Add support for variable speed coils for `SingleZoneVAV` control type in `AirLoopHVAC:UnitarySystem` #11142

lymereJ · 2025-08-05T21:16:24Z

Pull request overview

Addresses #10866: the EMS code developed by TRANE was similar to the calcSZVAVModel routine already implemented in EnergyPlus and available through the SingleZoneVAV control type for AirLoopHVAC:UnitarySystem objects. TRANE requested that we add support for variable speed coil objects.

Description of the purpose of this PR

The SingleZoneVAV control type for AirLoopHVAC:UnitarySystem currently only supports Coil:Cooling/Heating:Water or Coil:Cooling/Heating:DX:SingleSpeed coil objects. This new feature add support for Coil:Cooling/Heating:DX:VariableSpeed.

Pull Request Author

Title of PR should be user-synopsis style (clearly understandable in a standalone changelog context)
Label the PR with at least one of: Defect, Refactoring, NewFeature, Performance, and/or DoNoPublish
Pull requests that impact EnergyPlus code must also include unit tests to cover enhancement or defect repair
Author should provide a "walkthrough" of relevant code changes using a GitHub code review comment process
If any diffs are expected, author must demonstrate they are justified using plots and descriptions
If changes fix a defect, the fix should be demonstrated in plots and descriptions

Reviewer

src/EnergyPlus/SZVAVModel.cc

src/EnergyPlus/UnitarySystem.cc

lymereJ

Code walkthrough:

lymereJ · 2025-08-22T18:44:31Z

src/EnergyPlus/UnitarySystem.cc

+                } else if (this->m_SimASHRAEModel && this->m_SimASHRAEModelOn) {
+                    state.dataUnitarySystems->CompOnMassFlow = this->m_HeatMassFlowRate[this->m_NumOfSpeedHeating];
+                    state.dataUnitarySystems->CompOnFlowRatio = this->m_MSHeatingSpeedRatio[this->m_NumOfSpeedHeating];


Set the "compressor on" flow rate (and associated variable) to be the highest cooling/heating speed. This is only done when doing the SZVAV calculation. Changes below are similar.

lymereJ · 2025-08-22T18:48:56Z

tst/EnergyPlus/unit/SZVAVModel.unit.cc

+
+    state->dataGlobal->BeginEnvrnFlag = true;
+
+    // Test for zone 1 - Heating


Add new unit test and check all operating "zones" of the SZVAV control strategy for both heating and cooling operation.

lymereJ · 2025-08-22T18:49:46Z

testfiles/CMakeLists.txt

 add_simulation_test(IDF_FILE _5ZoneAirCooled_annual.idf EPW_FILE USA_IL_Chicago-OHare.Intl.AP.725300_TMY3.epw ANNUAL_SIMULATION)
 add_simulation_test(IDF_FILE _5ZoneAirCooled_LeapYear_annual.idf EPW_FILE MadeUpLeapYear.epw ANNUAL_SIMULATION)
 add_simulation_test(IDF_FILE UnitarySystem_MultiSpeedDX_EconoStaging.idf EPW_FILE USA_IL_Chicago-OHare.Intl.AP.725300_TMY3.epw ANNUAL_SIMULATION)
+add_simulation_test(IDF_FILE UnitarySystem_VariableSpeedDX_SZVAV.idf EPW_FILE USA_IL_Chicago-OHare.Intl.AP.725300_TMY3.epw ANNUAL_SIMULATION)


Add new example file; Results shown in the PR are generated from this file.

lymereJ · 2025-08-22T18:52:09Z

src/EnergyPlus/UnitarySystem.hh

        bool m_ValidASHRAEHeatCoil = false;
-        bool m_SimASHRAEModel = false; // flag denoting that ASHRAE model (SZVAV) should be used
+        bool m_SimASHRAEModel = false;   // flag denoting that ASHRAE model (SZVAV) should be used
+        bool m_SimASHRAEModelOn = false; // flag denoting that the SZVAV calculation is active


New flag to denote that the SZVAV calculation are being performed. This allows the initial calls to calcUnitarySystemToLoad in controlUnitarySystemOutput to not be based on the SZVAV flow rates, for example when determining which speed to pick to meet the load.

lymereJ · 2025-08-22T18:53:51Z

src/EnergyPlus/UnitarySystem.cc

+                if (this->m_SimASHRAEModel) {
+                    this->m_HeatingCycRatio = PartLoadRatio;
+                    this->m_HeatingSpeedRatio = 1.0;
+                } else if (this->m_SingleMode == 0) {


Single mode-like operation for SZVAV calcs.

lymereJ · 2025-08-22T18:54:45Z

src/EnergyPlus/UnitarySystem.cc

+            if (this->m_SimASHRAEModel) {
+                CoilPLR = PartLoadRatio;
+            } else if (this->m_CoolingSpeedNum > 1) {


Passing the actual PLR when running the SZVAV control strategy.

lymereJ · 2025-08-22T19:18:15Z

Results from the example file:

Cooling Operation

System flow rate (m3/s) as a function of predicated load (W)
- Single-speed DX Coil:
- Variable-speed (3-speed, represented by colors) DX Coil:
System supply air temperature (deg. C) as a function of predicated load (W), min SAT is set to 10 deg. C in the model
- Single-speed DX Coil:
- Variable-speed (3-speed, represented by colors) DX Coil:

Heating Operation

System flow rate (m3/s) as a function of predicated load (W)
- Single-speed DX Coil:
- Variable-speed (3-speed, represented by colors) DX Coil:
System supply air temperature (deg. C) as a function of predicated load (W), max SAT is set to 35 deg. C in the model
- Single-speed DX Coil:
- Variable-speed (3-speed, represented by colors) DX Coil:

nrel-bot-2 · 2025-09-24T16:01:03Z

@lymereJ @Myoldmopar it has been 33 days since this pull request was last updated.

mitchute · 2025-09-30T00:47:46Z

tst/EnergyPlus/unit/SZVAVModel.unit.cc

+        "  Coil:Cooling:DX:VariableSpeed,",
+        "    Sys 2 Furnace DX Cool Cooling Coil,                       !- Name",
+        "    Sys 2 Furnace DX Cool Mixed Air Outlet,                       !- Indoor Air Inlet Node Name",
+        "    Sys 2 Furnace DX Cool Cooling Coil Outlet,                       !- Indoor Air Outlet Node Name",
+        "    3,                         !- Number of Speeds {dimensionless}",
+        "    3,                         !- Nominal Speed Level {dimensionless}",
+        "    3513.72009,                  !- Gross Rated Total Cooling Capacity At Selected Nominal Speed Level {W}",


Not the most critical issue, but can you cleanup the comment alignment a bit here to help with readability?

@mitchute, done!

src/EnergyPlus/UnitarySystem.cc

mitchute · 2025-10-01T17:30:57Z

We'll try this one more time. It builds fine locally on my Arm64 Mac.

NPF and initial code changes.

eab5c6a

lymereJ added this to the EnergyPlus 25.2 IO Freeze milestone Aug 5, 2025

lymereJ added NewFeature Includes code to add a new feature to EnergyPlus NotIDDChange Code does not impact IDD (can be merged after IO freeze) labels Aug 5, 2025

rraustad reviewed Aug 12, 2025

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src/EnergyPlus/SZVAVModel.cc Outdated Show resolved Hide resolved

rraustad reviewed Aug 12, 2025

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src/EnergyPlus/UnitarySystem.cc Show resolved Hide resolved

lymereJ added 8 commits August 21, 2025 13:34

Add heating coil.

6bb2e1f

Add unit test.

b91f8dc

Update docs.

b70ef41

Add example file to the list of test files.

352f301

Update example file.

2376dee

Fix typos and remove tabs in the example file.

dc6591f

Only use adjusted flow rates for SZVAV calcs.

bdac766

Remove incorrect include statement.

f0d8224

lymereJ commented Aug 22, 2025

View reviewed changes

lymereJ marked this pull request as ready for review August 22, 2025 19:18

mitchute self-requested a review September 24, 2025 18:39

mjwitte modified the milestones: EnergyPlus 25.2 IO Freeze, EnergyPlus 25.2 Sep 25, 2025

Merge branch 'develop' into szvav_variable_speed

c558d8c

mitchute reviewed Sep 30, 2025

View reviewed changes

lymereJ and others added 3 commits September 30, 2025 13:13

Fix comment alignement.

d567ffd

Fix unit test failure after merge.

d894dcf

Merge branch 'develop' into szvav_variable_speed

dbf5c20

mitchute and others added 2 commits October 1, 2025 12:16

use logical AND instead of bitwise AND

a61fbf6

One more...

07f2868

rraustad approved these changes Oct 10, 2025

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		state->dataGlobal->BeginEnvrnFlag = true;

		// Test for zone 1 - Heating

Add support for variable speed coils for SingleZoneVAV control type in AirLoopHVAC:UnitarySystem #11142

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lymereJ commented Aug 5, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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Pull request overview

Description of the purpose of this PR

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lymereJ commented Aug 22, 2025

Cooling Operation

Heating Operation

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nrel-bot-2 commented Sep 24, 2025

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mitchute commented Oct 1, 2025

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Add support for variable speed coils for `SingleZoneVAV` control type in `AirLoopHVAC:UnitarySystem` #11142

Add support for variable speed coils for `SingleZoneVAV` control type in `AirLoopHVAC:UnitarySystem` #11142

lymereJ commented Aug 5, 2025 •

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