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Fix 10802 - Divide by zero error #11348

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Merged
mitchute merged 5 commits into from
Dec 17, 2025
Merged

Fix 10802 - Divide by zero error #11348

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c245793
Fix 10802 - Divide by zero error
kevin-moos Nov 21, 2025
e494b10
Merge branch 'develop' into Fix-10802-Divide-by-zero-in-controlCoolin…
kevin-moos Nov 21, 2025
25cb974
Fix 10802 - Unit tests for divide by zero error
kevin-moos Nov 24, 2025
3b384bd
Merge branch 'develop' into Fix-10802-Divide-by-zero-in-controlCoolin…
kevin-moos Dec 10, 2025
bc6b68b
Ensure m_CoolingSpeedNum is at least 1
kevin-moos Dec 10, 2025
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11 changes: 8 additions & 3 deletions src/EnergyPlus/UnitarySystem.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -14130,6 +14130,7 @@ namespace UnitarySystems {

if (OutletHumRatLS > DesOutHumRat) {
CycRatio = 1.0;
this->m_CoolingSpeedNum = std::max(1, this->m_CoolingSpeedNum);

for (int speedNum = this->m_CoolingSpeedNum; speedNum <= this->m_NumOfSpeedCooling; ++speedNum) {
VariableSpeedCoils::SimVariableSpeedCoils(state,
Expand Down Expand Up @@ -14331,7 +14332,8 @@ namespace UnitarySystems {
++this->warnIndex.m_SensPLRIter;
ShowWarningError(state,
format("{} - Iteration limit exceeded calculating part-load ratio for unit = {}", this->UnitType, this->Name));
ShowContinueError(state, format("Estimated part-load ratio = {:.3R}", (ReqOutput / FullOutput)));
ShowContinueError(state,
format("Estimated part-load ratio = {:.3R}", (FullOutput != 0 ? (ReqOutput / FullOutput) : PartLoadFrac)));
ShowContinueError(state, format("Calculated part-load ratio = {:.3R}", PartLoadFrac));
ShowContinueErrorTimeStamp(state, "The calculated part-load ratio will be used and the simulation continues. Occurrence info:");
} else {
Expand All @@ -14345,7 +14347,9 @@ namespace UnitarySystems {
}
}
} else if (SolFla == -2) {
PartLoadFrac = ReqOutput / FullOutput;
if (FullOutput != 0) {
PartLoadFrac = ReqOutput / FullOutput;
}
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@rraustad rraustad Nov 25, 2025 •
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There is likely a reason (a developer oversight) that FullOutput has not been calculated. Usually FullOutput is set near the beginning of this function when PLR is set to 1 and the system capacity is checked to see if that capacity exceeds the load, otherwise PLR = 1 and more calculations are not needed. See line 13249, 13681 and 13732. One thing is for sure, if this function is iterating with SolveRoot then the system does have a non-zero FullOutput (or maybe the coil is scheduled off? but I would think that would be caught early). To figure out the why you would need to step through this function at the time the error occurred.

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@mitchute mitchute Nov 26, 2025

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@rraustad I believe this catches the issue noted, but I'll hold this open for a little bit in case you want to take another quick look for any other higher level issues.

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@rraustad rraustad Dec 1, 2025 •
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I added at line 14133:

if (OutletHumRatLS > DesOutHumRat) {
    CycRatio = 1.0;
    this->m_CoolingSpeedNum = std::max(1, this->m_CoolingSpeedNum);  <-- new line since speedNum was 0

    for (int speedNum = this->m_CoolingSpeedNum; speedNum <= this->m_NumOfSpeedCooling; ++speedNum) {

The reason SolveRoot was failing was that a sensible load did not exist and FullOutput was not calculated and remained 0 from initialization (line 12999). There was, however, a latent load when there should NOT have been a latent load given these inputs. Adding the line above did allow SolveRoot to find the correct PLR and no longer fails with SolFla = -2. But there is another logic issue somewhere, I have not found that yet. I guess this change is OK but avoiding this calculation if FullOutput = 0 is not really the correct thing to do (because calling SolveRoot means the system is trying to meet a load and therefore the system should have a non-zero capacity) since if SolveRoot does fail with -2 then PLR will default to 0 and it would be prudent to at least try to find an operating PLR, as faulty as this could be (i.e., trying to find PLR based on PartLoadFrac = ReqOutput / FullOutput).

CoilSystem:Cooling:DX,
  GUID11_SYS0SCC-1System,
  None,   !- Dehumidification Control Type
  Yes,    !- Run on Sensible Load
  No,     !- Run on Latent Load

CoilSystem:Cooling:DX,
  A10,  \field Run on Latent Load
    \type choice
    \key Yes
    \key No
    \default No
    \note If Yes, unit will run if there is a latent load.
    \note even if there is no sensible load.
    \note If No, unit will not run only if there is a latent load.
    \note Dehumidification controls will be active if specified.

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@rraustad rraustad Dec 1, 2025

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A more important point is that SolveRoot should never fail with -2. That in itself is a developer error.

if (!state.dataGlobal->WarmupFlag) {
if (this->warnIndex.m_SensPLRFail < 1) {
++this->warnIndex.m_SensPLRFail;
Expand Down Expand Up @@ -14373,7 +14377,8 @@ namespace UnitarySystems {
++this->warnIndex.m_LatPLRIter;
ShowWarningError(
state, format("{} - Iteration limit exceeded calculating latent part-load ratio for unit = {}", this->UnitType, this->Name));
ShowContinueError(state, format("Estimated part-load ratio = {:.3R}", (ReqOutput / FullOutput)));
ShowContinueError(state,
format("Estimated part-load ratio = {:.3R}", (FullOutput != 0 ? (ReqOutput / FullOutput) : PartLoadFrac)));
ShowContinueError(state, format("Calculated part-load ratio = {:.3R}", PartLoadFrac));
ShowContinueErrorTimeStamp(state, "The calculated part-load ratio will be used and the simulation continues. Occurrence info:");
}
Expand Down
34 changes: 34 additions & 0 deletions tst/EnergyPlus/unit/HVACDXSystem.unit.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -639,6 +639,23 @@ TEST_F(EnergyPlusFixture, VariableSpeedCoils_RHControl)
EXPECT_NEAR(state->dataLoopNodes->Node(ControlNode).HumRat, state->dataLoopNodes->Node(ControlNode).HumRatMax, 0.0000001);
// latent load needed to increase compressor speed to speed 4
EXPECT_EQ(4, thisSys->m_CoolingSpeedNum);

// test for divide by zero error in controlCoolingSystemToSP
thisSys->m_DesiredOutletTemp = 23.888888888888900;
thisSys->m_DesiredOutletHumRat = 0.0092857142857142895;
state->dataLoopNodes->Node(InletNode).MassFlowRate = 1.3840962084222401;
state->dataLoopNodes->Node(InletNode).Temp = 16.566173051926114;
state->dataLoopNodes->Node(InletNode).HumRat = 0.0092873376541228961;
state->dataLoopNodes->Node(InletNode).HumRatMax = -999;

has_err_output(true);
EXPECT_NO_THROW(thisSys->controlCoolingSystemToSP(*state, airLoopNum, FirstHVACIteration, HXUnitOn, CompressorOn));
EXPECT_EQ(thisSys->m_CoolingPartLoadFrac, 0);
std::string const expected_error =
" ** Warning ** CoilSystem:Cooling:DX - sensible part-load ratio calculation failed: part-load ratio limits exceeded, for unit = DX "
"COOLING COIL SYSTEM\n ** ~~~ ** Estimated part-load ratio = 0.000\n ** ~~~ ** The estimated part-load ratio will be used and "
"the simulation continues. Occurrence info:\n ** ~~~ ** Environment=, at Simulation time= 00:-15 - 00:00\n";
compare_err_stream(expected_error, true);
}

TEST_F(EnergyPlusFixture, VariableSpeedCoils_LatentDegradation_Test)
Expand Down Expand Up @@ -1149,6 +1166,23 @@ TEST_F(EnergyPlusFixture, NewDXCoilModel_RHControl)
EXPECT_LT(outHumRat3, outHumRat1); // lower outlet humrat with multimode's alternate operating mode
EXPECT_NEAR(outHumRat1, 0.01166, 0.0001); // sensible control yields higher outlet humrat
EXPECT_NEAR(outHumRat3, 0.01119, 0.0001); // multimode control yields lower outlet humrat

// test for divide by zero error in controlCoolingSystemToSP
thisSys->m_DesiredOutletTemp = 23.888888888888900;
thisSys->m_DesiredOutletHumRat = 0.0092857142857142895;
state->dataLoopNodes->Node(InletNode).MassFlowRate = 1.3840962084222401;
state->dataLoopNodes->Node(InletNode).Temp = 16.566173051926114;
state->dataLoopNodes->Node(InletNode).HumRat = 0.0092873376541228961;
state->dataLoopNodes->Node(InletNode).HumRatMax = -999;

has_err_output(true);
EXPECT_NO_THROW(thisSys->controlCoolingSystemToSP(*state, airLoopNum, FirstHVACIteration, HXUnitOn, CompOn));
EXPECT_EQ(thisSys->m_CoolingPartLoadFrac, 0);
std::string const expected_error =
" ** Warning ** CoilSystem:Cooling:DX - sensible part-load ratio calculation failed: part-load ratio limits exceeded, for unit = DX "
"COOLING COIL SYSTEM\n ** ~~~ ** Estimated part-load ratio = 0.000\n ** ~~~ ** The estimated part-load ratio will be used and "
"the simulation continues. Occurrence info:\n ** ~~~ ** Environment=, at Simulation time= 00:-15 - 00:00\n";
compare_err_stream(expected_error, true);
}

} // namespace EnergyPlus