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Using the Indoor Air Quality option for DCV, why isn't the zone CO2 setpoint achieved?

asked 2017-02-20 12:23:21 -0500

Molly Curtz gravatar image

updated 2017-04-01 15:20:29 -0500

In Energy Plus, I am using the Indoor Air Quality (Contaminant concentration modeling) approach to simulate DCV. The zone CO2 setpoint of 1100 ppm is not met.

Below are graphs for one system, showing a "high" occupancy day in the first graph, and a "normal" occupancy day followed by two "low" occupancy days in the second graph; system cfm flow is plotted the left axis (blue = mixed air, orange = outside air) and zone C02 ppm (yellow) on the right axis. Looking at these it seems to me that although the zone CO2 setpoint is not met, the system airflow will not increase above what would be provided for space conditioning. On the "normal" day graph, the outside air (orange) is capped at the system minimum flow rate (sum of all connected terminal min flows). On the "high" day graph, the system flow increases above the system minimum, but this seems to be driven by space load rather than by CO2 concentration, as the zone CO2 setpoint is still not met.

"high" occupancy day:

image description

"Normal" followed by two "low" occupancy days:

image description

I found this post that I think is related, regarding using EMS to increase zone flow due to CO2 concentration. Is EMS the only way to do this?

As requested, here are some representative objects related to the DCV setup from my file:

DesignSpecification:OutdoorAir,
Exhall_OA,               !- Name
Sum,                     !- Outdoor Air Method
0.0031,                  !- Outdoor Air Flow per Person {m3/s-person}
0.0004;                  !- Outdoor Air Flow per Zone Floor Area {m3/s-m2}

ZoneControl:ContaminantController,
ExHallB2 DCV,            !- Name
ExHallB2:ExHallB2,       !- Zone Name
ALL ON,                  !- Carbon Dioxide Control Availability Schedule Name
Indoor CO2 Setpoint;     !- Carbon Dioxide Setpoint Schedule Name

Schedule:Constant,
Indoor CO2 Setpoint,     !- Name
Any Number,              !- Schedule Type Limits Name
1100;                    !- Hourly Value

Sizing:Zone,
ExHallB2:ExHallB2,       !- Zone or ZoneList Name
SupplyAirTemperature,    !- Zone Cooling Design Supply Air Temperature Input Method
15.3,                    !- Zone Cooling Design Supply Air Temperature {C}
,                        !- Zone Cooling Design Supply Air Temperature Difference {deltaC}
TemperatureDifference,   !- Zone Heating Design Supply Air Temperature Input Method
,                        !- Zone Heating Design Supply Air Temperature {C}
11,                      !- Zone Heating Design Supply Air Temperature Difference {deltaC}
0.0084,                  !- Zone Cooling Design Supply Air Humidity Ratio {kgWater/kgDryAir}
0.0026,                  !- Zone Heating Design Supply Air Humidity Ratio {kgWater/kgDryAir}
ExHall_OA,               !- Design Specification Outdoor Air Object Name
,                        !- Zone Heating Sizing Factor
,                        !- Zone Cooling Sizing Factor
DesignDay,               !- Cooling Design Air Flow Method
,                        !- Cooling Design Air Flow Rate {m3/s}
,                        !- Cooling Minimum Air Flow per Zone Floor Area {m3/s-m2}
,                        !- Cooling Minimum Air Flow {m3/s}
,                        !- Cooling Minimum Air Flow Fraction
DesignDay,               !- Heating Design Air Flow Method
,                        !- Heating Design Air Flow Rate {m3/s}
,                        !- Heating Maximum Air Flow per Zone Floor Area {m3/s-m2}
,                        !- Heating Maximum Air Flow {m3/s}
,                        !- Heating Maximum Air Flow Fraction
DSZAD;                   !- Design Specification Zone Air Distribution Object Name

Controller:OutdoorAir,
BF VAV System OA Controller,  !- Name
BF VAV System Relief Air Outlet,  !- Relief Air Outlet Node Name
BF VAV System Return Fan Outlet,  !- Return Air Node Name
BF VAV System Mixed Air Outlet,  !- Mixed Air ...
(more)
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Comments

Did you include the ZoneControl:ContaminantControllers? Can you post your Controller:MechanicalVentilation and some of your ZoneControl:ContaminantController?

Julien Marrec gravatar imageJulien Marrec ( 2017-02-20 16:49:52 -0500 )edit

Yes. I will edit my question to provide examples of the requested objects.Let me know if I left out any relevant ones. Sorry - I can't seem to get the Sizing:Zone object to format as a code block for some reason.

Molly Curtz gravatar imageMolly Curtz ( 2017-02-20 17:07:59 -0500 )edit

(a code block must start with at 4 spaces, you were just missing them for Sizing:Zone)

How is your AirLoopHVAC controlled? SetpointManager:Scheduled?

Julien Marrec gravatar imageJulien Marrec ( 2017-02-21 04:26:50 -0500 )edit

@Julien Marrec, thanks for improving the formatting on my question! A setpoint at the supply fan outlet node is set by a SetpointManager:Warmest (11.1C to 29.4C), and a setpoint at the supply path inlet node is set by a SetpointManager:Scheduled so that the heating coil will operate to provide a minimum supply air temp (10C).

Molly Curtz gravatar imageMolly Curtz ( 2017-02-21 13:11:59 -0500 )edit

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answered 2017-06-02 13:58:39 -0500

Molly Curtz gravatar image

updated 2017-10-25 11:20:32 -0500

Answering my own question, in the hopes it may help someone else in the future....

I found that a couple of things were happening here:

First, the ppm target (1100 ppm) that I had set was not reasonable for this high occupant density (exhibit hall) space occupancy. For information on how ventilation requirements correspond to ppm at steady state conditions Appendix A to ASHRAE 62.1 Users' Manual is helpful. Additionally, this article from Trane is helpful.

Second, neither the zone terminal supply air flow, nor the system supply air flow will increase in response to ppm exceedance at a zone, although the Controller:Outdoorair and the Controller:MechanicalVentilation will increase the outdoor air flow fraction to 100% of system flow.

Additional note which may be helpful: you can set a zone terminal outdoor air requirement, which will modulate the terminal minimum airflow if needed,(which will also impact total system airflow), using the DesignSpecification:OutdoorAir field at the air terminal object, as mentioned in this post. (The modulation would not be based directly on CO2 concentration though.)

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Asked: 2017-02-20 12:23:21 -0500

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Last updated: Oct 25 '17