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# Modeling Code Compliant DX with Two-Speed Airflow Control

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 ...

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I never did get this working in Version 8.6. Version 8.7 added a new fan model Fan:SystemModel that solves the issue. The economizer is a bit off with the economizer + 1st stage compression running the fan at low speed rather than high speed which is what most equipment (maybe all) is doing.

more

V8.7 also add a new control type to UnitarySystem

AirLoopHVAC:UnitarySystem,
A2,  \field Control Type
\type choice
\key SetPoint
\key SingleZoneVAV

( 2017-05-12 14:18:14 -0600 )edit