Changing Constant Volume RTU to use a two speed fan disproportionately increases heating energy
I am comparing a baseline constant volume (CV) RTU to a two-speed fan. This is a theoretical retrofit of the RTU systems on the 2004 Stand Alone Retail building from the DOE. All we intend to do is replace a CV fan with a two-speed fan that operates at 100% while heating and cooling, and 66% while meeting minimum ventilation rates.
Since the baseline case from the DOE model is all constant volume objects, we had to change the UnitaryHeatCool object to UnitarySystem. That allows us to use Fan:SystemModel to simulate a two speed motor.
The expected result is a large savings in fan energy with a modest gain in heating energy. We expect the extra heating energy to be needed since now there is less heat gain from the motor and fan in the airstream.
Instead the heating energy increases disproportionately to the savings in fan energy. This is counterintuitive but I can't figure out where and why the extra heating is needed. It's not a small amount either, one cooler climate example saved 70 MWh in fan energy, but heating energy increased 130 MWh.
After doing a detailed comparison of the baseline and new IDFs I am left with a few questions but I am a novice user so I am not sure where to go from here:
1) AirTerminal:SingleDuct:
- The baseline case uses Constant Volume duct while the new case was changed to VAV:NoReheat. Is that correct and necessary to model a two speed fan?
2) Coil:Cooling
The baseline case is DX:SingleSpeed but the new case is DX:MultiSpeed. I believe this was changed because the SingleSpeed object can't handle a two speed fan even if the 2nd speed is just for ventilation. Is that a true statement?
If using a MutiSpeed object was some sort of workaround to make a two-speed motor model, what should the speed 1 and 2 settings be? Furthermore, why did my predecessor have to change the cooling coil to multi-speed but the heating coil is unchanged?
3) Is there an easier/better way to convert a constant volume RTU to a slightly more modern model with a two speed fan for ventilation energy savings?
