Merge pull request #1644 from Hestia-Homes/fix/elmhurst-community-dhw-rate

Bill community-heating DHW at the network fuel rate (not lodged mains-gas)

domain/sap10_calculator/rdsap/cert_to_inputs.py

@@ -7698,9 +7698,16 @@ def _fuel_cost(
         if main_2_fuel_code is not None
         else main_1_high_rate_gbp_per_kwh
     )
+    # When DHW is supplied by a community heat network (WHC 901/902/914 on a
+    # heat-network main), the network heat — space AND water — bills at the
+    # main's Table-12 heat-network rate, NOT the cert-lodged HW fuel (which
+    # Elmhurst defaults to "Mains gas"). Mirror the .173 inherit already applied
+    # on the legacy/off-peak HW-rate paths (`_hot_water_fuel_cost_gbp_per_kwh`)
+    # into this §10a STANDARD-tariff path, which is the one that drives (255)/SAP.
     water_high_rate_gbp_per_kwh = (
-        table_32_unit_price_p_per_kwh(water_heating_fuel_code)
-        * _PENCE_TO_GBP
+        _fuel_cost_gbp_per_kwh(main, prices)
+        if _is_community_heating_hw_from_main(epc)
+        else table_32_unit_price_p_per_kwh(water_heating_fuel_code) * _PENCE_TO_GBP
     )
     # Secondary fuel cost: route through the cert's `secondary_fuel_type`
     # when lodged (e.g. mains-gas fire SAP code 605 → fuel 26 → Table 32

tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py

@@ -45,7 +45,11 @@ from domain.sap10_ml.tests._fixtures import (
     make_sap_heating,
     make_window,
 )
-from domain.sap10_calculator.calculator import Sap10Calculator, SapResult
+from domain.sap10_calculator.calculator import (
+    Sap10Calculator,
+    SapResult,
+    calculate_sap_from_inputs,
+)
 from domain.sap10_calculator.exceptions import (
     MissingMainFuelType,
     UnmappedSapCode,
@@ -69,6 +73,8 @@ from domain.sap10_calculator.rdsap.cert_to_inputs import (
     _TABLE_4G_DEFAULT_MEV_SFP_W_PER_L_PER_S,  # pyright: ignore[reportPrivateUsage]
     dimensions_from_cert,
     fuel_cost_section_from_cert,
+    _fuel_cost_gbp_per_kwh,  # pyright: ignore[reportPrivateUsage]
+    RDSAP_10_TABLE_32_PRICES,  # pyright: ignore[reportPrivateUsage]
     _table_12_factor_fuel_code,  # pyright: ignore[reportPrivateUsage]
     _table_12a_system_for_main,  # pyright: ignore[reportPrivateUsage]
     _is_electric_main,  # pyright: ignore[reportPrivateUsage]
@@ -252,6 +258,50 @@ def test_thermal_mass_wall_code_8_is_park_home_only_when_property_type_says_so()
     assert abs(tmp_system_built - 250.0) <= 1e-9
 
 
+def test_community_heating_dhw_bills_at_network_rate_not_lodged_fuel() -> None:
+    # Arrange — a community heat-network main (Table 4a code 301, cat 6) whose
+    # DHW is heated by the network (WHC 901, "from the main system") but whose
+    # lodged HW fuel is the Elmhurst "Mains gas" (26) placeholder. SAP 10.2 §C3
+    # bills network heat — space AND water — at the network's own Table-12 rate,
+    # so the DHW must inherit the main's heat-network rate, not the lodged fuel's.
+    main = MainHeatingDetail(
+        has_fghrs=False,
+        main_fuel_type=20,  # mains gas (community)
+        heat_emitter_type=1,
+        emitter_temperature=1,
+        main_heating_control=2306,
+        main_heating_category=6,
+        sap_main_heating_code=301,
+    )
+    epc = make_minimal_sap10_epc(
+        total_floor_area_m2=_TYPICAL_TFA_M2,
+        habitable_rooms_count=4,
+        country_code="ENG",
+        sap_building_parts=[make_building_part(construction_age_band="E")],
+        sap_heating=make_sap_heating(
+            main_heating_details=[main],
+            water_heating_code=901,  # DHW from the main (heat network)
+            water_heating_fuel=26,  # Elmhurst "Mains gas" placeholder
+        ),
+    )
+
+    # Act
+    section = fuel_cost_section_from_cert(epc)
+
+    # Assert — total DHW cost is priced at the main's heat-network rate, NOT the
+    # lodged mains-gas rate. Network fuel is single-rate, so (245)+(246)+(247) =
+    # HW kWh x network rate.
+    assert section is not None
+    hw_kwh = calculate_sap_from_inputs(cert_to_inputs(epc)).hot_water_kwh_per_yr
+    total_water_cost_gbp = (
+        section.water_high_rate_cost_gbp
+        + section.water_low_rate_cost_gbp
+        + section.water_other_fuel_cost_gbp
+    )
+    network_rate = _fuel_cost_gbp_per_kwh(main, RDSAP_10_TABLE_32_PRICES)
+    assert total_water_cost_gbp == pytest.approx(hw_kwh * network_rate, rel=1e-6)
+
+
 def test_heat_network_main_applies_table12c_dlf_to_main_heating_efficiency() -> None:
     # Arrange — heat-network main heating (Table 4a code 301 = community
     # heating with CHP/boilers; main_heating_category=6). Cert age band