fix(fuel): cost main heating system 2 at its own fuel price, not main 1's (SAP 10.2 §10a worksheet line 213)

Main heating system 2's space-heating fuel cost (worksheet (213)) was billed
at main system 1's Table 32 unit price (`main_2_high_rate_gbp_per_kwh` reused
`main_1_high_rate_gbp_per_kwh`). For a dual-FUEL pair this grossly mis-costs the
second main: cert 10032957680 "Copse Cottage" (main 1 electric room heaters
fuel 30, main 2 wood logs fuel 6) charged its 9481 kWh of wood at 13.19 p/kWh
instead of 4.23 p/kWh — +£850/yr → SAP 21.75 vs lodged 45.

Route main 2 through its own fuel code (`_main_fuel_code(details[1])`), mirroring
the existing secondary-fuel handling. Copse Cottage 21.75 -> 45.94. Corpus
within-0.5 holds 72.5%, SAP MAE 0.815 -> 0.793 (ratcheted ceiling 0.82 -> 0.80);
CO2/PE unchanged. Same-fuel dual mains (gas+gas) unaffected. Off-peak-tariff
dual-fuel mains still defer to the legacy scalar path (separate slice).

Spec-cited unit pin added (AAA). pyright not installed locally — strict type
gate not run.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

domain/sap10_calculator/rdsap/cert_to_inputs.py

@@ -6927,6 +6927,26 @@ def _fuel_cost(
     main_1_high_rate_gbp_per_kwh = (
         table_32_unit_price_p_per_kwh(main_fuel_code) * _PENCE_TO_GBP
     )
+    # Main heating system 2 is costed at ITS OWN fuel price (SAP 10.2 §10a
+    # worksheet line (213) bills main system 2's fuel separately from main 1's
+    # (211)) — Table 32 unit price keyed on main 2's fuel code. Pre-fix this
+    # column reused `main_1_high_rate_gbp_per_kwh`, charging a dual-fuel second
+    # main (e.g. wood logs SAP code 633, fuel 6 @ 4.23 p/kWh) at the main-1
+    # electric rate (13.19 p/kWh), grossly over-costing electric+wood
+    # room-heater dwellings (cert 10032957680 "Copse Cottage" +£850/yr ->
+    # SAP -23). Falls back to main 1's price only when no second main is lodged.
+    main_2_detail = (
+        epc.sap_heating.main_heating_details[1]
+        if epc.sap_heating
+        and len(epc.sap_heating.main_heating_details or []) >= 2
+        else None
+    )
+    main_2_fuel_code = _main_fuel_code(main_2_detail)
+    main_2_high_rate_gbp_per_kwh = (
+        table_32_unit_price_p_per_kwh(main_2_fuel_code) * _PENCE_TO_GBP
+        if main_2_fuel_code is not None
+        else main_1_high_rate_gbp_per_kwh
+    )
     water_high_rate_gbp_per_kwh = (
         table_32_unit_price_p_per_kwh(water_heating_fuel_code)
         * _PENCE_TO_GBP
@@ -6976,7 +6996,7 @@ def _fuel_cost(
         main_1_low_rate_gbp_per_kwh=0.0,
         main_1_high_rate_fraction=1.0,
         main_2_kwh_per_yr=main_2_kwh,
-        main_2_high_rate_gbp_per_kwh=main_1_high_rate_gbp_per_kwh,
+        main_2_high_rate_gbp_per_kwh=main_2_high_rate_gbp_per_kwh,
         main_2_low_rate_gbp_per_kwh=0.0,
         main_2_high_rate_fraction=1.0 if main_2_kwh > 0.0 else 0.0,
         secondary_kwh_per_yr=secondary_kwh,

tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py

@@ -473,6 +473,60 @@ def test_heat_network_primary_loss_uses_p1_h3_all_months_per_table_3() -> None:
     assert abs(sum(wh_result.primary_loss_monthly_kwh) - expected_primary_kwh) <= 1e-6
 
 
+def test_dual_main_system_2_costed_at_its_own_fuel_price() -> None:
+    # Arrange — SAP 10.2 §10a: main heating system 2's fuel cost (worksheet
+    # line (213)) is billed at ITS OWN Table 32 fuel price, separately from
+    # main system 1's (211). A dwelling with main-1 = electric room heaters
+    # (SAP 691, fuel 30 standard electricity @ 13.19 p/kWh) and main-2 =
+    # wood-log room heaters (SAP 633, fuel 6 @ 4.23 p/kWh), 50/50 split, on a
+    # single-rate meter (so the §10a standard path runs). Pre-fix the main-2
+    # column reused main-1's electric price, charging the wood at 13.19 p/kWh
+    # and grossly over-costing electric+wood dwellings (cert 10032957680
+    # "Copse Cottage" SAP 21.75 -> 45.94 vs lodged 45).
+    main_1_electric = MainHeatingDetail(
+        has_fghrs=False,
+        main_fuel_type=30,  # standard electricity
+        heat_emitter_type=0,
+        emitter_temperature="NA",
+        main_heating_control=2106,
+        main_heating_category=10,
+        sap_main_heating_code=691,
+        main_heating_fraction=50,
+    )
+    main_2_wood = MainHeatingDetail(
+        has_fghrs=False,
+        main_fuel_type=6,  # wood logs — Table 32 = 4.23 p/kWh
+        heat_emitter_type=0,
+        emitter_temperature="NA",
+        main_heating_control=2106,
+        main_heating_category=10,
+        sap_main_heating_code=633,
+        main_heating_fraction=50,
+    )
+    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="D")],
+        sap_heating=make_sap_heating(
+            main_heating_details=[main_1_electric, main_2_wood],
+        ),
+    )
+
+    # Act
+    inputs = cert_to_inputs(epc)
+    result = Sap10Calculator().calculate(epc)
+    fc = inputs.fuel_cost
+    main_2_kwh = result.main_2_heating_fuel_kwh_per_yr
+
+    # Assert — main-2 wood billed at 4.23 p/kWh (0.0423 £/kWh), NOT main-1's
+    # electric 13.19 p/kWh. (213) main_2_total_cost = kWh × wood price.
+    assert main_2_kwh > 0.0
+    assert abs(fc.main_2_total_cost_gbp - main_2_kwh * 0.0423) <= 1e-6
+    # And explicitly NOT the electric rate (the pre-fix value).
+    assert abs(fc.main_2_total_cost_gbp - main_2_kwh * 0.1319) > 1.0
+
+
 def test_cylinder_thermostat_assumed_present_per_sap_9_4_9() -> None:
     # Arrange — SAP 10.2 §9.4.9 (PDF p.32): "A cylinder thermostat should be
     # assumed to be present when the domestic hot water is obtained from a

tests/infrastructure/epc_client/test_sap_accuracy_corpus.py

@@ -194,7 +194,7 @@ _CORPUS = Path(
 # stress worksheet (simulated case 46): closed its last ventilation residual
 # (our Jan ACH 9.14 -> 9.0748 exact; SAP 29 -> 30 = accredited Elmhurst).
 _MIN_WITHIN_HALF_SAP = 0.72
-_MAX_SAP_MAE = 0.82
+_MAX_SAP_MAE = 0.80
 _MAX_CO2_MAE_TONNES = 0.09      # t CO2 / yr vs co2_emissions_current
 _MAX_PE_PER_M2_MAE = 3.7        # kWh / m2 / yr vs energy_consumption_current