NatLabRockies · mitchute · May 4, 2026 · Mar 17, 2026 · Mar 17, 2026 · Mar 17, 2026
diff --git a/design/FY2026/NFP_SkyLW_Actuator.md b/design/FY2026/NFP_SkyLW_Actuator.md
diff --git a/doc/input-output-reference/src/overview/group-surface-construction-elements.tex b/doc/input-output-reference/src/overview/group-surface-construction-elements.tex
@@ -4071,6 +4071,10 @@ \subsubsection{Outputs}\label{outputs-36-1}
 
 This output is the heat energy added after material layer N from the EMS heat flux actuator (Component type: ``CondFD Surface Material Layer''; Control type: ``Heat Flux''). Energy is aggregated on the electricity meter and is only valid for the CondFD solution algorithm.
 
+\paragraph{CondFD EMS Sky Longwave Radiation Override Heat Flux}
+
+This output is the user-specified net sky longwave radiation heat flux applied to the exterior surface via the EMS actuator (Component type: ``CondFD Surface''; Control type: ``Sky Longwave Radiation Override''). Reports 0.0 when not actuated. Only valid for exterior CondFD surfaces.
+
 \subsection{Construction:AirBoundary}\label{constructionairboundary}
 
 Construction:AirBoundary indicates an open boundary between two zones. It may be used for base surfaces and fenestration surfaces.

diff --git a/doc/readthedocs/sphinx/ems-application-guide/ems-application-guide.rst b/doc/readthedocs/sphinx/ems-application-guide/ems-application-guide.rst
@@ -2523,6 +2523,20 @@ described below.
    used to add energy (i.e. positive numbers only), and the energy used
    is added to the electric heating sub-meter.
 
+A separate actuator, called “CondFD Surface,” is available for exterior
+surfaces that use the Conduction Finite Difference solution algorithm.
+Unlike the material-layer actuators above, this actuator operates on the
+surface as a whole and is identified by the surface name alone (not
+SurfName:MatName).
+
+-  “Sky Longwave Radiation Override”. Has units of [W/m2]. Replaces
+   the default sky longwave radiation term (h_sky * (T_sky - T_surf))
+   in the exterior face heat balance with a user-specified net heat
+   flux. Positive values indicate net radiation into the surface;
+   negative values indicate net radiation leaving the surface (cooling).
+   Only applies to exterior CondFD surfaces. The actuated component
+   unique name is the surface name (e.g. “ZN001:ROOF001”).
+
 Conduction Finite Difference Outputs
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
@@ -2554,6 +2568,15 @@ CondFD EMS Heat Source Energy After Layer N
 
 This output reports the heat energy added after material layer N
 
+-  Zone,Average,CondFD EMS Sky Longwave Radiation Override Heat Flux [W/m2]
+
+CondFD EMS Sky Longwave Radiation Override Heat Flux
+''''''''''''''''''''''''''''''''''''''''''''''''''''
+
+This output reports the user-specified net sky longwave radiation heat
+flux applied to the exterior surface via the EMS actuator. Reports 0.0
+when not actuated.
+
 Air Movement
 ------------
 

diff --git a/src/EnergyPlus/HeatBalFiniteDiffManager.cc b/src/EnergyPlus/HeatBalFiniteDiffManager.cc
@@ -1038,6 +1038,24 @@ namespace HeatBalFiniteDiffManager {
                                         OutputProcessor::Group::Building,
                                         OutputProcessor::EndUseCat::Heating);
                 }
+
+                // Setup EMS Actuator for Sky LW Radiation Override (per-surface)
+                if (state.dataSurface->Surface(SurfNum).ExtBoundCond == DataSurfaces::ExternalEnvironment) {
+                    EnergyPlus::SetupEMSActuator(state,
+                                                 "CondFD Surface",
+                                                 state.dataSurface->Surface(SurfNum).Name,
+                                                 "Sky Longwave Radiation Override",
+                                                 "[W/m2]",
+                                                 SurfaceFD(SurfNum).enetActuator.isActuated,
+                                                 SurfaceFD(SurfNum).enetActuator.actuatedValue);
+                    SetupOutputVariable(state,
+                                        "CondFD EMS Sky Longwave Radiation Override Heat Flux",
+                                        Constant::Units::W_m2,
+                                        SurfaceFD(SurfNum).enetActuatorReport,
+                                        OutputProcessor::TimeStepType::Zone,
+                                        OutputProcessor::StoreType::Average,
+                                        state.dataSurface->Surface(SurfNum).Name);
+                }
             }
 
             int TotNodes = ConstructFD(state.dataSurface->Surface(SurfNum).Construction).TotNodes; // Full size nodes, start with outside face.
@@ -1594,6 +1612,8 @@ namespace HeatBalFiniteDiffManager {
 
         auto &s_hbfd = state.dataHeatBalFiniteDiffMgr;
         auto const &surface(state.dataSurface->Surface(Surf));
+        auto &surfaceFD = s_hbfd->SurfaceFD(Surf);
+        surfaceFD.enetActuatorReport = surfaceFD.enetActuator.isActuated ? surfaceFD.enetActuator.actuatedValue : 0.0;
         int const surface_ExtBoundCond(surface.ExtBoundCond);
 
         Real64 Tsky;
@@ -1625,7 +1645,6 @@ namespace HeatBalFiniteDiffManager {
             if (surface_ExtBoundCond == Surf) { // adiabatic surface, PT added since it is not the same as interzone wall
                 // as Outside Boundary Condition Object can be left blank.
 
-                auto &surfaceFD = s_hbfd->SurfaceFD(Surf);
                 InteriorBCEqns(state,
                                Delt,
                                NodeIn,
@@ -1699,7 +1718,6 @@ namespace HeatBalFiniteDiffManager {
 
             // Boundary Conditions from Simulation for Exterior
             Real64 const hconvo(state.dataMstBal->HConvExtFD(Surf));
-
             Real64 const hrad(state.dataMstBal->HAirFD(Surf));
             Real64 const hsky(state.dataMstBal->HSkyFD(Surf));
             Real64 const hgnd(state.dataMstBal->HGrndFD(Surf));
@@ -1708,6 +1726,11 @@ namespace HeatBalFiniteDiffManager {
             Real64 const Tgnd(Tgndsurface);
             Real64 const Tsurr(TsurrSurface);
 
+            // Sky longwave radiation actuator and reusable variables
+            auto const &enetAct = s_hbfd->SurfaceFD(Surf).enetActuator;
+            Real64 const eHsky = (enetAct.isActuated) ? 0.0 : hsky;
+            Real64 const eHskyTsky = (enetAct.isActuated) ? enetAct.actuatedValue : hsky * Tsky;
+
             if (surface.HeatTransferAlgorithm == DataSurfaces::HeatTransferModel::CondFD) {
 
                 int const ConstrNum(surface.Construction);
@@ -1724,8 +1747,8 @@ namespace HeatBalFiniteDiffManager {
                 if (mat->ROnly || mat->group == Material::Group::AirGap) { // R Layer or Air Layer  **********
                     // Use algebraic equation for TDT based on R
                     Real64 const Rlayer(mat->Resistance);
-                    TDT_i = (TDT_p + (QRadSWOutFD + hgnd * Tgnd + (hconvo + hrad) * Toa + hsky * Tsky + hsurr * Tsurr) * Rlayer) /
-                            (1.0 + (hconvo + hgnd + hrad + hsky + hsurr) * Rlayer);
+                    TDT_i = (TDT_p + (QRadSWOutFD + hgnd * Tgnd + (hconvo + hrad) * Toa + eHskyTsky + hsurr * Tsurr) * Rlayer) /
+                            (1.0 + (hconvo + hgnd + hrad + eHsky + hsurr) * Rlayer);
 
                 } else { // Regular or phase change material layer
 
@@ -1793,27 +1816,27 @@ namespace HeatBalFiniteDiffManager {
                         if (s_hbfd->CondFDSchemeType == CondFDScheme::CrankNicholsonSecondOrder) { // Second Order equation
                             Real64 const Cp_DelX_RhoS_2Delt(Cp * DelX * RhoS / (2.0 * Delt));
                             Real64 const kt_2DelX(kt / (2.0 * DelX));
-                            Real64 const hsum(0.5 * (hconvo + hgnd + hrad + hsky + hsurr));
+                            Real64 const hsum(0.5 * (hconvo + hgnd + hrad + eHsky + hsurr));
                             TDT_i = (QRadSWOutFD + Cp_DelX_RhoS_2Delt * TD_i + kt_2DelX * (TDT_p - TD_i + TD(i + 1)) + hgnd * Tgnd +
-                                     (hconvo + hrad) * Toa + hsky * Tsky + hsurr * Tsurr - hsum * TD_i) /
+                                     (hconvo + hrad) * Toa + eHskyTsky + hsurr * Tsurr - hsum * TD_i) /
                                     (hsum + kt_2DelX + Cp_DelX_RhoS_2Delt);
                         } else if (s_hbfd->CondFDSchemeType == CondFDScheme::FullyImplicitFirstOrder) { // First Order
                             Real64 const Two_Delt_DelX(2.0 * Delt_DelX);
                             Real64 const Cp_DelX2_RhoS(Cp * pow_2(DelX) * RhoS);
                             Real64 const Two_Delt_kt(2.0 * Delt * kt);
-                            TDT_i = (Two_Delt_DelX * (QRadSWOutFD + hgnd * Tgnd + (hconvo + hrad) * Toa + hsky * Tsky + hsurr * Tsurr) +
+                            TDT_i = (Two_Delt_DelX * (QRadSWOutFD + hgnd * Tgnd + (hconvo + hrad) * Toa + eHskyTsky + hsurr * Tsurr) +
                                      Cp_DelX2_RhoS * TD_i + Two_Delt_kt * TDT_p) /
-                                    (Two_Delt_DelX * (hconvo + hgnd + hrad + hsky + hsurr) + Two_Delt_kt + Cp_DelX2_RhoS);
+                                    (Two_Delt_DelX * (hconvo + hgnd + hrad + eHsky + hsurr) + Two_Delt_kt + Cp_DelX2_RhoS);
                         }
 
                     } else { // HMovInsul > 0.0: Transparent insulation on outside
                         // Transparent insulation additions
 
                         // Movable Insulation Layer Outside surface temp
 
-                        Real64 const TInsulOut(
-                            (QRadSWOutMvInsulFD + hgnd * Tgnd + HMovInsul * TDT_i + (hconvo + hrad) * Toa + hsky * Tsky + hsurr * Tsurr) /
-                            (hconvo + hgnd + HMovInsul + hrad + hsky + hsurr)); // Temperature of outside face of Outside Insulation
+                        Real64 const TInsulOut =
+                            (QRadSWOutMvInsulFD + hgnd * Tgnd + HMovInsul * TDT_i + (hconvo + hrad) * Toa + eHskyTsky + hsurr * Tsurr) /
+                            (hconvo + hgnd + HMovInsul + hrad + eHsky + hsurr);
                         Real64 const Two_Delt_DelX(2.0 * Delt_DelX);
                         Real64 const Cp_DelX2_RhoS(Cp * pow_2(DelX) * RhoS);
                         Real64 const Two_Delt_kt(2.0 * Delt * kt);
@@ -1844,15 +1867,15 @@ namespace HeatBalFiniteDiffManager {
             // One formulation that works for Fully Implicit and CrankNicholson and massless wall
 
             Real64 const Toa_TDT_i(Toa - TDT_i);
-            Real64 const QNetSurfFromOutside(
-                QRadSWOutFD + (hgnd * (-TDT_i + Tgnd) + (hconvo + hrad) * Toa_TDT_i + hsky * (-TDT_i + Tsky) + hsurr * (-TDT_i + Tsurr)));
+            Real64 const QNetSurfFromOutside =
+                QRadSWOutFD + (hgnd * (-TDT_i + Tgnd) + (hconvo + hrad) * Toa_TDT_i - eHsky * TDT_i + eHskyTsky + hsurr * (-TDT_i + Tsurr));
 
             // Same sign convention as CTFs
             state.dataHeatBalSurf->SurfOpaqOutFaceCondFlux(Surf) = -QNetSurfFromOutside;
 
             // Report all outside BC heat fluxes
             state.dataHeatBalSurf->SurfQdotRadOutRepPerArea(Surf) =
-                -(hgnd * (TDT_i - Tgnd) + hrad * (-Toa_TDT_i) + hsky * (TDT_i - Tsky) + hsurr * (TDT_i - Tsurr));
+                -(hgnd * (TDT_i - Tgnd) + hrad * (-Toa_TDT_i) + eHsky * TDT_i - eHskyTsky + hsurr * (TDT_i - Tsurr));
             state.dataHeatBalSurf->SurfQdotRadOutRep(Surf) = surface.Area * state.dataHeatBalSurf->SurfQdotRadOutRepPerArea(Surf);
             state.dataHeatBalSurf->SurfQRadOutReport(Surf) = state.dataHeatBalSurf->SurfQdotRadOutRep(Surf) * state.dataGlobal->TimeStepZoneSec;
 

diff --git a/src/EnergyPlus/HeatBalFiniteDiffManager.hh b/src/EnergyPlus/HeatBalFiniteDiffManager.hh
@@ -166,6 +166,8 @@ namespace HeatBalFiniteDiffManager {
         // Includes the EMS heat source
         Array1D<Real64> heatSourceEMSFluxLayerReport;
         Array1D<Real64> heatSourceEMSFluxEnergyLayerReport;
+        MaterialActuatorData enetActuator; // Sky LW radiation EMS override [W/m2]
+        Real64 enetActuatorReport = 0.0;   // Reported sky LW radiation EMS override [W/m2], zero when inactive
 
         // Default Constructor
         SurfaceDataFD() : SourceNodeNum(0), QSource(0.0), GSloopCounter(0), MaxNodeDelTemp(0.0), EnthalpyM(0.0), EnthalpyF(0.0), PhaseChangeState(0)

diff --git a/testfiles/CMakeLists.txt b/testfiles/CMakeLists.txt
@@ -494,6 +494,7 @@ if (LINK_WITH_PYTHON)
     add_simulation_test(IDF_FILE PythonPlugin_SingleFamilyHouse_TwoSpeed_MultiStageElectricSuppCoil.idf EPW_FILE USA_IL_Chicago-OHare.Intl.AP.725300_TMY3.epw)
     add_simulation_test(IDF_FILE PythonPlugin1ZoneUncontrolledCondFD.idf EPW_FILE USA_CO_Golden-NREL.724666_TMY3.epw)
     add_simulation_test(IDF_FILE PythonPlugin1ZoneUncontrolledTrackHistory.idf EPW_FILE USA_CO_Golden-NREL.724666_TMY3.epw)
+    add_simulation_test(IDF_FILE PythonPluginCondFD_Enet.idf EPW_FILE USA_CO_Golden-NREL.724666_TMY3.epw DESIGN_DAY_ONLY)
     add_simulation_test(IDF_FILE PythonPluginAirflowNetworkOpeningControlByHumidity.idf EPW_FILE USA_IL_Chicago-OHare.Intl.AP.725300_TMY3.epw)
     add_simulation_test(IDF_FILE PythonPluginConstantVolumePurchasedAir.idf EPW_FILE USA_IL_Chicago-OHare.Intl.AP.725300_TMY3.epw)
     add_simulation_test(IDF_FILE PythonPluginCurveOverride_PackagedTerminalHeatPump.idf EPW_FILE USA_FL_Miami.Intl.AP.722020_TMY3.epw)