diff --git a/tests/domain/epc/test_main_heating_system_overlay.py b/tests/domain/epc/test_main_heating_system_overlay.py
index 4421fefe1..af8af3bce 100644
--- a/tests/domain/epc/test_main_heating_system_overlay.py
+++ b/tests/domain/epc/test_main_heating_system_overlay.py
@@ -101,6 +101,19 @@ def test_solid_fuel_room_heaters_decode_off_gas_cpsu(
     assert simulation.heating.sap_main_heating_code == code
 
 
+def test_electric_boiler_decodes_to_191_and_drags_electricity() -> None:
+    # An "A rated NA" boiler (all-electric per its main_fuel override) was funnelled
+    # into Gas CPSU (120, mains gas). It is a direct-acting electric boiler (SAP 191)
+    # and unambiguously electric, so the archetype drags electricity (29) — never
+    # leaving a mains-gas fuel to bleed through (ADR-0045).
+    simulation = main_heating_overlay_for("Electric boiler", 0)
+
+    assert simulation is not None
+    assert simulation.heating is not None
+    assert simulation.heating.sap_main_heating_code == 191
+    assert simulation.heating.main_fuel_type == 29
+
+
 @pytest.mark.parametrize(
     ("main_heating_value", "code"),
     [