diff --git a/tests/domain/epc/test_main_heating_system_overlay.py b/tests/domain/epc/test_main_heating_system_overlay.py
index ea73c0f70..ca82a6ae1 100644
--- a/tests/domain/epc/test_main_heating_system_overlay.py
+++ b/tests/domain/epc/test_main_heating_system_overlay.py
@@ -383,3 +383,19 @@ def test_every_resolvable_main_heating_value_decodes(
 
     # Assert
     assert simulation is not None
+
+
+def test_solid_fuel_room_heater_decodes_to_the_closed_room_heater_code() -> None:
+    # A landlord-named solid-fuel room heater (e.g. a closed stove) is a
+    # recognised archetype, not a gas wet system — it must decode to its SAP
+    # Table 4a code (633, closed solid-fuel room heater), not overflow into the
+    # nearest wrong archetype the way the under-populated taxonomy did, sending
+    # "solid fuel room heaters: closed room heater" to Gas CPSU (ADR-0041).
+
+    # Act
+    simulation = main_heating_overlay_for("Solid fuel room heater, closed", 0)
+
+    # Assert — SAP Table 4a code 633 (closed solid-fuel room heater).
+    assert simulation is not None
+    assert simulation.heating is not None
+    assert simulation.heating.sap_main_heating_code == 633