DX Heating coil is running parallel to DX cooling coil

Hi all,

I am using HVACTemplate:System:UnitaryHeatPump:AirToAir to model a heatpump. It has a DX cooling and DX heating coil. I use the utpur variable Air System Total Heating Energy and Air System Total Cooling Energy to analyse when heating and cooling is required:

image description

Watching the eso file it seems that the heating coil is also operating during time while cooling is mostly required. Like in parallel. How can this happen?

The thermostat is set to operative temperatures and heating/cooling is only available during occupied hours. Here the detailed output variables plotting image description

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Xandrika

asked 2015-07-13 07:59:54 -0500

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__AmirRoth__

updated 2017-06-04 10:17:43 -0500

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@Xandrika I changed your air-to-air-heat-pump tag to the more generic air-source-heat-pump.

MatthewSteen (2015-07-13 08:07:00 -0500) edit

@MatthewSteen, thanks. your are right this is the better tag:-)

Xandrika (2015-07-13 08:14:45 -0500) edit

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You need to report at "Detailed" frequency. Those hourly values are showing the impact of smoothing so you can't tell what is happening at the system time step. See the Output:Variable documentation.

Kyle Benne

answered 2015-07-13 09:28:03 -0500

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@Kyle Benne, thanks for the hint. I have inserted the detailed output in the original question but I cannot find the reason why heating is happening while cooling. Or does a heatpump always heats a bit while DX cooling is operated? I am not an HVAC engineer and, do not really know the technical details of heatpump operation. Would be great if you could help me!

Xandrika (2015-07-20 04:15:36 -0500) edit

A heat pump will only provide heating or cooling at any given point in time. As your answer notes, using Output:Variable,*,Cooling(Heating) Coil Total Cooling Energy, detailed; will show the HP only provides cooling or heating in any single system time step.

The AirSys variables provide the sum of many components for example, fans, air loop coils, HX's, etc.

SysTotHTNG( AirLoopNum ) = SysFANCompHTNG( AirLoopNum ) + SysHCCompHTNG( AirLoopNum ) + SysHeatExHTNG( AirLoopNum ) + SysHumidHTNG( AirLoopNum ) + SysSolarCollectHeating( AirLoopNum ) + SysUserDefinedTerminalHeating( AirLoopNum );

rraustad (2015-07-27 09:03:53 -0500) edit

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After troubleshooting and reading more detailes about the HVAC systems in Energyplus, I found out that the output variables Air System Total Cooling Energy and Air System Total Heating Energy might include also the heating energy released by the system components of the Heatpump.

In Input-Output-Reference the following explanation is given:

System Loads Outputs

In this category, the total system load is reported. Two aspects are reported here: Heating and Cooling. The following two variables report the heat energy that system components (including packaged equipment, fans, main coils, reheat coils, humidifiers, desiccant dehumidifiers, evaporative coolers and heat exchangers) add or remove from the air loop. Each variable within these grouping wil

Air System Total Heating Energy

Heat Addition to the Air Loop (Sum of all components) in Joules

Air System Total Cooling Energy Heat Removal from the Air Loop (Sum of all components) in Joules In ‘Output:Table:Monthly’ format shown below.

Probably, it is better to use other output variable to monitor the operation of the heating and cooling coil, e.g.

Output:Variable,*,Cooling Coil Total Cooling Energy,hourly;

Output:Variable,*,Cooling Coil Sensible Cooling Rate,hourly;

Output:Variable,*,Cooling Coil Sensible Cooling Energy,hourly;

Output:Variable,*,Heating Coil Total Heating Rate,hourly;

Output:Variable,*,Heating Coil Total Heating Energy,hourly;

Output:Variable,*,Heating Coil Air Heating Energy,hourly;

Output:Variable,*,Heating Coil Air Heating Rate,hourly;

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Xandrika

answered 2015-07-20 04:40:13 -0500

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@Xandrika I typically use the ...Rate [W] output variable for checking heating and/or cooling status. If you use those are the coils still fighting each other?

MatthewSteen (2015-07-20 07:11:40 -0500) edit

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As Kyle mentioned above, reporting in "detailed" frequency would tell us more what is happening at system time step. So Output:Variable, should have detailed frequency instead of hourly or timestep. Also, how Air System Total Heating Energy and Air System Total Cooling Energy are calculated in EnergyPlus is described below. Reporting each of these outputs should answer the question.

SysTotHTNG = SysFANCompHTNG+ SysHCCompHTNG+ SysHeatExHTNG+ SysHumidHTNG+ SysSolarCollectHeating+ SysUserDefinedTerminalHeating;

SysTotCLNG = SysCCCompCLNG+ SysHeatExCLNG+ SysEvapCLNG+ DesDehumidCLNG+ SysSolarCollectCooling+ SysUserDefinedTerminalCooling;

SysTotHTNG : "Air System Total Heating Energy [J]"
SysTotCLNG : "Air System Total Cooling Energy [J]"

SysFANCompHTNG : "Air System Fan Air Heating Energy [J]"
SysHCCompHTNG : "Air System Heating Coil Total Heating Energy [J]"
SysHeatExHTNG : "Air System Heat Exchanger Total Heating Energy [J]"
SysHeatExCLNG : "Air System Heat Exchanger Total Cooling Energy [J]"
SysSolarCollectHeating : "Air System Solar Collector Total Heating Energy [J]"
SysSolarCollectCooling : "Air System Solar Collector Total Cooling Energy [J]"
SysUserDefinedTerminalHeating : "Air System User Defined Air Terminal Total Heating Energy [J]"
SysUserDefinedTerminalCooling : "Air System User Defined Air Terminal Total Cooling Energy [J]"
SysHumidHTNG : "Air System Humidifier Total Heating Energy [J]" 
DesDehumidCLNG : "Air System Desiccant Dehumidifier Total Cooling Energy [J]"