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AirloopHVAC:UnitarySystem Heating with EMS

asked 2020-03-05 14:35:41 -0500

tenna031's avatar

updated 2020-03-05 20:20:30 -0500

Hey all,

I'm using an AirloopHVAC:UnitarySystem to model an RTU so I can override the heating/cooling loads using the "Sensible Load Request" for my EMS application. However, I'm seeing that the effect of this actuator is dependent on the thermostat schedule for the zone selected in the "Controlling Zone or Thermostat Location". Why is that the case? If we're overriding the sensible load request anyway, am I right in assuming a temperature setpoint should no longer be needed (or am I missing something glaringly obvious)?

Setup here: image description

my_load_input =,"my_load_input", 0)
load_actuator =,"Unitary HVAC","Sensible Load Request")
program =

program_body = <<-EMS
  SET #{load_actuator.handle.to_s} = #{my_load_input.handle.to_s}
pcm =

Note: OpenStudio doesn't have a dxCooling-heatingGas-CVFan unitary system so I'm using the Unitary-Single Speed DX cooling-Gas heat-Cycling-Elec reheat object with the supplementary coil disabled using an Always Off schedule.


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answered 2020-03-06 11:58:33 -0500

tenna031's avatar

After reading through the EnergyPlus Engineering Reference 9.2.0, I found this:

While the unitary system may be configured to serve multiple zones, system operation is controlled by a thermostat located in a single “control” zone. ... If EnergyPlus determines that the unitary system must supply heating to the control zone to meet the zone air temperature setpoint, then the unitary system model computes the total sensible heating load to be delivered to the zones being served based on the control zone sensible heating load and the control zone airflow fraction.

UnitarySystemHeatingLoad = ControlZoneHeatingLoad / ControlZoneAirFlowFraction (16.275)

What I understood from this is that the thermostat setpoints are still significant even-though you are manually setting the sensible heating/cooling load. I changed the thermostat setpoint for the controlled zone via EMS and the heating energy delivered by the system reflected the input heating setpoint.

Hope this helps folks with similar issues. Also, if someone can confirm this that would be great.


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Asked: 2020-03-05 14:35:41 -0500

Seen: 214 times

Last updated: Mar 06 '20