diff --git a/packages/domain/src/domain/sap/rdsap/cert_to_inputs.py b/packages/domain/src/domain/sap/rdsap/cert_to_inputs.py
index cb8032bd..690d2368 100644
--- a/packages/domain/src/domain/sap/rdsap/cert_to_inputs.py
+++ b/packages/domain/src/domain/sap/rdsap/cert_to_inputs.py
@@ -50,7 +50,7 @@ from datatypes.epc.domain.epc_property_data import (
 from domain.ml.demand import predicted_hot_water_kwh, predicted_lighting_kwh
 from domain.ml.sap_efficiencies import (
     seasonal_efficiency,
-    water_heating_efficiency,
+    water_heating_efficiency as _legacy_water_heating_efficiency,
 )
 from domain.sap.calculator import CalculatorInputs, WindowInput
 from domain.sap.tables.table_12 import co2_factor_kg_per_kwh, unit_price_p_per_kwh
@@ -411,6 +411,35 @@ def _other_fuel_cost_gbp_per_kwh(prices: PriceTable) -> float:
     return prices.standard_electricity_p_per_kwh * _PENCE_TO_GBP
 
 
+# Water-heating codes that say "inherit from the main system" — the
+# `seasonal_efficiency` cascade returns 0 as a sentinel for these in the
+# legacy `domain.ml.sap_efficiencies` module. We need to inherit through
+# the SAME cascade the main heating uses, including the main_heating_
+# category fallback (e.g. heat pumps return 2.30 via category 4).
+_WATER_INHERIT_FROM_MAIN_CODES: Final[frozenset[int]] = frozenset({901, 902, 914})
+
+
+def _water_efficiency_with_category_inherit(
+    *,
+    water_heating_code: Optional[int],
+    main_code: Optional[int],
+    main_category: Optional[int],
+    main_fuel: Optional[int],
+) -> float:
+    """When the cert says "hot water comes from the main system" (codes
+    901 / 902 / 914), inherit the main system's efficiency — and crucially
+    inherit the cascade that maps `main_heating_category` to a default
+    when `sap_main_heating_code` is None. The legacy water_heating_efficiency
+    only passes main_code through and so collapses heat pumps (cat 4) +
+    no-code lodgements into the 0.80 gas-boiler default.
+    """
+    if water_heating_code is None:
+        return _legacy_water_heating_efficiency(None, main_code)
+    if water_heating_code in _WATER_INHERIT_FROM_MAIN_CODES:
+        return seasonal_efficiency(main_code, main_category, main_fuel)
+    return _legacy_water_heating_efficiency(water_heating_code, main_code)
+
+
 
 
 def _co2_factor_kg_per_kwh(main: Optional[MainHeatingDetail]) -> float:
@@ -499,7 +528,12 @@ def cert_to_inputs(
     )
 
     eff = seasonal_efficiency(main_code, main_category, main_fuel)
-    water_eff = water_heating_efficiency(epc.sap_heating.water_heating_code, main_code)
+    water_eff = _water_efficiency_with_category_inherit(
+        water_heating_code=epc.sap_heating.water_heating_code,
+        main_code=main_code,
+        main_category=main_category,
+        main_fuel=main_fuel,
+    )
     hw_kwh = predicted_hot_water_kwh(
         total_floor_area_m2=epc.total_floor_area_m2,
         seasonal_efficiency_water=water_eff,