#2650 CBECC-Com Issue - Question regarding Highrise Ventilation Modeling

[Felicia Lee]

Version = 2016.3.0_SP1 (started in Energy Pro, but completed in CBECC-COM)

Category = Other

I am getting some unexpected results and want to confirm the best modeling approach for the ventilation system in a high rise residential project.

The project is a high rise residential project in South San Francisco. Each unit has a heat pump for HVAC. Ventilation is being handled at the fan coil unit of each unit. The fan is at a continuous 100 cfm for ventilation then kicks up to 600 or 800 cfm (depending on heat pump size) for heating and cooling. I believe I’ve modeled it correctly, however, the results and forms indicate the ventilation cfm is at the high rate at all times.

Is this the proper way to model this ventilation system? If not, how?

[Nikhil Kapur]

2/27/18 JA - Can we even handle this type of system? I’m not familiar with any systems this size that do this…

If we cannot, would a reasonable approximation be to have the user model a separate dedicated OA supply fan for each space with the ~100 cfm, with similar fan static as the zonal heat pump, and then model the heat pump itself as cycling and used only for heating/cooling?

[Nikhil Kapur]

2/28/18 DR - Unfortunately, two speed fans are not supported for SZHPs. This is allowed for FPFCs, but not the DX coils ZnSys.

We could support what the user has outlined for ZnSys:Type = SZHP, SZAC, PTHP, and PTHP relatively easily with straightforward translator updates from NREL and a few rules. For most ZoneHVAC units, EnergyPlus supports specifying the supply & outdoor air flows for cooling, heating, and no load (vent), and these can all be different.

In the near-term, what you describe could be done, though, this approach would double-count (albeit small) amount of fan power for supply the ventilation air when the system is heating/cooling. If this route is taken, I'd recommend modeling it as a ZnSys exhaust fan, since the net impact is the same (fan power + OA load equivalent to the amount of ventilation air) as modeling the DOAS as an AirSys.

A few important practical and modeling for what is described:
• At the low speed, is the supply air flow equal to the ventilation air flow?
• What is the fan power at the low speed. ECM motor fans and PSC fans use much different amounts of power at reduced air flows
• Is this low speed provided by a smaller separate fan, or the supply fan? If by the supply fan, it may be difficult to achieve the proper OA balance at the low flow unless a damper on the return is closed, or, a continuously operating exhaust fan is provided to "pull" OA in through the OA duct.

[John Arent]

User Response: (3/6/18)

Thank you for getting back to me regarding the ventilation. I was thinking of modeling it as a dedicated ventilation fan as the most conservative path forward. Glad to hear that sounds like it should run and document best.

Here are the answers to your questions:

1) At the low speed, the supply air will be slightly over the ventilation requirements. Mechanical rounds up to the nearest 5 cfm.
2) I did not get the fan power at low speed from the mechanical engineer. I will ask.
3) The low speed ventilation is supplied by the main fan coil supply fan. They then have a continuously running exhaust fan at each bathroom running at 30 cfm.

Since the user has a reasonable means to model the system, it's considered closed.