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How to override heating supply temperature with EMS?

asked 2021-02-12 16:02:08 -0500

rafael.alonso gravatar image

updated 2021-04-07 13:06:15 -0500

Hi all,

I´m working on Energy plus in a model that supplies fresh air into a few occupied Zones, which is preheated in an unocupied underground zone.

The HVAC is only modelled at Ideal Air Loads level, to estimate the space heat demand and natural ventilation is modelled via AFN in the occupied spaces.

Since the AFN is not compatible with Zone Mixing nor Earth Tube objects, I have been exploring ways to introduce this preheated air for ventilation in the occupied zones. I found a good solution with the Ideal Air Loads/Air Temperature actuator in the EMS, which I have set to follow the air temperature in the underground zone.

The EMS programe looks like this, with a sensor for the Zone Mean Air temperature in the underground space:

*EnergyManagementSystem:Actuator,
FreshAirTemp_0AtelierS_0,
0AtelierS_0 Ideal Loads Air System,
Ideal Loads Air System,
Air temperature;

EnergyManagementSystem:Program,
0AtelierS_0,
Set FreshAirTemp_0AtelierS_0 = GalleryAirTemp;

But something strange happens. The results show a ten fold increase in heating demand. I checked and the Air Temperature in the Gallery is always higher than outdoors, so it doesn´t make too much sense to me.

After checking carefully, I found that the Ideal Air Loads system inputs air at 50ºC when heating is on. As my EMS code overrides that temperature with a lower one, heating demand increases (comparison graph below).

image description

My question, How can I implement this Ideal Air Loads to achieve the folllowing two objectives?

  • When heating is not required (zone temp > setpoint) , fresh air is supplied at the underground temperature. This works so far
  • When heating is required (zone temp < setpoint), fresh air is heated from the gallery temperature to supply temp. (instead of outdoors to supply temp) to reduce the load.

Thanks in advance, Rafael

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answered 2021-02-13 10:39:47 -0500

updated 2021-02-16 10:14:49 -0500

You will need to add two more EMS sensors:

  • Zone Mean Air Temperature of the occupied zone(s). Let's use an example sensor name of OccupiedAirTemp.
  • Schedule Value for the heating setpoint schedule assigned to occupied zone(s) thermostat (ZoneControl:Thermostat object, e.g.). Let's use an example sensor name of OccupiedHeatSetpt.

For the pre-heating of Gallery air, the simplest approach would be to assign the Gallery exhaust air node as the System Inlet Air Node Name of the Ideal Loads Air System object. That should mix Gallery air with outdoor air and occupied space exhaust air before being conditioned by the Ideal Loads Air System.

Then, your EMS program will use IF and ELSE statements to set up your logic.

EnergyManagementSystem:Program,
  0AtelierS_0,
  IF OccupiedAirTemp < OccupiedHeatSetpt, ! Heating required
  SET FreshAirTemp_0AtelierS_0 = 50,      ! Set Ideal Loads supply air temp to 50C
  ELSE,                                   ! Heating not required
  SET FreshAirTemp_0AtelierS_0 = GalleryAirTemp, ! Set Ideal Loads supply air temp to Gallery temp
  END;

If this approach doesn't work for pre-heating, then you will also need one more EMS Actuator:

Then, your EMS program will add a few lines to set OutdoorDrybulb:

EnergyManagementSystem:Program,
  0AtelierS_0,
  IF OccupiedAirTemp < OccupiedHeatSetpt, ! Heating required
  SET OutdoorDrybulb = GalleryAirTemp,      ! Set outdoor drybulb equal to Gallery temp (pre-heating)
  SET FreshAirTemp_0AtelierS_0 = 50,      ! Set Ideal Loads supply air temp to 50C
  ELSE,                                   ! Heating not required
  SET OutdoorDrybulb = NULL,      ! Reset outdoor drybulb to EPW data
  SET FreshAirTemp_0AtelierS_0 = GalleryAirTemp, ! Set Ideal Loads supply air temp to Gallery temp
  END;

Note the use of "NULL" to reset the OutdoorDrybulb actuator back to its original settings (EPW data). This "NULL" keyword can be applied to reset any actuator.

I should point out that the OutdoorDrybulb actuator should override the OutdoorAir:Node object assigned to the Ideal Loads Air System's Outdoor Air Inlet Node Name field. If you only have one OutdoorAir:Node object for the entire model, you should create a second one just for the Ideal Loads Air System that will be altered using the EMS program. The original OutdoorAir:Node object will then be unaffected and keep original outdoor air conditions throughout the rest of the model.

One more thing -- depending upon your pre-heating strategy (100% OA vs. mixing a specific portion of OA with Gallery air, e.g.), you can set the flow rate of pre-heated outdoor air provided from the Gallery to the occupied spaces using the "Outdoor Air Mass Flow Rate" actuator of the Ideal Loads Air System.

As with any project using EMS, I would definitely recommend analyzing outputs at a timestep frequency to test out this logic comparison of zone air temperature to heating setpoint schedule. It may work better to use one EMS sensor for predicted load to heating setpoint and check when that is positive instead of comparing the zone air temperature to heating setpoint schedule.

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Comments

Thanks for the tip Aaron.

Wouldn´t this solution ignore the fact that incoming outdoor air is pre-heated when Heating is ON (OccupiedAirTemp < OccupiedHeatSetpt) ?

There are some savings there as the starting air temperature is higher than outdoors by 5-7ºC, but Im not that sure any more if this can be accounted for using an Ideal Air Loads system.

rafael.alonso gravatar image rafael.alonso  ( 2021-02-15 03:31:25 -0500 )edit

Ah, I missed the pre-heating part of your question. You will need to connect the Gallery air as a Supply inlet air induced by the Ideal Loads Air System, or if that doesn't work, add an actuator for resetting the Outdoor Air Drybulb Temperature entering the Ideal Loads Air System to be equal to GalleryAirTemp. I've updated my answer.

Aaron Boranian gravatar image Aaron Boranian  ( 2021-02-16 10:15:03 -0500 )edit

Actually, with that OutdoorDrybulb actuator I can just replace the outdoor air node temperature in the Ideal Air Loads systems affected.

I have tested it and works pretty well:

  • When no additional heating is required, the air just passes through into the zone, at the Gallery temperature

  • When additional heating is ON, then that temperature is the starting point for the system.

Maybe I missed something, I have just set the Outdoor air node as the air temperature in the gallery all the time, without any other IF condition.

rafael.alonso gravatar image rafael.alonso  ( 2021-02-18 05:51:35 -0500 )edit

Sure, if the conditioned zones always have their supply air provided from the Gallery, that would make sense to set the OutdoorDrybulb actuator without an IF condition. Again, be careful that this isn't the only outdoor air node in the model that you are overriding, which would affect outdoor air conditions of other HVAC components in the model besides the Ideal Loads Air System.

Aaron Boranian gravatar image Aaron Boranian  ( 2021-02-18 08:25:27 -0500 )edit

Yes, I had to create an Outdoor node per Ideal Air Loads system that is fed from the gallery, since not all the zones are connected there. Thank you for your help.

rafael.alonso gravatar image rafael.alonso  ( 2021-02-18 08:39:33 -0500 )edit

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Asked: 2021-02-12 16:02:08 -0500

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Last updated: Feb 18