diff --git a/docs/adr/0041-landlord-heating-classification-targets-a-complete-modellable-taxonomy.md b/docs/adr/0041-landlord-heating-classification-targets-a-complete-modellable-taxonomy.md
index 3efcc1c9f..47614fe56 100644
--- a/docs/adr/0041-landlord-heating-classification-targets-a-complete-modellable-taxonomy.md
+++ b/docs/adr/0041-landlord-heating-classification-targets-a-complete-modellable-taxonomy.md
@@ -108,13 +108,15 @@ A room-heater / direct-acting archetype names a *system*, not its fuel — and f
 arrives as its own composable `main_fuel` override. Two rules keep the system
 self-coherent without coupling the two overlays:
 
-- **The heating overlay drags a natural fuel only where the archetype is
-  unambiguous** — electric room heaters / direct-acting / storage are
-  unambiguously electricity (RdSAP `main_fuel` code 29), a gas boiler is mains
-  gas (26). A **solid / oil room heater is fuel-ambiguous** (coal vs wood vs
-  smokeless), so it drags **no** natural fuel and relies on the `main_fuel`
-  override; dragging a guessed coal-vs-wood code would be *more* wrong (very
-  different carbon).
+- **Every archetype drags a natural fuel so the system self-coheres without a
+  `main_fuel` override.** Where the fuel is unambiguous it is exact — electric
+  room heaters / direct-acting / storage → electricity (RdSAP `main_fuel` 29), a
+  gas boiler → mains gas (26), an oil room heater → oil (28). **Solid fuel is
+  ambiguous** (coal / anthracite / smokeless / dual fuel / wood logs / pellets),
+  so it **defaults to house coal (33)** — the most common solid fuel — which a
+  specific `main_fuel` override then refines. (The `main_fuel` override's own
+  vocabulary must grow to carry the full solid-fuel list — a parallel taxonomy
+  gap, same shape as this one.)
 - **A present `main_fuel` override wins** by the applicator's last-wins
   composition (`apply_simulations` setattrs non-`None` fields in order), so the
   `main_fuel` overlay is applied **after** the heating overlay. The agreeing
diff --git a/domain/epc/property_overlays/main_heating_system_overlay.py b/domain/epc/property_overlays/main_heating_system_overlay.py
index bc00cf350..b08b230cf 100644
--- a/domain/epc/property_overlays/main_heating_system_overlay.py
+++ b/domain/epc/property_overlays/main_heating_system_overlay.py
@@ -84,9 +84,12 @@ _SOLID_FUEL_ROOM_HEATER_CODES = frozenset(range(631, 637))
 # 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.
+# main_fuel code 29). Solid fuel is ambiguous (coal / anthracite / smokeless /
+# dual fuel / wood logs / pellets), so it defaults to house coal (33) — the most
+# common solid fuel — when the landlord gives no `main_fuel` override; a specific
+# solid-fuel override still wins by last-wins composition.
 _ELECTRICITY_FUEL = 29
+_HOUSE_COAL_FUEL = 33
 _ELECTRIC_ROOM_HEATER_CODES = frozenset(range(691, 702))
 
 # SAP Table 4c full boiler-control code: programmer + room thermostat + TRVs. The
@@ -171,6 +174,8 @@ def _natural_fuel_for(code: int) -> Optional[int]:
     present `main_fuel` override wins by last-wins composition."""
     if code in _ELECTRIC_ROOM_HEATER_CODES:
         return _ELECTRICITY_FUEL
+    if code in _SOLID_FUEL_ROOM_HEATER_CODES:
+        return _HOUSE_COAL_FUEL
     return None