coil units with VAV fans providing cooling only, and terminal VAV boxes providing heating.

The way I initially modeled these FCUs was "Chilled Water VAV" - "Single Zone" with heating coil set to "Hot Water Hydronic" for office spaces, and "w/Terminal Boxes" with heating coil set to "No Heating Provided" for lab spaces. All FCU fans were set to "Variable Speed Drive." However, most spaces, both office and lab, had many heating unmet load hours, even when the model complied energy-wise. I tried multiple approaches to get rid of them, but nothing fixed the issue.

Then I started thinking I was modeling the system wrong. When I changed the system type to "Four Pipe Fan Coil System" for office spaces, the number of UMLHs dropped significantly, but the energy use increased, partially because this system type can't be modeled with variable speed drive fans (I had to use "Constant Volume" instead).

Does anyone know the right approach to model this? Is the second way described above the way to go? Any feedback would be helpful. Thank you!

Srecko Curkovic

asked 2023-10-17 18:46:47 -0500

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How is ventilation provided to each zone? through the fan coil unit or through the VAV ducted heating system? Is there economizing airflow capacity in the VAV ducted system?

While FPFC can only use a few types of fan control, either constant volume or cycling, if you have another system delivering ventilation to each zone with the fan coils cycling the energy penalty between cycling and a fully variable fan is a lot less. Not perfectly accurate, though closer to a 5% to 10% penalty on fan energy

Neil Bulger (2023-10-19 12:02:01 -0500) edit

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This may be a controversial suggestion, but here it is.. Your design is perfectly valid, but there is a limitation of the software engine that you're required to use. You might consider calculating what the average BHP would be using VAV fan controls, and then specify that in the software. Then use the "exceptional conditions" box to explain what you did. This is a reasonable energy modeling practice and you're up-front about it. Ultimately, it's up to the AHJ to review and approve the report but all the info is there so they can review and ask you about it (or deny it) if they so choose.

I personally wouldn't resort to something like that unless that is the change that makes the difference between complying or not. You'd want to have some backup on how you arrived at the average BHP - like using your VAV model and some hourly reports for airflow to determine how the fans would modulate, then apply the findings to the constant volume model.