diff --git a/domain/modelling/generators/heating_recommendation.py b/domain/modelling/generators/heating_recommendation.py
index d1300338..8e35eebb 100644
--- a/domain/modelling/generators/heating_recommendation.py
+++ b/domain/modelling/generators/heating_recommendation.py
@@ -66,6 +66,11 @@ _ASHP_OVERLAY = HeatingOverlay(
     main_heating_control=2210,
     main_heating_index_number=101413,
     main_heating_category=_HEAT_PUMP_CATEGORY,
+    # Hot water from the main heat-pump system via the new cylinder (code 901,
+    # "from main system"). Set absolutely so a combi (909/611) or electric
+    # (903/908) before is reset to the fixed HP end-state, not just the case
+    # where the before already lodged 901.
+    water_heating_code=901,
     water_heating_fuel=_ELECTRICITY_FUEL,
     cylinder_size=4,
     cylinder_insulation_type=1,
diff --git a/tests/domain/modelling/test_heating_recommendation.py b/tests/domain/modelling/test_heating_recommendation.py
index 90203d0f..52bf8b48 100644
--- a/tests/domain/modelling/test_heating_recommendation.py
+++ b/tests/domain/modelling/test_heating_recommendation.py
@@ -154,6 +154,7 @@ def test_gas_boiler_house_yields_an_ashp_bundle() -> None:
         main_heating_control=2210,
         main_heating_index_number=101413,
         main_heating_category=4,
+        water_heating_code=901,
         water_heating_fuel=30,
         cylinder_size=4,
         cylinder_insulation_type=1,