diff --git a/tests/domain/epc/test_main_heating_system_overlay.py b/tests/domain/epc/test_main_heating_system_overlay.py
index 4726d6866..0658bbf65 100644
--- a/tests/domain/epc/test_main_heating_system_overlay.py
+++ b/tests/domain/epc/test_main_heating_system_overlay.py
@@ -468,3 +468,22 @@ def test_air_source_heat_pump_decodes_to_the_default_ashp_code() -> None:
     assert simulation is not None
     assert simulation.heating is not None
     assert simulation.heating.sap_main_heating_code == 211
+
+
+def test_air_source_heat_pump_drags_its_coherent_companions() -> None:
+    # A heat pump is unambiguously electric (natural fuel 29) and SAP Table 4a
+    # category 4, billed on an off-peak (Dual) meter (§12 Rule 3 lists heat pumps
+    # 211-224). The overlay drags these code-derived companions so a system-only
+    # override is self-coherent on a cert that lodged a different system
+    # (ADR-0035 / ADR-0041).
+
+    # Act
+    simulation = main_heating_overlay_for("Air source heat pump", 0)
+
+    # Assert
+    assert simulation is not None
+    assert simulation.heating is not None
+    heating = simulation.heating
+    assert heating.main_heating_category == 4
+    assert heating.main_fuel_type == 29
+    assert heating.meter_type == "Dual"