diff --git a/tests/domain/epc/test_main_heating_system_overlay.py b/tests/domain/epc/test_main_heating_system_overlay.py
index 7bd0db524..9a329a321 100644
--- a/tests/domain/epc/test_main_heating_system_overlay.py
+++ b/tests/domain/epc/test_main_heating_system_overlay.py
@@ -436,3 +436,19 @@ def test_electric_room_heaters_drag_their_natural_electricity_fuel() -> None:
     assert simulation is not None
     assert simulation.heating is not None
     assert simulation.heating.main_fuel_type == 29
+
+
+def test_solid_fuel_room_heater_defaults_to_house_coal_as_its_natural_fuel() -> None:
+    # Solid fuel is fuel-ambiguous (coal / anthracite / smokeless / dual fuel /
+    # wood logs / pellets), but the system must still self-cohere when no
+    # main_fuel override is given. Default to house coal (RdSAP main_fuel code
+    # 33), the most common solid fuel; a main_fuel override naming a specific
+    # solid fuel still wins by last-wins composition (ADR-0041).
+
+    # Act
+    simulation = main_heating_overlay_for("Solid fuel room heater, closed", 0)
+
+    # Assert
+    assert simulation is not None
+    assert simulation.heating is not None
+    assert simulation.heating.main_fuel_type == 33