From eee31afbcef83448cd97cd25f09a9e6ca24192e6 Mon Sep 17 00:00:00 2001
From: Matt Mitchell <mitchute@gmail.com>
Date: Tue, 20 Oct 2026 13:02:24 -0600
Subject: [PATCH] fix third-order zone air heat balance storage reporting

---
 src/EnergyPlus/ZoneTempPredictorCorrector.cc  | 11 ++++------
 .../unit/ZoneTempPredictorCorrector.unit.cc   | 20 +++++++++++++++++++
 2 files changed, 24 insertions(+), 7 deletions(-)

diff --git a/src/EnergyPlus/ZoneTempPredictorCorrector.cc b/src/EnergyPlus/ZoneTempPredictorCorrector.cc
index a7cf4ffb8b2..875696ea819 100644
--- a/src/EnergyPlus/ZoneTempPredictorCorrector.cc
+++ b/src/EnergyPlus/ZoneTempPredictorCorrector.cc
@@ -5681,17 +5681,14 @@ void CalcZoneComponentLoadSums(EnergyPlusData &state,
             }
         }
     }
-    // now calculate air energy storage source term.
-    // capacitance is volume * density * heat capacity
-    Real64 CpAir = Psychrometrics::PsyCpAirFnW(thisHB->airHumRat);
-    Real64 RhoAir = Psychrometrics::PsyRhoAirFnPbTdbW(state, state.dataEnvrn->OutBaroPress, thisHB->MAT, thisHB->airHumRat);
-
+    // Calculate the air energy storage term using the same capacitance and time integration method as the zone air temperature solver.
     switch (state.dataHeatBal->ZoneAirSolutionAlgo) {
     case DataHeatBalance::SolutionAlgo::ThirdOrder: {
-        thisAirRpt.CzdTdt = RhoAir * CpAir * thisZone.Volume * thisZone.ZoneVolCapMultpSens * (thisHB->MAT - thisHB->ZTM[0]) / TimeStepSysSec;
-        // Exact solution
+        thisAirRpt.CzdTdt =
+            thisHB->AirPowerCap * ((11.0 / 6.0) * thisHB->MAT - 3.0 * thisHB->ZTM[0] + (3.0 / 2.0) * thisHB->ZTM[1] - (1.0 / 3.0) * thisHB->ZTM[2]);
     } break;
     case DataHeatBalance::SolutionAlgo::AnalyticalSolution: {
+        // Exact solution
         thisAirRpt.CzdTdt = thisHB->TempIndCoef - thisHB->TempDepCoef * thisHB->MAT;
     } break;
     case DataHeatBalance::SolutionAlgo::EulerMethod: {
diff --git a/tst/EnergyPlus/unit/ZoneTempPredictorCorrector.unit.cc b/tst/EnergyPlus/unit/ZoneTempPredictorCorrector.unit.cc
index dbac610447a..b2e66300930 100644
--- a/tst/EnergyPlus/unit/ZoneTempPredictorCorrector.unit.cc
+++ b/tst/EnergyPlus/unit/ZoneTempPredictorCorrector.unit.cc
@@ -1117,6 +1117,26 @@ TEST_F(EnergyPlusFixture, ZoneTempPredictorCorrector_calcZoneOrSpaceSums_SurfCon
     EXPECT_NEAR(7328.768356, thisZoneHB.SumSysMCpT, 0.0001);
 }
 
+TEST_F(EnergyPlusFixture, ZoneTempPredictorCorrector_ThirdOrderComponentLoadStorageTest)
+{
+    state->init_state(*state);
+
+    state->dataHeatBal->Zone.allocate(1);
+    state->dataHeatBalFanSys->SumConvHTRadSys.allocate(1);
+    state->dataHeatBalFanSys->SumConvPool.allocate(1);
+
+    ZoneTempPredictorCorrector::ZoneHeatBalanceData zoneHB;
+    zoneHB.AirPowerCap = 100.0;
+    zoneHB.MAT = 23.0;
+    zoneHB.ZTM = {20.0, 19.0, 18.0, 0.0};
+
+    state->dataHeatBal->ZoneAirSolutionAlgo = DataHeatBalance::SolutionAlgo::ThirdOrder;
+    DataHeatBalance::AirReportVars airReport;
+    CalcZoneComponentLoadSums(*state, 1, &zoneHB, airReport);
+
+    EXPECT_NEAR(1400.0 / 3.0, airReport.CzdTdt, 1.0e-10);
+}
+
 TEST_F(EnergyPlusFixture, ZoneTempPredictorCorrector_EMSOverrideSetpointTest)
 {
     // AUTHOR: L. Gu, FSEC