diff --git a/domain/epc/property_overlays/main_heating_system_overlay.py b/domain/epc/property_overlays/main_heating_system_overlay.py
index 6b7aca804..630c247a4 100644
--- a/domain/epc/property_overlays/main_heating_system_overlay.py
+++ b/domain/epc/property_overlays/main_heating_system_overlay.py
@@ -19,10 +19,21 @@ from __future__ import annotations
 
 from typing import Optional
 
-from domain.modelling.simulation import EpcSimulation
+from domain.modelling.simulation import EpcSimulation, HeatingOverlay
+
+# Canonical system archetype → representative `sap_main_heating_code` (SAP Table
+# 4b boiler rows / Table 4a). Codes map to the modern/condensing variant (A-G
+# efficiency deferred): 102 regular condensing, 104 condensing combi, 120 CPSU,
+# 404 fan storage heaters, 191 direct-acting electric boiler.
+_MAIN_HEATING_CODES: dict[str, int] = {
+    "Gas boiler, combi": 104,
+}
 
 
 def main_heating_overlay_for(
     main_heating_value: str, building_part: int
 ) -> Optional[EpcSimulation]:
-    raise NotImplementedError
+    code = _MAIN_HEATING_CODES.get(main_heating_value)
+    if code is None:
+        return None
+    return EpcSimulation(heating=HeatingOverlay(sap_main_heating_code=code))