Multispeed Fan & Unitary System

Hi all,

I want to transpose a DOE-2 model to Energy+. What I want to model is a multi-speed fan as part of a unitary system. For that, I am using a Fan:OnOff and AirLoopHVAC:UnitarySystem. I read the input/output reference guide and assigned the same "fan power ratio function of speed ratio curve" to the Fan:OnOff object as in my DOE-2 model. I would like my minimum supply flow to be 60% of the maximum airflow rate. The I/O reference guide states for Fan:OnOff:

"If modeling multi-speed fan operation, this object must be used as part of a compound object that allows multiple fan speeds (e.g., AirLoopHVAC:Unitary:Furnace:HeatCool, ZoneHVAC:PackagedTerminalAirConditioner, etc.).In this case, the ratio of the compound object air flow rate to the fan s maximum air flow rate is used to determine the power at alternate fan speeds."

My understanding is that I need to have the maximum air flow rate set to my Fan:On:Off object and the minimum to my AirLoopHVAC:UnitarySystem. I did that but when I run a simulation and create a hourly report I can see that my fan is always running at constant speed (expect when cycling due to the night cycle control).

What am I missing?

Thanks a lot for your help!

1.6k

Jeremy

asked 2015-05-21 18:36:41 -0500

4.4k

__AmirRoth__

updated 2015-11-14 16:13:05 -0500

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A good place to start would be reviewing input file MultiSpeedHeatPump.idf for multi speed unitary system setup. AirLoopHVAC:UnitarySystem (+ UnitarySystemPerformance:HeatPump:Multispeed) can also model multi speed unitary system. Fan:OnOff object will have maximum air flow rate and air flow rate at different speed should be specified either at parent object level in case of multi-speed heat pump or in UnitarySystemPerformance:HeatPump:Multispeed object in case of AirLoopHVAC:UnitarySystem. Also PackagedTerminalHeatPumpVSAS.idf illustrates "Fan Power Ratio Function of Speed Ratio Curve Name".

Chandan Sharma (2015-05-23 05:02:39 -0500) edit

Thanks @Chandan Sharma you for your input! It looks like I had everything setup right. I had just an error in my hourly report.

Jeremy (2015-05-27 17:22:26 -0500) edit

Jeremy, is this question now answered?

rraustad (2015-07-23 17:39:50 -0500) edit

@rraustad Yes, it is.

Jeremy (2015-07-23 18:01:39 -0500) edit

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This question has been lingering unanswered for some time. Answered with a comment, but technically still unanswered.

The multi-speed system in E+ is a heat pump. To properly model a multi-speed fan, a system that models a change in speed must be used. This was originally named a multi-speed heat pump. The heat pump uses a multi-speed cooling coil (Coil:Cooling:DX:MultiSpeed) with a multi-speed or multi-stage heating coil. Other heating coil types are allowed with this parent object but the point is that there are discrete speeds associated with the performance of this equipment type. The following example uses the input data dictionary (IDD) to show specific input syntax.

AirLoopHVAC:UnitaryHeatPump:AirToAir:MultiSpeed,
  N9, \field Number of Speeds for Heating
  N10, \field Number of Speeds for Cooling
  N11, \field Heating Speed 1 Supply Air Flow Rate
  N12, \field Heating Speed 2 Supply Air Flow Rate
  N13, \field Heating Speed 3 Supply Air Flow Rate
  N14, \field Heating Speed 4 Supply Air Flow Rate
  N15, \field Cooling Speed 1 Supply Air Flow Rate
  N16, \field Cooling Speed 2 Supply Air Flow Rate
  N17, \field Cooling Speed 3 Supply Air Flow Rate
  N18; \field Cooling Speed 4 Supply Air Flow Rate

This multi-speed heat pump must also use a multi-speed cooling coil, with inputs similar to those shown above, and choices for heating coil types, multi-speed or otherwise.

A new model, one which combines all coil types into a single parent object, can also model multi-speed coils. This is the AirloopHVAC:UnitarySystem. A field at the end of this object allows a user to specify a multi-speed performance object.

AirLoopHVAC:UnitarySystem,
  A26, \field Design Specification Multispeed Object Type
    \key UnitarySystemPerformance:Multispeed
  A27; \field Design Specification Multispeed Object Name

The multi-speed performance object looks identical to the inputs for the AirLoopHVAC:UnitaryHeatPump:AirToAir:MultiSpeed object except that flow ratios are used instead of actual flow rates.

UnitarySystemPerformance:Multispeed,
  A1 , \field Name
  N1 , \field Number of Speeds for Heating
  N2 , \field Number of Speeds for Cooling
  N3 , \field Heating Speed 1 Supply Air Flow Ratio
  N4 , \field Cooling Speed 1 Supply Air Flow Ratio
  N5 , \field Heating Speed 2 Supply Air Flow Ratio
  N6 , \field Cooling Speed 2 Supply Air Flow Ratio
  N7 , \field Heating Speed 3 Supply Air Flow Ratio
  N8 , \field Cooling Speed 3 Supply Air Flow Ratio
  N9 , \field Heating Speed 4 Supply Air Flow Ratio
  N10; \field Cooling Speed 4 Supply Air Flow Ratio

The difference between these two methods for modeling a multi-speed system is very subtle.

For the AirLoopHVAC:UnitaryHeatPump:AirToAir:MultiSpeed object, the speed inputs must be entered in two locations, the parent and the child object. If these inputs are autosized, the system performance is the same as that specified in the coil object. If the inputs are not autosized, the air flow rate is dictated by the parent object and the performance changes based on the parent object flow rates. See the Speed 1 Total Cooling Capacity Function of Flow Fraction Curve Name ... (more)