VRF & DOAS Design & Operation

edit

1) If you are looking for the size of the outdoor unit then this information is in the eio file or table reports (html file) if that input is autosized.

AirConditioner:VariableRefrigerantFlow,
  VRF Heat Pump,           !- Heat Pump Name
  autosize,                !- Gross Rated Total Cooling Capacity {W}
  autosize,                !- Gross Rated Heating Capacity {W}

! <Component Sizing Information>, Component Type, Component Name, Input Field Description, Value
Component Sizing Information, AirConditioner:VariableRefrigerantFlow, VRF HEAT PUMP, Design Size Rated Total Cooling Capacity (gross) [W], 37248.19743
Component Sizing Information, AirConditioner:VariableRefrigerantFlow, VRF HEAT PUMP, Design Size Rated Total Heating Capacity [W], 37248.19743

If you are talking about the capacity during the simulation then that information would be available using the Output:Variable object defined in your input file.

Output:Variable, *, VRF Heat Pump Total Cooling Rate, timestep;
Output:Variable, *, VRF Heat Pump Total Heating Rate, timestep;

2) The Account for Dedicated Outdoor Air System input field is in the Sizing:Zone object. Turning this input on will impact the zone loads used during sizing calculations. The zone loads used during sizing calculations primarily affect the zone air flow rate in cooling and heating mode, which in turn are used to size the HVAC system. The VRF system terminal units have a dedicated mixer object which accounts for outdoor air during operation. When the terminal unit is on the mixer provides the specified outdoor air.

3) The higher efficiency of the VRF system is attributed to lower fan power when some terminal units turn off when not needed and lower external static pressure of the non-ducted terminal units. Purported higher efficiency of the outdoor unit can also reduce energy consumption when some terminal units turn off. And of course there is the heat recovery aspect of VRF systems. Fan power savings is a given in the VRF system while higher operating efficiency at part-load is reflected in the performance curve for EIR as a function of PLR.

AirConditioner:VariableRefrigerantFlow,
  VRF Heat Pump,           !- Heat Pump Name
  CoolingEIRLowPLR,        !- Cooling Energy Input Ratio Modifier Function of Low Part-Load Ratio Curve Name
  CoolingEIRHiPLR,         !- Cooling Energy Input Ratio Modifier Function of High Part-Load Ratio Curve Name
  HeatingEIRLowPLR,        !- Heating Energy Input Ratio Modifier Function of Low Part-Load Ratio Curve Name
  HeatingEIRHiPLR,         !- Heating Energy Input Ratio Modifier Function of High Part-Load Ratio Curve Name

I have tested these systems in both lab and field environments and the efficiency of those systems did not represent what was reported in the product literature. The reason for this difference is wrapped up in the control logic used by each manufacturer. As these tests were years ago maybe things have gotten better over time. A discussion of the EIRfPLR performance curve is presented in the publication here in Figure 10 on page 32.