Bill water-only community DHW on the heat-pump network fuel 🟩

A water-only community scheme (water_heating_code 950/951/952) is served
by its own heat network, independent of the space main. _water_heating_fuel_code
now resolves the network's SAP Table 12 community fuel (e.g. 41 = heat from
electric heat pump @ 4.24 p/kWh, COP already priced into the row) instead of
defaulting to the space main's fuel. For a direct-electric main this had
mis-billed communal heat-pump DHW at the 13.19p standard-electric rate.

Dovestone Apt 4 (cert 9556-3047-3304-5355-1200): hot-water cost £353 → £113,
SAP 67 → 80.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DX8oAGsGkBHq3U4dsxYRzz

datatypes/epc/domain/tests/test_from_sap_schema.py

@@ -1079,3 +1079,18 @@ class TestSapWaterOnlyCommunityDhw:
         assert epc.sap_heating.community_water_heating_efficiency == pytest.approx(
             301.6
         )
+
+    def test_prices_community_dhw_on_the_heat_pump_network(self) -> None:
+        # Arrange — communal heat-pump DHW; the space main is direct electric.
+        from domain.sap10_calculator.calculator import Sap10Calculator
+
+        raw = load("sap_18_0_water_community_hp.json")
+        epc = EpcPropertyDataMapper.from_api_response(raw)
+
+        # Act
+        result = Sap10Calculator().calculate(epc)
+
+        # Assert — DHW bills on the community heat-pump fuel (SAP Table 32 code
+        # 41 @ 4.24 p/kWh, COP baked in), NOT the direct-electric main (code 39
+        # @ 13.19 p/kWh) it was defaulting to.
+        assert result.hot_water_fuel_code == 41

domain/sap10_calculator/rdsap/cert_to_inputs.py

@@ -1943,6 +1943,18 @@ def _water_heating_fuel_code(epc: EpcPropertyData) -> Optional[int]:
     no fuel_type lodged, so the old fallback resolved to None and CO2/
     PE/cost lookups returned defaults instead of the gas-combi values.
     """
+    # A water-ONLY community scheme (WHC 950/951/952) is served by its own heat
+    # network, independent of the space main. Bill DHW on the network's SAP
+    # Table 12 community fuel (e.g. 41 = heat from electric heat pump @ 4.24
+    # p/kWh — the COP is already priced into the community row) rather than
+    # defaulting to the space main's fuel, which for a direct-electric main
+    # mis-billed communal heat-pump DHW at the 13.19p standard-electric rate.
+    community_water_fuel = epc.sap_heating.community_water_heating_fuel
+    if (
+        epc.sap_heating.water_heating_code in _WATER_HEAT_NETWORK_ONLY_CODES
+        and community_water_fuel is not None
+    ):
+        return community_water_fuel
     if epc.sap_heating.water_heating_fuel:
         fuel = epc.sap_heating.water_heating_fuel
         # When DHW is on a heat network, the colliding community fuels