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Correct VAV maximum air flow fraction during reheat for heating dominated use cases #10763

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Feb 7, 2025
Merged

Correct VAV maximum air flow fraction during reheat for heating dominated use cases #10763

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90f2846
Correct VAV maximum air flow fraction during reheat
rraustad Sep 24, 2024
73fcfff
Revised based on using DesHeatVolFlowMax as limit
rraustad Sep 24, 2024
d29faa1
Update 2 unit tests
rraustad Sep 24, 2024
3473759
Fix reheat coil air flow rate used for coil sizing
rraustad Oct 23, 2024
1129a59
Merge branch 'develop' of https://github.com/NREL/EnergyPlus into 107…
rraustad Oct 23, 2024
9051fe9
Correct ReverseWithLimits heating coil air flow used for sizing
rraustad Feb 4, 2025
83351a9
Merge branch 'develop' of https://github.com/NREL/EnergyPlus into 107…
rraustad Feb 4, 2025
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39 changes: 31 additions & 8 deletions src/EnergyPlus/SingleDuct.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -2335,8 +2335,15 @@ void SingleDuctAirTerminal::SizeSys(EnergyPlusData &state)

CheckZoneSizing(state, this->sysType, this->SysName);

MaxAirVolFlowRateDes = max(state.dataSize->TermUnitFinalZoneSizing(state.dataSize->CurTermUnitSizingNum).DesCoolVolFlow,
state.dataSize->TermUnitFinalZoneSizing(state.dataSize->CurTermUnitSizingNum).DesHeatVolFlow);
Real64 heatingMaxFlow;
if (this->DamperHeatingAction == Action::ReverseWithLimits &&
state.dataSize->TermUnitFinalZoneSizing(state.dataSize->CurTermUnitSizingNum).DesHeatVolFlow >
state.dataSize->TermUnitFinalZoneSizing(state.dataSize->CurTermUnitSizingNum).DesHeatVolFlowMax) {
heatingMaxFlow = state.dataSize->TermUnitFinalZoneSizing(state.dataSize->CurTermUnitSizingNum).DesHeatVolFlowMax;
} else {
heatingMaxFlow = state.dataSize->TermUnitFinalZoneSizing(state.dataSize->CurTermUnitSizingNum).DesHeatVolFlow;
}
MaxAirVolFlowRateDes = max(state.dataSize->TermUnitFinalZoneSizing(state.dataSize->CurTermUnitSizingNum).DesCoolVolFlow, heatingMaxFlow);
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If ReverseWithLimits uses a different heating air flow rate from the zone design heating air flow rate then that flow rate should be used as the value for the VAV box heating air flow rate.

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@rraustad rraustad Sep 24, 2024 •
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Well, I was expecting DesHeatVolFlowMax to always be less than DesHeatVolFlow. For 5ZoneBoilerOutsideAirReset that's not the case. The diffs show that Zone 5 TU max air flow rate increased, I was only expecting decreases in max air flow rate for heating dominated zones. Zone cooling design air flow is greater than zone heating design air flow, but DesHeatVolFlowMax is larger than both of these.

image

image

// set the zone maximum heating supply air flow rate. This will be used for autosizing VAV terminal unit
// max heating flow rates
zsFinalSizing.DesHeatVolFlowMax = max(zsFinalSizing.DesHeatMaxAirFlow,
                                      zsFinalSizing.DesHeatMaxAirFlow2,
                                      max(zsFinalSizing.DesCoolVolFlow, zsFinalSizing.DesHeatVolFlow) * zsFinalSizing.DesHeatMaxAirFlowFrac);

zsFinalSizing.DesHeatMaxAirFlow2 = zsFinalSizing.DesHeatMaxAirFlowPerArea * floorArea * zoneMult;
                                 = 0.002032 * 182.49 * 1 = 0.37082

N8 , \field Maximum Flow Fraction During Reheat
     \type real
     \autosizable
     \default autosize
     \note Used only when Reheat Coil Object Type = Coil:Heating:Water and Damper Heating Action = ReverseWithLimits
     \note When autocalculating, the maximum flow fraction is set to the ratio of
     \note 0.002032 m3/s-m2 (0.4 cfm/sqft) multiplied by the zone floor area and the
     \note Maximum Air Flow Rate.
     \note This optional field limits the maximum flow allowed in reheat mode.
     \note At no time will the maximum flow rate calculated here exceed the value of
     \note Maximum Air Flow Rate.

So is DesHeatVolFlowMax just an upper limit? or is it the expected heating air flow rate?

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@rraustad rraustad Sep 24, 2024 •
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So, since DesHeatVolFlowMax looks like a limit and not a flow rate, maybe I should change this line and line 3224 to?:

Real64 heatingMaxFlow = (DesHeatVolFlowMax > DesHeatVolFlow) ? state.dataSize->TermUnitFinalZoneSizing(state.dataSize->CurTermUnitSizingNum).DesHeatVolFlow :
    state.dataSize->TermUnitFinalZoneSizing(state.dataSize->CurTermUnitSizingNum).DesHeatVolFlowMax);

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Based on this discussion I changed to treat DesHeatVolFlowMax as an upper limit. This should still correct the issue and result in fewer diffs.


if (MaxAirVolFlowRateDes < SmallAirVolFlow) {
MaxAirVolFlowRateDes = 0.0;
Expand Down Expand Up @@ -2385,7 +2392,13 @@ void SingleDuctAirTerminal::SizeSys(EnergyPlusData &state)
}
} else {
CheckZoneSizing(state, this->sysType, this->SysName);
MaxHeatAirVolFlowRateDes = state.dataSize->TermUnitFinalZoneSizing(state.dataSize->CurTermUnitSizingNum).DesHeatVolFlow;
if (this->DamperHeatingAction == Action::ReverseWithLimits &&
state.dataSize->TermUnitFinalZoneSizing(state.dataSize->CurTermUnitSizingNum).DesHeatVolFlow >
state.dataSize->TermUnitFinalZoneSizing(state.dataSize->CurTermUnitSizingNum).DesHeatVolFlowMax) {
MaxHeatAirVolFlowRateDes = state.dataSize->TermUnitFinalZoneSizing(state.dataSize->CurTermUnitSizingNum).DesHeatVolFlowMax;
} else {
MaxHeatAirVolFlowRateDes = state.dataSize->TermUnitFinalZoneSizing(state.dataSize->CurTermUnitSizingNum).DesHeatVolFlow;
}
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Below at line 3224, ReverseWithLimits uses DesHeatVolFlowMax. That value should also be used here.

3224: MaxAirVolFlowRateDuringReheatDes = state.dataSize->TermUnitFinalZoneSizing(state.dataSize->CurTermUnitSizingNum).DesHeatVolFlowMax;

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@rraustad rraustad Sep 25, 2024 •
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As discussed above, these 2 areas of code, above (2937) and below (3231), were changed to use DesHeatVolFlowMax as a limit.

if (MaxHeatAirVolFlowRateDes < SmallAirVolFlow) {
MaxHeatAirVolFlowRateDes = 0.0;
}
Expand Down Expand Up @@ -2672,7 +2685,13 @@ void SingleDuctAirTerminal::SizeSys(EnergyPlusData &state)
if (state.dataSize->ZoneSizingRunDone) {
if (state.dataSize->CurTermUnitSizingNum > 0) {
// if zone sizing run done, set the design max reheat air flow to the value from the design calcs
MaxAirVolFlowRateDuringReheatDes = state.dataSize->TermUnitFinalZoneSizing(state.dataSize->CurTermUnitSizingNum).DesHeatVolFlowMax;
if (state.dataSize->TermUnitFinalZoneSizing(state.dataSize->CurTermUnitSizingNum).DesHeatVolFlow >
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Not sure why we need this same (or very similar logic) in multiple places, but not holding up this PR for that. It works as-is.

state.dataSize->TermUnitFinalZoneSizing(state.dataSize->CurTermUnitSizingNum).DesHeatVolFlowMax) {
MaxAirVolFlowRateDuringReheatDes =
state.dataSize->TermUnitFinalZoneSizing(state.dataSize->CurTermUnitSizingNum).DesHeatVolFlowMax;
} else {
MaxAirVolFlowRateDuringReheatDes = state.dataSize->TermUnitFinalZoneSizing(state.dataSize->CurTermUnitSizingNum).DesHeatVolFlow;
}
}
} else {
// if no design calc use 0.002032 [m3/s-m2] times floor area. That's .40 cfm/ft2
Expand Down Expand Up @@ -2868,9 +2887,13 @@ void SingleDuctAirTerminal::SizeSys(EnergyPlusData &state)
max(state.dataSize->TermUnitFinalZoneSizing(state.dataSize->CurTermUnitSizingNum).NonAirSysDesHeatVolFlow,
this->MaxAirVolFlowRate * this->ZoneTurndownMinAirFrac);
} else {
TermUnitSizing(state.dataSize->CurTermUnitSizingNum).AirVolFlow =
max(state.dataSize->TermUnitFinalZoneSizing(state.dataSize->CurTermUnitSizingNum).NonAirSysDesHeatVolFlow,
this->MaxAirVolFlowRate * this->ZoneMinAirFracDes * this->ZoneTurndownMinAirFrac);
if (this->SysType_Num == SysType::SingleDuctVAVReheat && this->DamperHeatingAction == Action::ReverseWithLimits) {
TermUnitSizing(state.dataSize->CurTermUnitSizingNum).AirVolFlow = this->MaxAirVolFlowRateDuringReheat;
} else {
TermUnitSizing(state.dataSize->CurTermUnitSizingNum).AirVolFlow =
max(state.dataSize->TermUnitFinalZoneSizing(state.dataSize->CurTermUnitSizingNum).NonAirSysDesHeatVolFlow,
this->MaxAirVolFlowRate * this->ZoneMinAirFracDes * this->ZoneTurndownMinAirFrac);
}
}
} else {
if (this->SysType_Num == SysType::SingleDuctVAVReheatVSFan) {
Expand Down Expand Up @@ -3530,7 +3553,7 @@ void SingleDuctAirTerminal::SimVAV(EnergyPlusData &state, bool const FirstHVACIt
this->ControlCompTypeNum,
this->CompErrIndex,
ZoneNodeNum,
SysOutletNode); // why not QZnReq ?
SysOutletNode);
// air flow controller, not on plant, don't pass plant topology info
// reset terminal unit inlet air mass flow to new value.
state.dataLoopNodes->Node(this->OutletNodeNum).MassFlowRateMaxAvail = this->sd_airterminalInlet.AirMassFlowRateMaxAvail;
Expand Down
64 changes: 55 additions & 9 deletions tst/EnergyPlus/unit/SingleDuct.unit.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -1294,7 +1294,7 @@ TEST_F(EnergyPlusFixture, SingleDuct_ZeroFloorAreaTest)
// zone floor area of zone 1 = 0, zone 2 > 0. Expect TU MaxAirVolFlowRateDuringReheat = 0 only for zone 1.
// this test isn't relevant anymore since defaulting is done differently
Real64 MaxAirVolFlowRateDuringReheatDes =
min(state->dataSize->FinalZoneSizing(1).DesHeatVolFlowMax, state->dataSingleDuct->sd_airterminal(1).MaxAirVolFlowRate);
min(state->dataSize->FinalZoneSizing(1).DesHeatVolFlow, state->dataSingleDuct->sd_airterminal(1).MaxAirVolFlowRate);
// Real64 MaxAirVolFlowRateDuringReheatDes = min( 0.002032 * state->dataSingleDuct->sd_airterminal( 1 ).ZoneFloorArea,
// state->dataSingleDuct->sd_airterminal( 1 ).MaxAirVolFlowRate ); apply limit based on min stop
MaxAirVolFlowRateDuringReheatDes =
Expand All @@ -1303,7 +1303,7 @@ TEST_F(EnergyPlusFixture, SingleDuct_ZeroFloorAreaTest)

// This isn't relevant any more since the default is calculated differently
Real64 MaxAirVolFractionDuringReheatDes =
min(1.0, (state->dataSize->FinalZoneSizing(1).DesHeatVolFlowMax / state->dataSingleDuct->sd_airterminal(1).MaxAirVolFlowRate));
min(1.0, (state->dataSize->FinalZoneSizing(1).DesHeatVolFlow / state->dataSingleDuct->sd_airterminal(1).MaxAirVolFlowRate));
// Real64 MaxAirVolFractionDuringReheatDes = min( 1.0, ( 0.002032 * state->dataSingleDuct->sd_airterminal( 1 ).ZoneFloorArea /
// state->dataSingleDuct->sd_airterminal( 1 ).MaxAirVolFlowRate )
// ); apply limit based on min stop
Expand All @@ -1318,12 +1318,12 @@ TEST_F(EnergyPlusFixture, SingleDuct_ZeroFloorAreaTest)
EXPECT_NEAR(MaxAirVolFractionDuringReheatDes, state->dataSingleDuct->sd_airterminal(1).MaxAirVolFractionDuringReheat, 0.0000000000001);

MaxAirVolFlowRateDuringReheatDes =
min(state->dataSize->FinalZoneSizing(2).DesHeatVolFlowMax, state->dataSingleDuct->sd_airterminal(2).MaxAirVolFlowRate);
min(state->dataSize->FinalZoneSizing(2).DesHeatVolFlow, state->dataSingleDuct->sd_airterminal(2).MaxAirVolFlowRate);
MaxAirVolFlowRateDuringReheatDes =
max(MaxAirVolFlowRateDuringReheatDes,
(state->dataSingleDuct->sd_airterminal(2).MaxAirVolFlowRate * state->dataSingleDuct->sd_airterminal(2).ZoneMinAirFrac));
MaxAirVolFractionDuringReheatDes =
min(1.0, (state->dataSize->FinalZoneSizing(2).DesHeatVolFlowMax / state->dataSingleDuct->sd_airterminal(2).MaxAirVolFlowRate));
min(1.0, (state->dataSize->FinalZoneSizing(2).DesHeatVolFlow / state->dataSingleDuct->sd_airterminal(2).MaxAirVolFlowRate));
MaxAirVolFractionDuringReheatDes = max(MaxAirVolFractionDuringReheatDes, state->dataSingleDuct->sd_airterminal(2).ZoneMinAirFrac);
MaxAirVolFlowRateDuringReheatDes =
min(max(MaxAirVolFlowRateDuringReheatDes, MaxAirVolFractionDuringReheatDes * state->dataSingleDuct->sd_airterminal(2).MaxAirVolFlowRate),
Expand Down Expand Up @@ -2750,23 +2750,69 @@ TEST_F(EnergyPlusFixture, VAVReheatTerminal_SizeMinFrac)
EXPECT_TRUE(compare_err_stream(""));

int SysNum = 1;
auto &thisSys = state->dataSingleDuct->sd_airterminal(SysNum);

// First test - design min flow < max flow
state->dataSize->ZoneSizingRunDone = true;
state->dataSize->CurZoneEqNum = 1;
state->dataSize->CurTermUnitSizingNum = 1;
state->dataSize->TermUnitFinalZoneSizing(1).DesCoolVolFlowMin = 0.5;
state->dataSingleDuct->sd_airterminal(SysNum).SizeSys(*state);
EXPECT_EQ(0.5, state->dataSingleDuct->sd_airterminal(SysNum).ZoneMinAirFracDes);
thisSys.SizeSys(*state);
EXPECT_EQ(0.5, thisSys.ZoneMinAirFracDes);

// Second test - design min flow > max flow
state->dataSize->ZoneSizingRunDone = true;
state->dataSize->CurZoneEqNum = 1;
state->dataSize->CurTermUnitSizingNum = 1;
state->dataSingleDuct->sd_airterminal(SysNum).ZoneMinAirFracDes = AutoSize; // need to reset this so it sizes again
thisSys.ZoneMinAirFracDes = AutoSize; // need to reset this so it sizes again
state->dataSize->TermUnitFinalZoneSizing(1).DesCoolVolFlowMin = 1.5;
state->dataSingleDuct->sd_airterminal(SysNum).SizeSys(*state);
EXPECT_EQ(1.0, state->dataSingleDuct->sd_airterminal(SysNum).ZoneMinAirFracDes);
thisSys.SizeSys(*state);
EXPECT_EQ(1.0, thisSys.ZoneMinAirFracDes);

// test Maximum Flow Fraction During Reheat for heating dominated
thisSys.MaxAirVolFlowRate = DataSizing::AutoSize;
thisSys.ZoneMinAirFracDes = DataSizing::AutoSize;
thisSys.MaxAirVolFlowRateDuringReheat = DataSizing::AutoSize;
thisSys.MaxAirVolFractionDuringReheat = DataSizing::AutoSize;
state->dataSize->TermUnitFinalZoneSizing(1).DesHeatVolFlow = 1.7;
state->dataSize->TermUnitFinalZoneSizing(1).DesHeatVolFlowMax = 1.6;
thisSys.SizeSys(*state);
Real64 expectedZoneMinAirFracDes = std::min(1.0, state->dataSize->TermUnitFinalZoneSizing(1).DesCoolVolFlowMin / thisSys.MaxAirVolFlowRate);
// DesHeatVolFlowMax is limiting flow rate
Real64 expectedMaxAirVolFractionDuringReheat = state->dataSize->TermUnitFinalZoneSizing(1).DesHeatVolFlowMax / thisSys.MaxAirVolFlowRate;
EXPECT_EQ(expectedZoneMinAirFracDes, thisSys.ZoneMinAirFracDes);
EXPECT_EQ(1.0, thisSys.MaxAirVolFractionDuringReheat);
EXPECT_EQ(expectedMaxAirVolFractionDuringReheat, thisSys.MaxAirVolFractionDuringReheat);

// switch magnitude of DesHeatVolFlow and DesHeatVolFlowMax, still heating dominated
thisSys.MaxAirVolFlowRate = DataSizing::AutoSize;
thisSys.ZoneMinAirFracDes = DataSizing::AutoSize;
thisSys.MaxAirVolFlowRateDuringReheat = DataSizing::AutoSize;
thisSys.MaxAirVolFractionDuringReheat = DataSizing::AutoSize;
state->dataSize->TermUnitFinalZoneSizing(1).DesHeatVolFlow = 1.6;
state->dataSize->TermUnitFinalZoneSizing(1).DesHeatVolFlowMax = 1.7;
thisSys.SizeSys(*state);
expectedZoneMinAirFracDes = std::min(1.0, state->dataSize->TermUnitFinalZoneSizing(1).DesCoolVolFlowMin / thisSys.MaxAirVolFlowRate);
// DesHeatVolFlowMax is NOT limiting flow rate
expectedMaxAirVolFractionDuringReheat = state->dataSize->TermUnitFinalZoneSizing(1).DesHeatVolFlow / thisSys.MaxAirVolFlowRate;
EXPECT_EQ(expectedZoneMinAirFracDes, thisSys.ZoneMinAirFracDes);
EXPECT_EQ(1.0, thisSys.MaxAirVolFractionDuringReheat);
EXPECT_EQ(expectedMaxAirVolFractionDuringReheat, thisSys.MaxAirVolFractionDuringReheat);

// test Maximum Flow Fraction During Reheat for cooling dominated
thisSys.MaxAirVolFlowRate = DataSizing::AutoSize;
thisSys.ZoneMinAirFracDes = DataSizing::AutoSize;
thisSys.MaxAirVolFlowRateDuringReheat = DataSizing::AutoSize;
thisSys.MaxAirVolFractionDuringReheat = DataSizing::AutoSize;
state->dataSize->TermUnitFinalZoneSizing(1).DesHeatVolFlow = 1.4;
state->dataSize->TermUnitFinalZoneSizing(1).DesHeatVolFlowMax = 1.3;
thisSys.SizeSys(*state);
expectedZoneMinAirFracDes = std::min(1.0, state->dataSize->TermUnitFinalZoneSizing(1).DesCoolVolFlowMin / thisSys.MaxAirVolFlowRate);
// DesHeatVolFlowMax is limiting flow rate
expectedMaxAirVolFractionDuringReheat = state->dataSize->TermUnitFinalZoneSizing(1).DesHeatVolFlowMax / thisSys.MaxAirVolFlowRate;
EXPECT_EQ(expectedZoneMinAirFracDes, thisSys.ZoneMinAirFracDes);
EXPECT_EQ(1.0, thisSys.MaxAirVolFractionDuringReheat);
EXPECT_EQ(expectedMaxAirVolFractionDuringReheat, thisSys.MaxAirVolFractionDuringReheat);
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Added 3 new tests for maximum air flow fraction during reheat to this unit test.

}

TEST_F(EnergyPlusFixture, setATMixerSizingProperties_Test)
Expand Down
6 changes: 5 additions & 1 deletion tst/EnergyPlus/unit/VAVDefMinMaxFlow.unit.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -483,8 +483,12 @@ TEST_F(EnergyPlusFixture, VAVDefMinMaxFlowTestSizing2)
state->dataSingleDuct->sd_airterminal(1).ZoneFloorArea = state->dataHeatBal->Zone(1).FloorArea;
UpdateTermUnitFinalZoneSizing(*state); // Fills the TermUnitFinalZoneSizing array
state->dataSingleDuct->sd_airterminal(1).SizeSys(*state);
Real64 heatVolFlow = (state->dataSize->FinalZoneSizing(state->dataSize->CurZoneEqNum).DesHeatVolFlow >
state->dataSize->FinalZoneSizing(state->dataSize->CurZoneEqNum).DesHeatVolFlowMax)
? state->dataSize->FinalZoneSizing(state->dataSize->CurZoneEqNum).DesHeatVolFlowMax
: state->dataSize->FinalZoneSizing(state->dataSize->CurZoneEqNum).DesHeatVolFlow;
EXPECT_NEAR(state->dataSingleDuct->sd_airterminal(state->dataSize->CurZoneEqNum).ZoneMinAirFracDes, 0.348739, 0.000001);
EXPECT_NEAR(state->dataSingleDuct->sd_airterminal(state->dataSize->CurZoneEqNum).MaxAirVolFlowRateDuringReheat, 0.196047, 0.000001);
EXPECT_NEAR(state->dataSingleDuct->sd_airterminal(state->dataSize->CurZoneEqNum).MaxAirVolFlowRateDuringReheat, heatVolFlow, 0.000001);

state->dataLoopNodes->Node.deallocate();
state->dataZoneEquip->ZoneEquipConfig.deallocate();
Expand Down
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