My goal is to model energy code required air flow control. Energy codes regulate over 5 ton single zone DX system flows as a fraction of design flow as follows: 66% during venting, 66% during low cooling, 100% during high cooling. Heating is not regulated but I believe it is typically 100% in gas furnace units.
I thought this would be easy with the airloophvac:unitarysystem object but am stuck in a house of mirrors. Does anyone have suggestions for how this is supposed to be done? Or know of a good example. I did look up the 90.1 Determination models but they seem to use a large EMS program.
What I'm doing that gets close:
airloophvac:unitarysystem
- cooling design supply air flow rate method = SupplyAirFlowRate
- cooling design supply air flow rate = Autosize
- heating design supply air flow rate method = SupplyAirFlowRate
- heating design supply air flow rate = Autosize
- supply air flow rate method when no cooling or heating is required = SupplyAirFlowRate
- supply air flow rate when no cooling or heating is required = Autosize
UnitarySystemPerformance:MultiSpeed,
- Single Mod Operation = Yes or No
- Heating Speed 1 Supply Air Flow Ratio = 1
- Cooling Speed 1 Supply Air Flow Ratio = 0.66
- Heating Speed 2 Supply Air Flow Ratio = 1
- Cooling Speed 2 Supply Air Flow Ratio = 1
Coil:Cooling:DX:MultiSpeed,
- Rated Air Flow Rate, Speed 1 {m3/s} = Autosize
- Rated Air Flow Rate, Speed 2 {m3/s} = Autosize
With the above inputs and single mode = Yes, the mass flow in low cooling is constant and 66% of the mass flow in heating, venting and high cooling. Fan runtime fraction is 1 and the fan power is reduced to account for the lower mass flow. Just as I would expect it. Only issue is the ventilation flow is at 100%
With the above inputs and single mode = No, in heating and venting mode the mass flow is constant and the fan runtime is 1. In low cooling, the mass flow is constant and 66% but the fan runtime fraction is suddenly less than 1. is constant and 66% of the mass flow in heating, venting and high cooling. Fan runtime fraction is 1 and the fan power is reduced . I do not understand why the fan runtime is not 1.
To get the reduced ventilation flow I was thinking I needed to set the following:
airloophvac:unitarysystem
- Supply Air Flow Rate Method When No Cooling or Heating is Required = FractionOfAutosizedCoolingValue
- No Load Fraction of Autosized Cooling Supply Air Flow Rate = 0.66
But with this input the mass flow, fan runtime, and fan energy behave in unexpected ways. With single mode = Yes or No the heating mode mass flow is no longer constant and fan runtimes are less than 0. In ventilation mode the mass flow rate is constant just a few percent below the heating mode mass flow rate (rather than 0.66). The fan runtime is 0.66. The fan energy is a few percent below the heating mode fan energy. In low cooling mode with single mode =yes, the mass flow is constant and just below that of venting and heating and the runtime is 1.0 and the fan energy is reduced 40% rather than the 60% of the first two cases. With single mode=no the mass flow is constant and just below that of venting and heating. The fan runtime is a small fractional number and the fan energy is not decreased.
