diff --git a/IDEAS/Buildings/Components/BoundaryWall.mo b/IDEAS/Buildings/Components/BoundaryWall.mo
index 8b440c7d9..1ae6169c2 100644
--- a/IDEAS/Buildings/Components/BoundaryWall.mo
+++ b/IDEAS/Buildings/Components/BoundaryWall.mo
@@ -4,31 +4,36 @@ model BoundaryWall "Opaque wall with optional prescribed heat flow rate or tempe
     final custom_q50=0,
     final use_custom_q50=true,
     final nWin=1,
-    QTra_design=U_value*A*(273.15 + 21 - TRef_a),
     dT_nominal_a=-1,
     add_cracks=false,
+    final QTra_design(fixed=false),
     layMul(disableInitPortB=use_T_in or use_T_fixed, monLay(monLayDyn(each
             addRes_b=(sim.lineariseDymola and (use_T_in or use_T_fixed))))));
 
+  parameter Boolean use_T_in = false
+    "Use a temperature boundary condition which is read from the input connector T_in"
+    annotation(Dialog(group="Boundary conditions"));
   parameter Boolean use_T_fixed = false
-    "Get the boundary temperature from the input connector"
+    "Use a fixed temperature boundary condition which is read from the parameter T_fixed"
     annotation(Dialog(group="Boundary conditions"));
   parameter Modelica.Units.SI.Temperature T_fixed=294.15
     "Fixed boundary temperature"
-    annotation (Dialog(group="Boundary conditions", enable=use_T_fixed));
-  parameter Boolean use_T_in = false
-    "Get the boundary temperature from the input connector"
-    annotation(Dialog(group="Boundary conditions"));
+    annotation (Dialog(group="Boundary conditions",enable=use_T_fixed));
+  parameter Modelica.Units.SI.Temperature T_in_nom=T_fixed
+    "Nominal boundary temperature, for calculation of design heat loss"
+    annotation (Dialog(group="Design power", tab="Advanced",enable=use_T_fixed or use_T_in));
   parameter Boolean use_Q_in = false
-    "Get the boundary heat flux from the input connector"
+    "Use a heat flow boundary condition which is read from the input connection Q_in"
     annotation(Dialog(group="Boundary conditions"));
-
+  parameter Modelica.Units.SI.HeatFlowRate Q_in_nom=0
+    "Nominal boundary heat flux, for calculation of design heat loss (positive if entering the wall)"
+    annotation (Dialog(group="Design power", tab="Advanced", enable=use_Q_in));
   Modelica.Blocks.Interfaces.RealInput T if use_T_in
     "Input for boundary temperature"                 annotation (Placement(
         transformation(extent={{-120,10},{-100,30}}),iconTransformation(extent={{-120,10},
             {-100,30}})));
   Modelica.Blocks.Interfaces.RealInput Q_flow if use_Q_in
-    "Input for boundary heat flow rate entering the wall" annotation (Placement(
+    "Input for boundary heat flow rate entering the wall (positive)" annotation (Placement(
         transformation(extent={{-120,-30},{-100,-10}}),
                                                     iconTransformation(extent={{-120,
             -30},{-100,-10}})));
@@ -64,6 +69,8 @@ protected
   IDEAS.Buildings.Components.Interfaces.WeaBus weaBus(final numSolBus=sim.numIncAndAziInBus,
       outputAngles=sim.outputAngles)                  "Weather bus"
     annotation (Placement(transformation(extent={{40,-80},{60,-60}})));
+initial equation
+    QTra_design=if use_T_in or use_T_fixed then U_value*A*(TRefZon - T_in_nom) else -Q_in_nom;
 equation
   assert(not (use_T_in and use_Q_in or use_T_in and use_T_fixed or use_Q_in and use_T_fixed),
     "In "+getInstanceName()+": Only one of the following options can be used simultaneously: use_T_in, use_Q_in, use_T_fixed");
@@ -169,11 +176,18 @@ to enable an input for a prescribed boundary condition temperature or heat flow
 Alternatively, parameters <code>use_T_fixed</code> and <code>T_fixed</code> can be used
 to specify a fixed boundary condition temperature.
 It is not allowed to enabled multiple of these three options. 
-If all are disabled then an adiabatic boundary (<code>Q_flow=0</code>) is used.
+If all are disabled, an adiabatic boundary (<code>Q_flow=0</code>) is used.</p>
+Parameters <code>T_in_nom</code> and <code>Q_in_nom</code> are used for the calculation 
+of heat losses, when the temperature boundary condition and heat flow boundary condition are applied, respectively.  
 </p>
 </html>", revisions="<html>
 <ul>
 <li>
+November 7, 2024, by Anna Dell'Isola and Jelger Jansen:<br/>
+Update calculation of transmission design losses.
+See <a href=\"https://github.com/open-ideas/IDEAS/issues/1337\">#1337</a>
+</li>
+<li>
 April 26, 2020, by Filip Jorissen:<br/>
 Refactored <code>SolBus</code> to avoid many instances in <code>PropsBus</code>.
 See <a href=\"https://github.com/open-ideas/IDEAS/issues/1131\">
diff --git a/IDEAS/Buildings/Components/Interfaces/DummyConnection.mo b/IDEAS/Buildings/Components/Interfaces/DummyConnection.mo
index 30d1ada8c..742bf741e 100644
--- a/IDEAS/Buildings/Components/Interfaces/DummyConnection.mo
+++ b/IDEAS/Buildings/Components/Interfaces/DummyConnection.mo
@@ -54,6 +54,8 @@ model DummyConnection "Source generator/sink for propsbus"
     annotation (Placement(transformation(extent={{-80,-100},{-60,-80}})));
   Modelica.Blocks.Sources.BooleanConstant custom_q50(k=false) if not isZone
     annotation (Placement(transformation(extent={{-80,-130},{-60,-110}})));
+  Modelica.Blocks.Sources.Constant TRefZon(k=T)      if isZone
+    annotation (Placement(transformation(extent={{-40,60},{-20,80}})));
 equation
   connect(prescribedHeatFlow[1].port, zoneBus.surfCon) annotation (Line(
       points={{-50,20},{62,20},{62,-1.9},{100.1,-1.9}},
@@ -132,6 +134,8 @@ equation
   connect(custom_q50.y, zoneBus.use_custom_q50) annotation (Line(points={{-59,
           -120},{100.1,-120},{100.1,-1.9}},
                                      color={255,0,255}));
+  connect(TRefZon.y, zoneBus.TRefZon) annotation (Line(points={{-19,70},{100.1,70},
+          {100.1,-1.9}}, color={0,0,127}));
    annotation (Diagram(coordinateSystem(preserveAspectRatio=false, extent={{-100,
             -100},{100,100}})),           Icon(coordinateSystem(
           preserveAspectRatio=false, extent={{-100,-100},{100,100}}), graphics={
@@ -145,6 +149,11 @@ equation
     Documentation(revisions="<html>
 <ul>
 <li>
+November 7, 2024, by Anna Dell'Isola and Jelger Jansen:<br/>
+Assign value for <code>TRefZon</code> in the bus connector.
+See <a href=\"https://github.com/open-ideas/IDEAS/issues/1337\">#1337</a>
+</li>
+<li>
 May 23, 2022, by Filip Jorissen:<br/>
 Added missing medium declaration.
 See <a href=\"https://github.com/open-ideas/IDEAS/issues/1254\">
diff --git a/IDEAS/Buildings/Components/Interfaces/PartialSurface.mo b/IDEAS/Buildings/Components/Interfaces/PartialSurface.mo
index 3a8b0e77d..32877ce92 100644
--- a/IDEAS/Buildings/Components/Interfaces/PartialSurface.mo
+++ b/IDEAS/Buildings/Components/Interfaces/PartialSurface.mo
@@ -26,10 +26,6 @@ partial model PartialSurface "Partial model for building envelope component"
   parameter Modelica.Units.SI.Temperature T_start=293.15
     "Start temperature for each of the layers"
     annotation (Dialog(tab="Dynamics", group="Initial condition"));
-
-  parameter Modelica.Units.SI.Temperature TRef_a=291.15
-    "Reference temperature of zone on side of propsBus_a, for calculation of design heat loss"
-    annotation (Dialog(group="Design power", tab="Advanced"));
   parameter Boolean linIntCon_a=sim.linIntCon
     "= true, if convective heat transfer should be linearised"
     annotation (Dialog(tab="Convection"));
@@ -138,6 +134,9 @@ protected
   Modelica.Thermal.HeatTransfer.Sources.PrescribedHeatFlow prescribedHeatFlowQgai
     "Component for computing conservation of energy";
 
+  Modelica.Units.SI.Temperature TRefZon=propsBusInt.TRefZon
+      "Reference zone temperature for calculation of design heat load";
+
   IDEAS.Buildings.Components.Interfaces.ZoneBusVarMultiplicator gain(redeclare
       package Medium = Medium,                                       k=nWin)
     "Gain for all propsBus variable to represent nWin surfaces instead of 1"
@@ -279,7 +278,6 @@ equation
       points={{56.09,19.91},{46,19.91},{46,0},{40,0}},
       color={191,0,0},
       smooth=Smooth.None));
-
   connect(layMul.port_a, propsBusInt.surfRad) annotation (Line(
       points={{10,0},{16,0},{16,19.91},{56.09,19.91}},
       color={191,0,0},
@@ -335,14 +333,18 @@ equation
   connect(setArea.v50, propsBus_a.v50) annotation (Line(points={{79.4,-83.2},{
           79.4,-82},{56,-82},{56,0},{100.1,0},{100.1,19.9}}, color={0,0,127}));
   annotation (
-    Diagram(graphics,
-            coordinateSystem(preserveAspectRatio=false, extent={{-100,-100},{
+    Diagram(coordinateSystem(preserveAspectRatio=false, extent={{-100,-100},{
             100,100}})),
     Icon(graphics,
          coordinateSystem(preserveAspectRatio=false, extent={{-50,-100},{50,100}})),
     Documentation(revisions="<html>
 <ul>
 <li>
+November 7, 2024, by Anna Dell'Isola and Jelger Jansen:<br/>
+Add variable <code>TRefZon</code> to be used when calculating <code>QTra_design</code>.
+See <a href=\"https://github.com/open-ideas/IDEAS/issues/1337\">#1337</a>
+</li>
+<li>
 August 10, 2020, by Filip Jorissen:<br/>
 Modifications for supporting interzonal airflow.
 See <a href=\"https://github.com/open-ideas/IDEAS/issues/1066\">
diff --git a/IDEAS/Buildings/Components/Interfaces/PartialZone.mo b/IDEAS/Buildings/Components/Interfaces/PartialZone.mo
index 9008164a2..fcd38bc3b 100644
--- a/IDEAS/Buildings/Components/Interfaces/PartialZone.mo
+++ b/IDEAS/Buildings/Components/Interfaces/PartialZone.mo
@@ -34,13 +34,16 @@ model PartialZone "Building zone model"
   parameter Boolean calculateViewFactor = false
     "Explicit calculation of view factors: works well only for rectangular zones!"
     annotation(Dialog(tab="Advanced", group="Radiative heat exchange"));
+  parameter Modelica.Units.SI.Temperature TZon_design=294.15
+    "Reference zone temperature for calculation of design heat load"
+    annotation (Dialog(group="Design heat load", tab="Advanced"));
   final parameter Modelica.Units.SI.Power QInf_design=1012*1.204*V/3600*n50_int
-      /n50toAch*(273.15 + 21 - sim.Tdes)
+      /n50toAch*(TZon_design - sim.Tdes)
     "Design heat losses from infiltration at reference outdoor temperature";
   final parameter Modelica.Units.SI.Power QRH_design=A*fRH
     "Additional power required to compensate for the effects of intermittent heating";
   final parameter Modelica.Units.SI.Power Q_design(fixed=false)
-    "Total design heat losses for the zone";
+    "Total design heat losses for the zone (including transmission, infiltration, and reheating; excluding ventilation)";
   parameter Medium.Temperature T_start=Medium.T_default
     "Start value of temperature"
     annotation(Dialog(tab = "Initialization"));
@@ -192,6 +195,9 @@ model PartialZone "Building zone model"
 
 
 protected
+  Modelica.Blocks.Sources.RealExpression TRefZon[nSurf](each y=TZon_design)
+    "Reference zone temperature for the surfaces connected to this zone, for calculation of design heat load";
+
   parameter Real n50_int(unit="1/h",min=0.01,fixed= false)
     "n50 value cfr airtightness, i.e. the ACH at a pressure diffence of 50 Pa"
     annotation(Dialog(enable=use_custom_n50,tab="Airflow", group="Airtightness"));
@@ -298,11 +304,11 @@ end Setq50;
 
 
 initial equation
-
-
   n50_int = if use_custom_n50 and not setq50.allSurfacesCustom then n50 else sum(propsBusInt.v50)/V;
 
-  Q_design=QInf_design+QRH_design+QTra_design; //Total design load for zone (additional ventilation losses are calculated in the ventilation system)
+  Q_design=QInf_design+QRH_design+QTra_design;
+  //Total design load for zone (excluding ventilation losses, these are assumed to be calculated in the ventilation system
+  //and should be added afterwards to obtain the total design heat load). See for example IDEAS.Templates.Interfaces.Building.
 
 equation
   if interzonalAirFlow.verifyBothPortsConnected then
@@ -504,8 +510,8 @@ end for;
   connect(setq50.use_custom_n50s, propsBusInt.use_custom_n50) annotation (Line(points={{-60.8,
           -92.8},{-60,-92.8},{-60,-92},{-80.1,-92},{-80.1,39.9}},       color={
           255,0,255}));
-  annotation (Placement(transformation(extent={{
-            140,48},{100,88}})),
+  connect(TRefZon.y, propsBusInt.TRefZon);
+    annotation (Placement(transformation(extent={{140,48},{100,88}})),
     Icon(coordinateSystem(preserveAspectRatio=false, extent={{-100,-100},{100,100}}),
          graphics),
     Documentation(info="<html>
@@ -518,6 +524,11 @@ Change Medium to IDEAS.Media.Air and use 'constrainedby' for Modelica.Media.Inte
 This is for <a href=https://github.com/open-ideas/IDEAS/issues/1375>#1375</a>.
 </li>
 <li>
+November 7, 2024, by Anna Dell'Isola and Jelger Jansen:<br/>
+Add parameter <code>TZon_design</code> to be used when calculating <code>Q_design</code>.
+See <a href=\"https://github.com/open-ideas/IDEAS/issues/1337\">#1337</a>
+</li>
+<li>
 April 26, 2024 by Jelger Jansen:<br/>
 Added parameter <code>ignAss</code> to ignore view factor asserts for non-physical unit test models.
 This is for <a href=https://github.com/open-ideas/IDEAS/issues/1272>#1272</a>.
diff --git a/IDEAS/Buildings/Components/Interfaces/ZoneBus.mo b/IDEAS/Buildings/Components/Interfaces/ZoneBus.mo
index 3298d24a3..b490cf1ad 100644
--- a/IDEAS/Buildings/Components/Interfaces/ZoneBus.mo
+++ b/IDEAS/Buildings/Components/Interfaces/ZoneBus.mo
@@ -48,6 +48,9 @@ expandable connector ZoneBus
   IDEAS.Buildings.Components.Interfaces.RealConnector q50_zone(final unit="m3/(h.m2)") "v50 of the surface" annotation ();
   IDEAS.Buildings.Components.Interfaces.BooleanConnector use_custom_q50 "true if custome q50 value is assigned to surface" annotation ();
   IDEAS.Buildings.Components.Interfaces.BooleanConnector use_custom_n50 "true if the zone n50 is a custom value";
+  IDEAS.Buildings.Components.Interfaces.RealConnector TRefZon(
+    final quantity="Temperature",
+    final unit="K") annotation ();
   annotation (Documentation(info="<html>
 <p>
 Connector that contains a weather bus and further
@@ -57,6 +60,11 @@ heat and information between a zone and a surface.
 </html>", revisions="<html>
 <ul>
 <li>
+November 7, 2024, by Anna Dell'Isola and Jelger Jansen:<br/>
+Add connector <code>TRefZon</code> to be used when calculating <code>QTra_design</code>.
+See <a href=\"https://github.com/open-ideas/IDEAS/issues/1337\">#1337</a>
+</li>
+<li>
 April 20, 2023 by Jelger Jansen:<br/>
 Make the connector expandable to avoid (pedantic check) warnings in Dymola 2022x.
 See issue <a href=https://github.com/open-ideas/IDEAS/issues/1317>#1317</a>
diff --git a/IDEAS/Buildings/Components/Interfaces/ZoneBusVarMultiplicator.mo b/IDEAS/Buildings/Components/Interfaces/ZoneBusVarMultiplicator.mo
index 0abbedf94..c4f9412f8 100644
--- a/IDEAS/Buildings/Components/Interfaces/ZoneBusVarMultiplicator.mo
+++ b/IDEAS/Buildings/Components/Interfaces/ZoneBusVarMultiplicator.mo
@@ -41,7 +41,7 @@ protected
      if sim.interZonalAirFlowType <> IDEAS.BoundaryConditions.Types.InterZonalAirFlow.None
     "Mass flow rate multiplier for port 1"
     annotation (Placement(transformation(extent={{-10,-170},{10,-150}})));
-  Modelica.Blocks.Math.Gain QTra_desgin(k=k) "Design heat flow rate"
+  Modelica.Blocks.Math.Gain QTra_design(k=k) "Design heat flow rate"
     annotation (Placement(transformation(extent={{-10,178},{10,198}})));
   Modelica.Blocks.Math.Gain area(k=k) "Heat exchange surface area"
     annotation (Placement(transformation(extent={{-10,150},{10,170}})));
@@ -80,10 +80,13 @@ protected
   Modelica.Blocks.Routing.BooleanPassThrough use_custom_q50
     "0 if the surface has a custom q50"
     annotation (Placement(transformation(extent={{-12,-296},{8,-276}})));
+  Modelica.Blocks.Routing.RealPassThrough TRefZon
+    "Reference zone temperature for calculation of design heat load"
+    annotation (Placement(transformation(extent={{8,-356},{-12,-336}})));
 equation
-  connect(QTra_desgin.u, propsBus_a.QTra_design) annotation (Line(points={{-12,188},
+  connect(QTra_design.u, propsBus_a.QTra_design) annotation (Line(points={{-12,188},
           {-100.1,188},{-100.1,0.1}},         color={0,0,127}));
-  connect(QTra_desgin.y, propsBus_b.QTra_design) annotation (Line(points={{11,188},
+  connect(QTra_design.y, propsBus_b.QTra_design) annotation (Line(points={{11,188},
           {100.1,188},{100.1,-0.1}},color={0,0,127}));
   connect(area.u, propsBus_a.area) annotation (Line(points={{-12,160},{-100.1,
           160},{-100.1,0.1}},color={0,0,127}));
@@ -162,6 +165,10 @@ equation
   connect(use_custom_n50.y, propsBus_a.use_custom_n50) annotation (Line(points={{-13,
           -314},{-100,-314},{-100,0.1},{-100.1,0.1}},
                                                 color={255,0,255}));
+  connect(TRefZon.u, propsBus_b.TRefZon) annotation (Line(points={{10,-346},{
+          100.1,-346},{100.1,-0.1}},  color={0,0,127}));
+  connect(TRefZon.y, propsBus_a.TRefZon) annotation (Line(points={{-13,-346},{
+          -100.1,-346},{-100.1,0.1}},  color={0,0,127}));
   annotation (Icon(coordinateSystem(preserveAspectRatio=false, extent={{-100,-180},
             {100,200}}), graphics={
         Polygon(
@@ -186,6 +193,11 @@ equation
     Documentation(revisions="<html>
 <ul>
 <li>
+November 7, 2024, by Anna Dell'Isola and Jelger Jansen:<br/>
+Add connector <code>TRefZon</code> to be used when calculating <code>QTra_design</code>.
+See <a href=\"https://github.com/open-ideas/IDEAS/issues/1337\">#1337</a>
+</li>
+<li>
 August 10, 2020, by Filip Jorissen:<br/>
 Modifications for supporting interzonal airflow.
 See <a href=\"https://github.com/open-ideas/IDEAS/issues/1066\">
diff --git a/IDEAS/Buildings/Components/InternalWall.mo b/IDEAS/Buildings/Components/InternalWall.mo
index fa54b615d..fcfc5be08 100644
--- a/IDEAS/Buildings/Components/InternalWall.mo
+++ b/IDEAS/Buildings/Components/InternalWall.mo
@@ -8,8 +8,8 @@ model InternalWall "interior opaque wall between two zones"
     E(y=if sim.computeConservationOfEnergy then layMul.E else 0),
     Qgai(y=(if sim.openSystemConservationOfEnergy or not sim.computeConservationOfEnergy
            then 0 else sum(port_emb.Q_flow))),
-    final QTra_design=U_value*A*(TRef_a - TRef_b),
-    q50_zone(v50_surf=0));
+    q50_zone(v50_surf=0),
+    final QTra_design(fixed=false));
   //using custom q50 since this model is not an external component
 
   parameter Boolean linIntCon_b=sim.linIntCon
@@ -18,10 +18,8 @@ model InternalWall "interior opaque wall between two zones"
   parameter Modelica.Units.SI.TemperatureDifference dT_nominal_b=1
     "Nominal temperature difference used for linearisation, negative temperatures indicate the solid is colder"
     annotation (Dialog(tab="Convection"));
-  parameter Modelica.Units.SI.Temperature TRef_b=291.15
-    "Reference temperature of zone on side of propsBus_b, for calculation of design heat loss"
-    annotation (Dialog(group="Design power", tab="Advanced"));
-
+  Modelica.Units.SI.Temperature TRef_b = propsBus_b.TRefZon
+    "Reference temperature of zone on side of propsBus_b, for calculation of design heat loss";
   // open door modelling
   parameter Boolean hasCavity = false
     "=true, to model open door or cavity in wall"
@@ -98,7 +96,7 @@ protected
     "convective surface heat transimission on the interior side of the wall"
     annotation (Placement(transformation(extent={{-22,-10},{-42,10}})));
   Modelica.Blocks.Sources.RealExpression QDesign_b(y=-QTra_design);
-  //Negative, because of its losses from zone side b to zone side a, oposite of calculation of QTra_design
+  //Negative, because of its losses from zone side b to zone side a, opposite of calculation of QTra_design
 
   Modelica.Blocks.Sources.RealExpression incExp1(y=incInt + Modelica.Constants.pi)
     "Inclination angle";
@@ -141,6 +139,10 @@ public
     if hasCavity and sim.interZonalAirFlowType == IDEAS.BoundaryConditions.Types.InterZonalAirFlow.OnePort
     "1-port model for open door"
     annotation (Placement(transformation(extent={{-10,58},{10,78}})));
+initial equation
+  QTra_design=U_value*A*(TRefZon - TRef_b)
+    "TRefZon is the reference temperature for heat loss calculations of the zone connected to propsbus_a,
+     TRef_b is the reference temperature for heat loss calculations of the zone connected to propsBus_b";
 equation
   assert(hasCavity == false or IDEAS.Utilities.Math.Functions.isAngle(incInt, IDEAS.Types.Tilt.Wall),
     "In " + getInstanceName() + ": Cavities are only supported for vertical walls, but inc=" + String(incInt) + ". The model is not accurate.",
@@ -203,7 +205,6 @@ equation
           -58},{56,20.1},{-100.1,20.1}},   color={0,0,127}));
   connect(q50_zone.using_custom_q50, propsBus_b.use_custom_q50) annotation (Line(points={{79,-52},
           {56,-52},{56,20.1},{-100.1,20.1}},      color={0,0,127}));
-
   annotation (
     Icon(coordinateSystem(preserveAspectRatio=false,extent={{-60,-100},{60,100}}),
         graphics={
@@ -264,6 +265,11 @@ We assume that the value of <code>A</code> excludes the surface area of the cavi
 </html>", revisions="<html>
 <ul>
 <li>
+November 7, 2024, by Anna Dell'Isola and Jelger Jansen:<br/>
+Update calculation of transmission design losses.
+See <a href=\"https://github.com/open-ideas/IDEAS/issues/1337\">#1337</a>
+</li>
+<li>
 August 2, 2022, by Filip Jorissen:<br/>
 Activating thermal model when using OnePorts.
 See <a href=\"https://github.com/open-ideas/IDEAS/issues/1291\">
diff --git a/IDEAS/Buildings/Components/OuterWall.mo b/IDEAS/Buildings/Components/OuterWall.mo
index 2d52cf657..54114e40f 100644
--- a/IDEAS/Buildings/Components/OuterWall.mo
+++ b/IDEAS/Buildings/Components/OuterWall.mo
@@ -4,7 +4,7 @@ model OuterWall "Opaque building envelope construction"
      setArea(A=A),
      final nWin=1,
      dT_nominal_a=-3,
-     QTra_design(fixed=false));
+     final QTra_design(fixed=false));
 
   parameter Boolean linExtCon=sim.linExtCon
     "= true, if exterior convective heat transfer should be linearised (uses average wind speed)"
@@ -96,7 +96,7 @@ protected
     "Outside air model"
     annotation (Placement(transformation(extent={{-100,-60},{-80,-40}})));
 initial equation
-  QTra_design =U_value*A*(273.15 + 21 - Tdes.y);
+  QTra_design =U_value*A*(TRefZon - Tdes.y);
 
 equation
   if hasBuildingShade then
@@ -268,6 +268,11 @@ The correct shading parameter values should then be passed through the redeclara
 </html>", revisions="<html>
 <ul>
 <li>
+November 7, 2024, by Anna Dell'Isola and Jelger Jansen:<br/>
+Update calculation of transmission design losses.
+See <a href=\"https://github.com/open-ideas/IDEAS/issues/1337\">#1337</a>
+</li>
+<li>
 July 18, 2022, by Filip Jorissen:<br/>
 Revised code for supporting new shading model.
 See <a href=\"https://github.com/open-ideas/IDEAS/issues/1270\">
diff --git a/IDEAS/Buildings/Components/SlabOnGround.mo b/IDEAS/Buildings/Components/SlabOnGround.mo
index 0a778334b..f86676fd3 100644
--- a/IDEAS/Buildings/Components/SlabOnGround.mo
+++ b/IDEAS/Buildings/Components/SlabOnGround.mo
@@ -4,9 +4,9 @@ model SlabOnGround "opaque floor on ground slab"
     custom_q50=0,
     final use_custom_q50=true,
     final nWin=1,
-    QTra_design=UEqui*A*(273.15 + 21 - sim.Tdes),
     add_cracks=false,
     dT_nominal_a=-3,
+    final QTra_design(fixed=false),
     inc=IDEAS.Types.Tilt.Floor,
     azi=0,
     redeclare replaceable Data.Constructions.FloorOnGround constructionType,
@@ -36,7 +36,8 @@ model SlabOnGround "opaque floor on ground slab"
       cos(2*3.1415/12*(i - 1 + alfa)) + Lpe*dTeAvg*cos(2*3.1415/12*(i - 1 -
       beta)) for i in 1:12})/12 "Two-dimensional correction for edge flow";
 
-//Calculation of heat loss based on ISO 13370
+  Modelica.Blocks.Routing.RealPassThrough TdesGround "Design temperature passthrough";
+
   IDEAS.Fluid.Sources.MassFlowSource_T boundary1(
     redeclare package Medium = Medium,
     nPorts=1,
@@ -66,7 +67,7 @@ protected
   final parameter Real alfa=1.5 - 12/(2*3.14)*atan(dt/(dt + delta));
   final parameter Real beta=1.5 - 0.42*log(delta/(dt + 1));
   final parameter Real delta=sqrt(3.15*10^7*ground1.k/3.14/ground1.rho/ground1.c);
-  final parameter Real Lpi=A    *ground1.k/dt*sqrt(1/((1 + delta/dt)^2 + 1));
+  final parameter Real Lpi=A*ground1.k/dt*sqrt(1/((1 + delta/dt)^2 + 1));
   final parameter Real Lpe=0.37*PWall*ground1.k*log(delta/dt + 1);
   Real m = sim.solTim.y/3.1536e7*12 "time in months";
   final parameter Integer nLayGro = layGro.nLay "Number of ground layers";
@@ -96,8 +97,11 @@ protected
       outputAngles=sim.outputAngles)
     "Weather data bus connectable to weaBus connector from Buildings Library"
     annotation (Placement(transformation(extent={{46,-90},{66,-70}})));
-equation
 
+initial equation
+    QTra_design=UEqui*A*(TRefZon - weaBus.TGroundDes);
+equation
+  connect(TdesGround.u, weaBus.TGroundDes);
   connect(periodicFlow.port, layMul.port_b) annotation (Line(points={{-20,22},{
           -14,22},{-14,0},{-10,0}}, color={191,0,0}));
   connect(layGro.port_a, layMul.port_b)
@@ -181,6 +185,11 @@ zone that is surrounded by air at the ambient temperature.
 </html>", revisions="<html>
 <ul>
 <li>
+November 7, 2024, by Anna Dell'Isola and Jelger Jansen:<br/>
+Update calculation of transmission design losses.
+See <a href=\"https://github.com/open-ideas/IDEAS/issues/1337\">#1337</a>
+</li>
+<li>
 May 16, 2024, by Lucas Verleyen:<br>
 Created final and protected parameter <code>T_start_gro</code> for initial temperature of the ground (<code>layGro</code>).<br>
 See <a href=https://github.com/open-ideas/IDEAS/issues/1292>#1292</a> for more information.
diff --git a/IDEAS/Buildings/Components/Window.mo b/IDEAS/Buildings/Components/Window.mo
index 3c2e29506..e9d75baba 100644
--- a/IDEAS/Buildings/Components/Window.mo
+++ b/IDEAS/Buildings/Components/Window.mo
@@ -12,7 +12,7 @@ model Window "Multipane window"
            A*(1 - frac),
            linearise=linIntCon_a or sim.linearise,
            dT_nominal=dT_nominal_a),
-    QTra_design(fixed=false),
+    final QTra_design(fixed=false),
     Qgai(y=if sim.computeConservationOfEnergy then
                                                   (gain.propsBus_a.surfCon.Q_flow +
         gain.propsBus_a.surfRad.Q_flow + gain.propsBus_a.iSolDif.Q_flow + gain.propsBus_a.iSolDir.Q_flow) else 0),
@@ -74,10 +74,8 @@ model Window "Multipane window"
   Modelica.Blocks.Interfaces.RealInput Ctrl if controlled
     "Control signal between 0 and 1, i.e. 1 is fully closed" annotation (
       Placement(visible = true,transformation(
-        
         origin={-50,-110},extent={{20,-20},{-20,20}},
         rotation=-90), iconTransformation(
-        
         origin={-40,-100},extent={{10,-10},{-10,10}},
         rotation=-90)));
 
@@ -190,7 +188,7 @@ protected
     "Outside air model"
     annotation (Placement(transformation(extent={{-40,-100},{-20,-80}})));
 initial equation
-  QTra_design = (U_value*A + (if fraType.briTyp.present then fraType.briTyp.G else 0)) *(273.15 + 21 - Tdes.y);
+  QTra_design = (U_value*A + (if fraType.briTyp.present then fraType.briTyp.G else 0)) *(TRefZon - Tdes.y);
 
   assert(not use_trickle_vent or sim.interZonalAirFlowType <> IDEAS.BoundaryConditions.Types.InterZonalAirFlow.None,
     "In " + getInstanceName() + ": Trickle vents can only be enabled when sim.interZonalAirFlowType is not None.");
@@ -271,8 +269,9 @@ equation
   connect(shaType.hForcedConExt, radSolData.hForcedConExt) annotation (
     Line(points={{-68.5,-32.7043},{-76,-32.7043},{-76,-62.2},{-79.4,-62.2}},
                                                                     color = {0, 0, 127}));
-  connect(outsideAir.TDryBul_in, shaType.TDryBul) annotation(
-    Line(points = {{-42, -90}, {-46, -90}, {-46, -48}, {-58, -48}}, color = {0, 0, 127}));
+  connect(outsideAir.TDryBul_in, shaType.TDryBul) annotation (
+    Line(points={{-42,-90},{-46,-90},{-46,-49.4895},{-57.5,-49.4895}},
+                                                                    color = {0, 0, 127}));
   annotation (
     Icon(coordinateSystem(preserveAspectRatio=true, extent={{-60,-100},{60,100}}),
         graphics={Rectangle(fillColor = {255, 255, 255}, pattern = LinePattern.None, fillPattern = FillPattern.Solid, extent = {{-50, -90}, {50, 100}}),
@@ -340,6 +339,11 @@ IDEAS.Buildings.Components.Validations.WindowEN673</a>
 </html>", revisions="<html>
 <ul>
 <li>
+November 7, 2024, by Anna Dell'Isola and Jelger Jansen:<br/>
+Update calculation of transmission design losses.
+See <a href=\"https://github.com/open-ideas/IDEAS/issues/1337\">#1337</a>
+</li>
+<li>
 May 22, 2022, by Filip Jorissen:<br/>
 Fixed Modelica specification compatibility issue.
 See <a href=\"https://github.com/open-ideas/IDEAS/issues/1254\">
@@ -467,4 +471,4 @@ Adjusted implementation for grouping of solar calculations.
 </li>
 </ul>
 </html>"));
-end Window;
\ No newline at end of file
+end Window;
diff --git a/IDEAS/Buildings/Components/Zone.mo b/IDEAS/Buildings/Components/Zone.mo
index ef1c99063..f1e707bfd 100644
--- a/IDEAS/Buildings/Components/Zone.mo
+++ b/IDEAS/Buildings/Components/Zone.mo
@@ -117,6 +117,13 @@ The model <code>ligCtr</code> determines how the lighting is controlled.
 See <a href=\"modelica://IDEAS.Buildings.Components.LightingControl\">
 IDEAS.Buildings.Components.LightingControl</a> for the available options.
 </p>
+<p>
+The design heat losses <code>Q_design</code> are calculated via an initial equation based on, but not exactly according to, NBN EN 12831-1 ANB:2020.
+This value includes heat losses via transmission (<code>QTra_design</code>) and infiltration (<code>QInf_design</code>) and accounts for reheating (<code>QRH_design</code>).
+Ventilation losses are not considered as this system is typically modelled in a seperate block, 
+see for example <a href=\"modelica://IDEAS.Templates.Interfaces.Building\"> IDEAS.Templates.Interfaces.Building</a>.
+In this model, the total design losses are calculated as the sum of the design losses from the building envelope (transmission, infiltration, reheating) and the ventilation system.
+</p>
 </html>", revisions="<html>
 <ul>
 <li>