diff --git a/tests/domain/epc/test_main_heating_system_overlay.py b/tests/domain/epc/test_main_heating_system_overlay.py
index e102336e..af31e810 100644
--- a/tests/domain/epc/test_main_heating_system_overlay.py
+++ b/tests/domain/epc/test_main_heating_system_overlay.py
@@ -318,6 +318,19 @@ def test_the_three_heating_overrides_compose_without_conflict() -> None:
     assert result.sap_heating.water_heating_fuel == 29
 
 
+def test_electric_room_heaters_member_decodes_to_the_room_heater_code() -> None:
+    # Arrange — the canonical landlord archetype for direct-acting room heaters
+    member = MainHeatingSystemType.ELECTRIC_ROOM_HEATERS
+
+    # Act
+    simulation = main_heating_overlay_for(member.value, 0)
+
+    # Assert — member value stays in lock-step with the overlay (code 691)
+    assert simulation is not None
+    assert simulation.heating is not None
+    assert simulation.heating.sap_main_heating_code == 691
+
+
 @pytest.mark.parametrize(
     "member",
     [m for m in MainHeatingSystemType if m is not MainHeatingSystemType.UNKNOWN],