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# Two-speed fan object?

Is it possible to model a two-speed fan in EnergyPlus? I can see only on-off and VAV fan objects. I am modelling a residential two-stage unitary-system where at low loads the compressor and fan is at 70% and cycling and at high loads, the fan and compressor is at 100%. I think I need a 2-speed fan for this, but if there is some other way to model the system, would appreciate it if someone can point it out.

Update: I finally got back to this. Based on the comments (I tried all, other than EMS), it looks like the fan does not cycle. Here are the results:

• The RTF is varying between 1-0.56.
• The supply temp is varying between 14-20C.
• The supply flow/RTF comes to the 100% value.
• The st1 speed is 64% of st2.
• The unitary system control Is based on Load, the fan type is OnOff.

So looks like the flow is not varying at all and only temp is varying. If I look at the times when the RTF is 1 – the supply temp is still varying between 14-20C – in reality the supply temp should be near 14C, and the RTF would be less than 1.The same coil (curve for 100%) used as single speed works as expected – the temp varies slightly between 12C-13C, the flow/RTF is constant at the max value.

Is there anyway to vary the flow to the st1 cfm or st2 cfm using a two-speed coil?

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See the two-speed coil object (Coil:Cooling:DX:TwoSpeed) and the parent object CoilSystem:Cooling:DX.

The speeds entered in the coil object will be used to select the air flow rates and therefore the fan speed.

The example file 5ZoneAutoDXVAV shows how this coil type is used in the coil system.

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So you are saying that even if the fan used is VAV, it will work only on the two speeds that the coil has? It needs a fan curve - I guess any curve is fine as long as the kW at 70% and 100% is what is in the specs. Thanks.

( 2015-08-07 15:41:13 -0600 )edit

The unit also has a dehumidification mode - here the compressor is at 100% and the flow is at 70%. Can I use the Coil:Cooling:DX:TwoStageWithHumidityControlMode for this? Thanks.

( 2015-08-07 16:21:00 -0600 )edit

If dehumidification is required, use the Coil:Cooling:DX:TwoStageWithHumidityControlMode object. If you look at the CoilSystem:Cooling:DX object you will see that these cooling coil types are allowed:

CoilSystem:Cooling:DX, Coil:Cooling:DX:SingleSpeed CoilSystem:Cooling:DX:HeatExchangerAssisted Coil:Cooling:DX:TwoSpeed Coil:Cooling:DX:TwoStageWithHumidityControlMode Coil:Cooling:DX:VariableSpeed Coil:Cooling:DX:SingleSpeed:ThermalStorage

The operation of a 2-speed fan is controlled in the parent object, not the fan. VAV fan should work.

( 2015-08-07 16:53:06 -0600 )edit

Is there a way to figure out if the fan is working only at the two speeds and not varying between the two speeds? It is difficult to know if the m3/s volume flow output is due to varying speed or runtime fraction.

( 2015-08-18 12:45:16 -0600 )edit

Report the coil outlet node volume flow rate and temperature and Cooling Coil Runtime Fraction. A cycling DX coil will report the full load temperature with an average volume flow rate. Compare the outlet temperature to what you expect at full load and compare the node volume flow rate to the max expected based on RTF. Report these variables using a detailed reporting frequency.

( 2015-08-18 13:53:23 -0600 )edit

I think it takes EMS to fully model a two-speed fan. The coil control is not a problem but getting an air terminal to request a constant, low speed flow is not simple. The uncontrolled direct air terminal has an EMS actuator that makes it possible to fix the air flow at the low speed using EMS runtime programming. Then the VAV fan will follow the low speed flow requested at the air terminal. Cycling the low speed fan is even more challenging but can be done with short timesteps and EMS programming for explict thermostat-based control.

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Multi-speed air handlers are modeled using one of several unitary system objects which have a Fan:OnOff child component. The unitary system controls the speed of the fan to meet the load.

For a multi-speed air-to-air heat pump or multi-speed furnace with DX cooling, use AirLoopHVAC:UnitaryHeatPump:AirToAir:MultiSpeed. This can be used with the standard thermostat or with a staged thermostat. See example files AirflowNetwork_MultiZone_House_TwoSpeed.idf, MultiSpeedACFurnace.idf, MultiSpeedHP_StagedThermostat.idf, and others. But this unit does not support humidity control.

AirLoopHVAC:UnitarySystem can model the same options plus it supports humidity control (but there are not as many example files yet to show these combinations). For the type of humidity control you describe, use Coil:Cooling:DX:TwoStageWithHumidityControlMode and Dehumidification Control Type = MultiMode.
Note: I think there is an error in the documentation which states that MultiMode only works with cooling coil type = CoilSystem:Cooling:DX:HeatExchangerAssisted. I am checking on this.

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From the source code I can see that MultiMode works with cooling coil type = CoilSystem:Cooling:DX:HeatExchangerAssisted or Coil:Cooling:DX:TwoStageWithHumidityControlMode

( 2015-09-09 11:37:42 -0600 )edit

To model a two-speed fan, you need the two-speed coil object (Coil:Cooling:DX:TwoSpeed) with a VAV fan (Fan:VariableVolume), and VAV No Reheat terminal units at the zone (AirTerminal:SingleDuct:VAV:NoReheat). The minimum air flow fraction on the terminal unit should match the Fan Power Minimum Flow Fraction on your fan, the coil's Rated Low Speed Air Flow Rate, and the Central Heating Maximum System Air Flow Ratio in the System:Sizing object (so your heating coil is sized to meet load at low speed). The best example of two speed fans are in the CBECC-Com prototype models (see Retail-Med).

Plot below is Fan Flow (blue) and OA flow (orange) from the Retail-Med CBECC-Com prototype model.

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