diff --git a/domain/epc/property_overlays/main_heating_system_overlay.py b/domain/epc/property_overlays/main_heating_system_overlay.py
index a2d9bc634..bc00cf350 100644
--- a/domain/epc/property_overlays/main_heating_system_overlay.py
+++ b/domain/epc/property_overlays/main_heating_system_overlay.py
@@ -79,6 +79,16 @@ _ROOM_HEATER_CATEGORY = 10
 _ROOM_HEATER_CONTROL = 2601
 _SOLID_FUEL_ROOM_HEATER_CODES = frozenset(range(631, 637))
 
+# Natural-fuel coherence (ADR-0041): where an archetype unambiguously implies a
+# fuel, the overlay drags it so a system-only override is self-coherent on a cert
+# that lodged a different fuel. A later `main_fuel` override still wins (last-wins
+# composition); a contradicting landlord fuel is logged, not silently overridden.
+# Electric room heaters (Table 4a 691-701) are unambiguously electricity (RdSAP
+# main_fuel code 29). Solid/oil room heaters are fuel-ambiguous (coal vs wood vs
+# smokeless), so they drag no natural fuel and rely on the `main_fuel` override.
+_ELECTRICITY_FUEL = 29
+_ELECTRIC_ROOM_HEATER_CODES = frozenset(range(691, 702))
+
 # SAP Table 4c full boiler-control code: programmer + room thermostat + TRVs. The
 # landlord names the boiler, not its controls — but a gas boiler installed under
 # modern Building Regs must carry compliant controls, and this overlay already
@@ -155,6 +165,15 @@ def _category_for(code: int) -> Optional[int]:
     return None
 
 
+def _natural_fuel_for(code: int) -> Optional[int]:
+    """The fuel an archetype unambiguously implies (a coherent default), or None
+    where the fuel is ambiguous and must come from the `main_fuel` override. A
+    present `main_fuel` override wins by last-wins composition."""
+    if code in _ELECTRIC_ROOM_HEATER_CODES:
+        return _ELECTRICITY_FUEL
+    return None
+
+
 def _gas_boiler_overlay(code: int) -> HeatingOverlay:
     """The coherent gas-boiler companion set: a mains-gas connection + gas main
     fuel, the gas-boiler heating category, a fanned room-sealed flue, full modern
@@ -186,6 +205,7 @@ def main_heating_overlay_for(
         heating=HeatingOverlay(
             sap_main_heating_code=code,
             main_heating_category=_category_for(code),
+            main_fuel_type=_natural_fuel_for(code),
             meter_type=_meter_for(code),
             main_heating_control=_control_for(code),
             # A landlord override describes the existing dwelling, so its assumed