diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index 681480feb..27ed37fb0 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -6066,6 +6066,20 @@ def _map_sap_heating(
         )
     )
 
+    # Water heating is only typed for cylinder dwellings; a combi leaves the WHC
+    # None so HW inherits the main system (issue #1565).
+    cylinder_present = heating.water_heating.cylinder_size != "No Cylinder"
+    water_heating_code = (
+        _pashub_water_heating_code(heating.water_heating.system)
+        if cylinder_present
+        else None
+    )
+    water_heating_fuel = (
+        _PASHUB_STANDARD_ELECTRICITY_FUEL
+        if water_heating_code == _PASHUB_WHC_ELECTRIC_IMMERSION
+        else None
+    )
+
     return SapHeating(
         instantaneous_wwhrs=InstantaneousWwhrs(),
         main_heating_details=[
@@ -6087,12 +6101,16 @@ def _map_sap_heating(
         shower_outlets=shower_outlets,
         cylinder_size=(
             heating.water_heating.cylinder_size
-            if heating.water_heating.cylinder_size != "No Cylinder"
+            if cylinder_present
             else None
         ),
         cylinder_insulation_type=heating.water_heating.insulation_type,
         cylinder_insulation_thickness_mm=heating.water_heating.insulation_thickness_mm,
-        immersion_heating_type=heating.water_heating.immersion_type,
+        water_heating_code=water_heating_code,
+        water_heating_fuel=water_heating_fuel,
+        immersion_heating_type=_pashub_immersion_type_code(
+            heating.water_heating.immersion_type, cylinder_present
+        ),
     )
 
 
@@ -7276,6 +7294,48 @@ def _pashub_glazing_type_int(label: Optional[str]) -> Union[int, str]:
     return _PASHUB_GLAZING_TYPE_TO_SAP10[key]
 
 
+# PasHub water-heating `system` label → SAP10 water-heating code (WHC) that the
+# calculator's water cascade keys off (cert_to_inputs: `_WHC_FROM_MAIN_HEATING`
+# 901 inherits the main fuel; `_WHC_ELECTRIC_IMMERSION` 903; heat-network water
+# 950). Only lodged for cylinder dwellings — combi dwellings leave it None and HW
+# inherits from the main system (issue #1565).
+_PASHUB_WATER_HEATING_SYSTEM_TO_SAP10: Dict[str, int] = {
+    "From main heating 1": 901,
+    "Electric immersion": 903,
+    "Hot water only community scheme - boilers": 950,
+}
+_PASHUB_WHC_ELECTRIC_IMMERSION: Final[int] = 903
+_PASHUB_STANDARD_ELECTRICITY_FUEL: Final[int] = 30  # Table 32 standard electricity
+
+
+def _pashub_water_heating_code(system_label: Optional[str]) -> Optional[int]:
+    """Resolve a PasHub water-heating `system` label to its SAP10 WHC code. Blank
+    is "no lodging" (None); an unmapped label strict-raises `UnmappedPasHubLabel`.
+    Call only for cylinder dwellings (#1565)."""
+    if system_label is None or not system_label.strip():
+        return None
+    key = system_label.strip()
+    if key not in _PASHUB_WATER_HEATING_SYSTEM_TO_SAP10:
+        raise UnmappedPasHubLabel("water heating system", system_label)
+    return _PASHUB_WATER_HEATING_SYSTEM_TO_SAP10[key]
+
+
+def _pashub_immersion_type_code(
+    immersion_label: Optional[str], cylinder_present: bool
+) -> Optional[int]:
+    """Resolve a PasHub `immersion_type` label to the SAP10 `immersion_heating_type`
+    code, reusing the Elmhurst map (1 = dual, 2 = single, RdSAP 10 §10.5). None
+    when no cylinder or no immersion lodged; unmapped label strict-raises."""
+    if not cylinder_present or immersion_label is None:
+        return None
+    key = immersion_label.strip()
+    if key in ("", "None"):
+        return None
+    if key not in _ELMHURST_IMMERSION_TYPE_LABEL_TO_SAP10:
+        raise UnmappedPasHubLabel("immersion type", immersion_label)
+    return _ELMHURST_IMMERSION_TYPE_LABEL_TO_SAP10[key]
+
+
 def _resolve_elmhurst_underfloor_subtype(
     main_floor: ElmhurstFloorDetails,
     main_age_band: str,
diff --git a/datatypes/epc/domain/tests/test_from_site_notes.py b/datatypes/epc/domain/tests/test_from_site_notes.py
index 52e6d22ba..04e13eeec 100644
--- a/datatypes/epc/domain/tests/test_from_site_notes.py
+++ b/datatypes/epc/domain/tests/test_from_site_notes.py
@@ -223,6 +223,63 @@ class TestGlazingTypeCoding:
             EpcPropertyDataMapper.from_site_notes(survey)
 
 
+class TestWaterHeatingCoding:
+    """`from_site_notes` must int-code the PAS Hub water-heating fields for
+    **cylinder** dwellings — `water_heating_code` (WHC), `water_heating_fuel`,
+    `immersion_heating_type` — not leave them None/raw. Combi dwellings (no
+    cylinder) keep the None default (HW inherits from the main system), so they
+    are untouched (issue #1565). example1 is a cylinder dwelling.
+    """
+
+    def _survey(self, **water: object) -> PasHubRdSapSiteNotes:
+        data = load("pashub_rdsap_site_notes_example1.json")
+        data["heating_and_hot_water"]["water_heating"].update(water)
+        return from_dict(PasHubRdSapSiteNotes, data)
+
+    def test_cylinder_from_main_maps_whc_901(self) -> None:
+        result = EpcPropertyDataMapper.from_site_notes(
+            self._survey(system="From main heating 1", cylinder_size="Normal (90-130 litres)")
+        )
+        assert result.sap_heating.water_heating_code == 901
+        assert result.sap_heating.water_heating_fuel is None
+
+    def test_electric_immersion_maps_903_electric_fuel_and_single_immersion(self) -> None:
+        result = EpcPropertyDataMapper.from_site_notes(
+            self._survey(
+                system="Electric immersion",
+                cylinder_size="Normal (90-130 litres)",
+                immersion_type="Single",
+            )
+        )
+        assert result.sap_heating.water_heating_code == 903
+        assert result.sap_heating.water_heating_fuel == 30  # standard electricity
+        assert result.sap_heating.immersion_heating_type == 2  # single
+
+    def test_community_scheme_maps_950(self) -> None:
+        result = EpcPropertyDataMapper.from_site_notes(
+            self._survey(
+                system="Hot water only community scheme - boilers",
+                cylinder_size="Normal (90-130 litres)",
+            )
+        )
+        assert result.sap_heating.water_heating_code == 950
+
+    def test_combi_leaves_whc_none(self) -> None:
+        result = EpcPropertyDataMapper.from_site_notes(
+            self._survey(system="From main heating 1", cylinder_size="No Cylinder")
+        )
+        assert result.sap_heating.water_heating_code is None
+
+    def test_unknown_cylinder_system_strict_raises(self) -> None:
+        with pytest.raises(UnmappedPasHubLabel):
+            EpcPropertyDataMapper.from_site_notes(
+                self._survey(
+                    system="Perpetual motion boiler",
+                    cylinder_size="Normal (90-130 litres)",
+                )
+            )
+
+
 class TestFromSiteNotesExample1:
     """
     Fixture: pashub_rdsap_site_notes_example1.json
@@ -581,6 +638,7 @@ class TestFromSiteNotesExample1:
                 cylinder_size="Normal (90-130 litres)",
                 cylinder_insulation_type="Factory fitted",
                 cylinder_insulation_thickness_mm=12,
+                water_heating_code=901,  # cylinder + "From main heating 1" → WHC 901
                 shower_outlets=ShowerOutlets(
                     shower_outlet=ShowerOutlet(
                         shower_outlet_type="Non-Electric Shower"