diff --git a/tests/domain/epc/test_main_heating_system_overlay.py b/tests/domain/epc/test_main_heating_system_overlay.py
index dae992cba..27eb6625b 100644
--- a/tests/domain/epc/test_main_heating_system_overlay.py
+++ b/tests/domain/epc/test_main_heating_system_overlay.py
@@ -58,6 +58,22 @@ def test_electric_room_heaters_assume_a_dual_economy7_meter() -> None:
     assert simulation.heating.meter_type == "Dual"
 
 
+def test_electric_room_heaters_overlay_the_room_heater_category() -> None:
+    # A landlord "Electric room heaters" override replaces the lodged system, so
+    # the overlay must stamp the room-heater heating category (SAP Table 4a
+    # Category 10). Leaving it unset lets a replaced storage heater's category 7
+    # survive onto the effective cert, which the SAP 10.2 Table 12a resolver
+    # (keyed on category) mis-reads as off-peak storage and bills the peaky room
+    # heaters at the all-night low rate.
+    # Act
+    simulation = main_heating_overlay_for("Electric room heaters", 0)
+
+    # Assert — SAP Table 4a Category 10 (Room heaters).
+    assert simulation is not None
+    assert simulation.heating is not None
+    assert simulation.heating.main_heating_category == 10
+
+
 @pytest.mark.parametrize(
     ("main_heating_value", "code"),
     [