diff --git a/recommendations/Costs.py b/recommendations/Costs.py
index 08b05a8a..afcd48d9 100644
--- a/recommendations/Costs.py
+++ b/recommendations/Costs.py
@@ -1143,7 +1143,7 @@ class Costs:
             "labour_days": labour_days,
         }
 
-    def air_source_heat_pump(self):
+    def air_source_heat_pump(self, ashp_size):
         """
         Based on the region and type of property, this function will produce a cost estimation for an air source heat
         pump. This cost will include the boiler upgrade scheme grant
@@ -1151,14 +1151,19 @@ class Costs:
         """
 
         # This is the average cost of a project, we'll add some additional contingency
-        regional_cost = MCS_AIR_SOURCE_HEAT_PUMP_COST_DATA[self.region]
 
-        total_cost = regional_cost * (1 + self.CONTINGENCY) - BOILER_UPGRADE_SCHEME_ASHP_VALUE
+        if ashp_size is None:
+            cost = [x for x in INSTALLER_ASHP_COSTS if x["capacity_kw"] is None][0]["cost"]
+        else:
+            cost = [x for x in INSTALLER_ASHP_COSTS if x][0]["cost"]
+
+        # We add some contingency since there are additional costs such as resizing radiators, that could be required
+        total_cost = cost * (1 + self.CONTINGENCY)
         subtotal_before_vat = total_cost / (1 + self.VAT_RATE)
         vat = total_cost - subtotal_before_vat
 
-        # We assume 3 days installation
-        labour_days = 3
+        # We assume 5 days installation
+        labour_days = 5
         labour_hours = labour_days * 8
 
         return {
diff --git a/recommendations/HeatingRecommender.py b/recommendations/HeatingRecommender.py
index 203bab87..bdc73e3b 100644
--- a/recommendations/HeatingRecommender.py
+++ b/recommendations/HeatingRecommender.py
@@ -1,3 +1,4 @@
+import re
 from recommendations.Costs import Costs, BOILER_UPGRADE_SCHEME_ASHP_VALUE
 from recommendations.recommendation_utils import (
     check_simulation_difference, override_costs, combine_recommendation_configs
@@ -368,6 +369,75 @@ class HeatingRecommender:
             description = ("Replace the existing boiler and cylinder without a thermostat with a new electric combi "
                            "boiler")
 
+    def size_heat_pump(self):
+        """
+        Given the methodology by installers (SCIS) this function will perform a basic heat loss calculation and
+        produce a recommendation for the size of the heat pump
+        :return:
+        """
+
+        floor_area = self.property.floor_area
+
+        # We use the default heat loss W/m2 values are specified by the insaller, depending on the property type
+
+        def remap_to_heat_loss(construction_age_band):
+            if "before 1900" in construction_age_band:
+                return "Pre 1900 (solid stone)"
+            elif "1900-1929" in construction_age_band:
+                return "Early 1900s (solid brick)"
+            elif re.search(r'1930|1949|1950|1966|1967|1975', construction_age_band):
+                return "1950-1980 (cavity void)"
+            elif re.search(r'1976|1982|1983|1990', construction_age_band):
+                return "Post 1980 (cavity wall construction)"
+            elif re.search(r'1991|1995|1996|2002|2003|2011', construction_age_band):
+                return "2000-2018"
+            elif "2012 onwards" in construction_age_band:
+                return "New build (2018+)"
+            else:
+                return None
+
+        def select_heatpump_size(heat_loss_calculation):
+            """
+            This function calculates the size of the heat pump based on the heat loss calculation, mapping
+            the heat loss calculation to the size of the heat pump in KW
+            :param heat_loss_calculation: This is calcualted as the floor area multipled by the heat loss constant,
+            divided by 1000
+            """
+            if heat_loss_calculation < 5:
+                return 5
+            elif 5 <= heat_loss_calculation < 6:
+                return 6
+            elif 6 <= heat_loss_calculation < 8.5:
+                return 8.5
+            elif 8.5 <= heat_loss_calculation < 11.2:
+                return 11.2
+            elif 11.2 <= heat_loss_calculation < 14:
+                return 14
+            elif 14 <= heat_loss_calculation < 17:
+                return 17
+            elif 17 <= heat_loss_calculation < 20:
+                return 20
+            else:
+                return None
+
+        heat_loss_constants = {
+            "New build (2018+)": 35,
+            "2000-2018": 50,
+            "Post 1980 (cavity wall construction)": 60,
+            "1950-1980 (cavity void)": 70,
+            "Early 1900s (solid brick)": 80,
+            "Pre 1900 (solid stone)": 90
+        }
+
+        heat_loss_group = remap_to_heat_loss(self.property.construction_age_band)
+        heat_loss_constant = heat_loss_constants[heat_loss_group]
+
+        heat_loss_calculation = floor_area * heat_loss_constant / 1000
+
+        heat_pump_size = select_heatpump_size(heat_loss_calculation)
+
+        return heat_pump_size
+
     def recommend_air_source_heat_pump(self, phase, has_cavity_or_loft_recommendations, _return=False):
         """
         This method will implement the recommendation for an air source heat pump
@@ -383,8 +453,9 @@ class HeatingRecommender:
 
         controls_recommender = HeatingControlRecommender(self.property)
         controls_recommender.recommend(heating_description="Air source heat pump, radiators, electric")
+        ashp_size = self.size_heat_pump()
 
-        ashp_costs = self.costs.air_source_heat_pump()
+        ashp_costs = self.costs.air_source_heat_pump(ashp_size)
         if non_intrusive_recommendation:
             # Update with non-intrusive recommendation
             if non_intrusive_recommendation.get("cost"):