diff --git a/tests/domain/epc/test_main_heating_system_overlay.py b/tests/domain/epc/test_main_heating_system_overlay.py
index e4bd544c..e102336e 100644
--- a/tests/domain/epc/test_main_heating_system_overlay.py
+++ b/tests/domain/epc/test_main_heating_system_overlay.py
@@ -36,6 +36,16 @@ def test_gas_combi_overlays_the_primary_heating_code() -> None:
     assert simulation.heating.sap_main_heating_code == 104
 
 
+def test_electric_room_heaters_overlay_the_direct_acting_room_heater_code() -> None:
+    # Act — panel/convector/radiant direct-acting electric room heaters
+    simulation = main_heating_overlay_for("Electric room heaters", 0)
+
+    # Assert — SAP Table 4a code 691, NOT convector storage (403).
+    assert simulation is not None
+    assert simulation.heating is not None
+    assert simulation.heating.sap_main_heating_code == 691
+
+
 @pytest.mark.parametrize(
     ("main_heating_value", "code"),
     [