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 690d2368..4a13fa6a 100644
--- a/packages/domain/src/domain/sap/rdsap/cert_to_inputs.py
+++ b/packages/domain/src/domain/sap/rdsap/cert_to_inputs.py
@@ -128,6 +128,12 @@ _DEFAULT_THERMAL_MASS_PARAMETER_KJ_PER_M2_K: Final[float] = 250.0
 _DEFAULT_PUMPS_FANS_KWH_PER_YR: Final[float] = 130.0
 
 
+# Water-heating codes for instantaneous (no-cylinder) systems — SAP §4
+# Appendix J skips cylinder-storage + primary-pipework losses for these
+# because there's no cylinder and no primary circuit.
+_INSTANTANEOUS_WATER_CODES: Final[frozenset[int]] = frozenset({907, 909})
+
+
 @dataclass(frozen=True)
 class PriceTable:
     """Seam between the spec-correct SAP 10.2/10.3 Table 12 prices and
@@ -534,13 +540,22 @@ def cert_to_inputs(
         main_category=main_category,
         main_fuel=main_fuel,
     )
+    is_instantaneous = epc.sap_heating.water_heating_code in _INSTANTANEOUS_WATER_CODES
     hw_kwh = predicted_hot_water_kwh(
         total_floor_area_m2=epc.total_floor_area_m2,
         seasonal_efficiency_water=water_eff,
-        cylinder_size=_int_or_none(epc.sap_heating.cylinder_size),
-        cylinder_insulation_thickness_mm=epc.sap_heating.cylinder_insulation_thickness_mm,
-        cylinder_insulation_type=_int_or_none(epc.sap_heating.cylinder_insulation_type),
-        age_band=primary_age,
+        # Instantaneous HW systems (codes 907/909) have no cylinder and
+        # no primary circuit — pass None for both to suppress storage
+        # and primary losses in the demand model.
+        cylinder_size=None if is_instantaneous else _int_or_none(epc.sap_heating.cylinder_size),
+        cylinder_insulation_thickness_mm=(
+            None if is_instantaneous else epc.sap_heating.cylinder_insulation_thickness_mm
+        ),
+        cylinder_insulation_type=(
+            None if is_instantaneous
+            else _int_or_none(epc.sap_heating.cylinder_insulation_type)
+        ),
+        age_band=None if is_instantaneous else primary_age,
         has_wwhrs=False,
         has_solar_water_heating=epc.solar_water_heating,
     )