Design Size Speed overwriting User-Specified Speed without obvious autosizing???

asked 2019-07-18 13:29:07 -0500

mldichter gravatar image

updated 2019-07-22 16:58:12 -0500

I have an IDF file where I changed the cooling and heating thermostat setpoints and now I'm getting this warning.

SizeDXCoil: Potential issue with equipment sizing for Coil:Cooling:DX:MultiSpeed COOLING COIL
User-Specified Speed 1 Rated Air Flow Rate [m3/s] = 0.49829
differs from Design Size Speed 1 Rated Air Flow Rate [m3/s] = 0.37600
This may, or may not, indicate mismatched component sizes.
Verify that the value entered is intended and is consistent with other components.

Plus a bunch of these warnings.

SimHVAC: Maximum iterations (20) exceeded for all HVAC loops

All of my values are hardcoded in the IDF as far as I know, and I haven't been able to find any object fields that would calculate this value for the simulation. I pasted the relevant IDF objects as text below. In case you need the whole IDF file, here is a google drive link. (The IDF objects are a little weird since we're trying to verify some experimental data matches our EnergyPlus simulation data by replicating all the environment and zone conditions and comparing the experimental to simulation data.)

BEGIN UPDATE
The 0.37600 [m3/s] Design Size is manifesting itself as another discrete Outdoor Air flow rate. The objects model a discrete speed heat pump. There are Ventilation, Low-Speed, and High-Speed modes. These modes correspond to three different supply air flows. When the Design Size Speed warning is reported, another discrete outdoor air flow appears between the Low-Speed and High-Speed air flows corresponding to the 0.37600 [m3/s]. So the warning is definitely legitimate and my user specified flow rates are being overwritten.

THIS GRAPH IN kg/s, not m3/s DENSITY OF AIR ~ 1.22 kg/m3
0.3760 m3/s ~ 0.45 kg/s which is off from the graph
image description
END UPDATE

BEGIN WORKAROUND
I was able to force the correct OA flow rate using an EMS variable specifically for that case. I am still getting that Warning though, so EnergyPlus is still trying to Design Size the OA flow, but EMS is overwriting it at every iteration of the HVAC loop.
END WORKAROUND

How do I prevent the Design Size Speed 1 Rated Air Flow Rate [m3/s] from being calculated?

!-   ===========  ALL OBJECTS IN CLASS: VERSION ===========

Version,
    9.1;                     !- Version Identifier


!-   ===========  ALL OBJECTS IN CLASS: SIMULATIONCONTROL ===========

SimulationControl,
    No,                      !- Do Zone Sizing Calculation
    No,                      !- Do System Sizing Calculation
    No,                      !- Do Plant Sizing Calculation
    No,                      !- Run Simulation for Sizing Periods
    Yes,                     !- Run Simulation for Weather File Run Periods
    No,                      !- Do HVAC Sizing Simulation for Sizing Periods
    1;                       !- Maximum Number of HVAC Sizing Simulation Passes


!-   ===========  ALL OBJECTS IN CLASS: AIRTERMINAL:SINGLEDUCT:UNCONTROLLED ===========

AirTerminal:SingleDuct:Uncontrolled,
    Air Terminal Single Duct Uncontrolled 1,  !- Name
    ,                        !- Availability Schedule Name
    Node 6,                  !- Zone Supply Air Node Name
    0.752;                   !- Maximum Air Flow Rate {m3/s}


!-   ===========  ALL OBJECTS IN CLASS: FAN:SYSTEMMODEL ===========

Fan:SystemModel,
    Fan,                     !- Name
    School in Session Boolean,  !- Availability Schedule Name
    Mixed Air Node,          !- Air Inlet Node Name
    Fan Outlet,              !- Air Outlet Node ...
(more)
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