diff --git a/cara/apps/calculator/model_generator.py b/cara/apps/calculator/model_generator.py
index 5a26afd0..ddaca9c9 100644
--- a/cara/apps/calculator/model_generator.py
+++ b/cara/apps/calculator/model_generator.py
@@ -83,8 +83,32 @@ class FormData:
         return model_from_form(self, tmp_raw_form_data)
 
     def ventilation(self) -> models.Ventilation:
-        # TODO
-        pass
+        # Initializes a ventilation instance as a window if 'natural' is selected, or as a HEPA-filter otherwise
+        if self.ventilation_type == 'natural':
+            if self.windows_open == 'always':
+                period, duration = 120, 120
+            else:
+                period, duration = 15, 120
+            # I multiply the opening width by the number of windows to simulate the correct window area
+            if self.event_type == 'single_event':
+                month_number = int(self.single_event_date.split('/')[1])
+                month = ['Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec'][month_number]
+            else:
+                month = self.recurrent_event_month[:3]
+
+            inside_temp = models.PiecewiseConstant((0, 24), (293,))
+            outside_temp = models.GenevaTemperatures[month]
+
+            ventilation = models.WindowOpening(active=models.PeriodicInterval(period=period, duration=duration),
+                                               inside_temp=inside_temp, outside_temp=outside_temp, cd_b=0.6,
+                                               window_height=self.window_height,
+                                               opening_length=self.opening_distance * self.windows_number)
+        else:
+            q_air_mech = self.air_changes + self.air_supply
+            ventilation = models.HEPAFilter(active=models.PeriodicInterval(period=120, duration=120),
+                                            q_air_mech=q_air_mech)
+
+        return ventilation
 
     def present_interval(self) -> models.Interval:
         coffee_period = (self.activity_finish - self.activity_start) // self.coffee_breaks
@@ -149,25 +173,6 @@ def model_from_form(form: FormData, tmp_raw_form_data) -> models.Model:
         volume = int(float(d['floor_area']) * form.ceiling_height)
     room = models.Room(volume=volume)
 
-    # Initializes a ventilation instance as a window if 'natural' is selected, or as a HEPA-filter otherwise
-    if d['ventilation_type'] == 'natural':
-        if d['windows_open'] == 'always':
-            period, duration = 120, 120
-        else:
-            period, duration = 15, 120
-        # I multiply the opening width by the number of windows to simulate the correct window area
-        ventilation = models.WindowOpening(active=models.PeriodicInterval(period=period, duration=duration),
-                                           inside_temp=models.PiecewiseConstant((0, 24), (293, )),
-                                           # TODO: This should be based on the month etc.
-                                           outside_temp=models.PiecewiseConstant((0, 24), (283, )),
-                                           cd_b=0.6,
-                                           window_height=float(d['window_height']),
-                                           opening_length=float(d['opening_distance']) * int(d['windows_number']))
-    else:
-        q_air_mech = float(d['air_changes']) if d['air_type'] == 'air_changes' else float(d['air_supply'])
-        ventilation = models.HEPAFilter(active=models.PeriodicInterval(period=120, duration=120),
-                                        q_air_mech=q_air_mech)
-
     # Initializes the virus as SARS_Cov_2
     virus = models.Virus.types['SARS_CoV_2']
 
@@ -195,7 +200,7 @@ def model_from_form(form: FormData, tmp_raw_form_data) -> models.Model:
     # Initializes and returns a model with the attributes defined above
     return models.Model(
         room=room,
-        ventilation=ventilation,
+        ventilation=form.ventilation(),
         infected=models.InfectedPerson(
             virus=virus,
             presence=form.present_interval(),