Hi Seamus,
My first thoughts would be that modeling these may be somewhat outside the intended scope of BS806, as this is based on normal outlets, as you mention. So you may struggle to effectively model all components from the plant room, to HIU to outlets, in one single project.
You may need to consider modelling the system over two projects:
You would just need to export the results from each project to Excel and combine the relevant parts. This should mean that the diversity is taken into account at local level, and the pipework is not oversized at plant / HIU level.
Hopefully that helps, please let me know how you get on.
Shane Regan | Principle Building Performance Support EngineerHevacomp to OpenBuildings - A complete guide for every user | Bentley Communities