fix(tariff): include MVHR fan electricity in the off-peak Grid 2 fan split

SAP 10.2 Table 12a Grid 2 (PDF p.191) bills "Fans for mechanical ventilation
systems" at 0.71 (7-hour) / 0.58 (10-hour), distinct from "All other uses"
(0.90 / 0.80) which covers circulation pumps, flue fans and the solar HW pump.

The cost-split mech-vent kWh (`mev_kwh_for_cost_split`) only summed the
decentralised-MEV (230b) fans, not the (230a) MVHR fan electricity — even
though the total pumps/fans bucket adds both. So an MVHR dwelling on an
off-peak tariff billed its fan electricity at the 0.90/0.80 "all other uses"
rate instead of 0.71/0.58. The comment already said "MEV/MVHR-fan portion";
only the MEV term was wired when MVHR landed. Fixed to mirror both
mechanical-ventilation fan terms summed into the total.

Worksheet-proven on simulated case 50 (000565 semi + MVHR Vent Axia + dual
electric immersion, Unknown meter -> 7-hour via the §12 dual-immersion
trigger): the fan bucket (315.64 kWh, 100% MVHR per worksheet line 230a) was
billing at 14.311 p/kWh (0.90) vs Elmhurst's 12.451 p/kWh (0.71) — +£5.87/yr,
-0.23 SAP. After the fix our existing-dwelling rating reconciles to Elmhurst
EXACTLY: SAP value 38.8426 (=Elmhurst 38.8426 -> 39), total cost £1317.0116
(=Elmhurst £1317.0116 to the penny).

Same `mev_kwh_for_cost_split` feeds the CO2 + PE cascades, so all three split
consistently. 0 corpus impact (all 3 corpus MVHR certs are standard tariff);
gauge unchanged 73.3% / MAE 0.774 / CO2 0.08 / PE 3.4.

Pin: test_mvhr_fan_electricity_bills_at_grid2_fan_fraction_on_off_peak.
pyright strict gate not run locally (pyright not installed in this container).

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

domain/sap10_calculator/rdsap/cert_to_inputs.py

@@ -7318,10 +7318,21 @@ def cert_to_inputs(
             pumps_fans_kwh += _table_4f_circulation_pump_kwh(_pumps_main_2)
     pumps_fans_kwh += _table_4f_additive_components(epc)
     # Track the MEV/MVHR-fan portion separately so the cost cascade can
-    # apply Table 12a Grid 2 `FANS_FOR_MECH_VENT` (0.58 high-frac on
-    # 10-hour) instead of `ALL_OTHER_USES` (0.80) — see
-    # `_pumps_fans_fuel_cost_gbp_per_kwh`. Zero when no MEV is lodged.
-    mev_kwh_for_cost_split = _mev_decentralised_kwh_per_yr_from_cert(epc)
+    # apply Table 12a Grid 2 `FANS_FOR_MECH_VENT` (0.71 high-frac on
+    # 7-hour, 0.58 on 10-hour) instead of `ALL_OTHER_USES` (0.90 / 0.80)
+    # — see `_pumps_fans_fuel_cost_gbp_per_kwh`. Must mirror the
+    # mechanical-ventilation fan terms summed into the total pumps/fans
+    # at `_table_4f_additive_components` (230b decentralised MEV/extract)
+    # + the (230a) MVHR fan: both are "Fans for mechanical ventilation
+    # systems" in Grid 2, while flue fans / circulation pumps / solar HW
+    # pump are "All other uses". The MVHR term was omitted when MVHR
+    # landed, so an MVHR dwelling on off-peak billed its fan electricity
+    # at 0.90 instead of 0.71 (case 50: +£5.87/yr, -0.23 SAP). Zero when
+    # no mechanical-ventilation fan is lodged.
+    mev_kwh_for_cost_split = (
+        _mev_decentralised_kwh_per_yr_from_cert(epc)
+        + _mvhr_fan_kwh_per_yr_from_cert(epc)
+    )
     primary_age = _dwelling_age_band(epc)
 
     # SAP 10.2 Appendix D2.1: if the cert lodges a PCDB index number that

tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py

@@ -3876,6 +3876,71 @@ def test_mvhr_system_values_apply_pcdb_wet_room_point_and_table_329_iufs() -> No
     assert abs(fan_kwh - expected_fan_kwh) <= 1e-6
 
 
+def test_mvhr_fan_electricity_bills_at_grid2_fan_fraction_on_off_peak() -> None:
+    # Arrange — an MVHR dwelling on an off-peak tariff. SAP 10.2 Table 12a
+    # Grid 2 (PDF p.191) bills "Fans for mechanical ventilation systems" at
+    # 0.71 (7-hour) / 0.58 (10-hour) high-rate fraction, distinct from "All
+    # other uses" (0.90 / 0.80) which covers circulation pumps + flue fans.
+    # The MVHR fan electricity (230a) was summed into the pumps/fans total
+    # but NOT into the cost-split mech-vent kWh, so on off-peak it billed at
+    # the 0.90/0.80 "all other uses" rate. Worksheet-proven on simulated
+    # case 50 (000565 + MVHR + dual immersion): +£5.87/yr, -0.23 SAP.
+    import dataclasses
+
+    from domain.sap10_calculator.rdsap.cert_to_inputs import (
+        _mvhr_fan_kwh_per_yr_from_cert,  # pyright: ignore[reportPrivateUsage]
+        _tariff_high_low_rates_p_per_kwh,  # pyright: ignore[reportPrivateUsage]
+    )
+    from domain.sap10_calculator.tables.table_12a import (
+        OtherUse,
+        Tariff,
+        other_use_high_rate_fraction,
+    )
+    from tests.domain.sap10_calculator.worksheet import (
+        _elmhurst_worksheet_000565 as _w000565,
+    )
+
+    base = _w000565.build_epc()
+    assert base.sap_ventilation is not None
+    epc = dataclasses.replace(
+        base,
+        mechanical_ventilation_index_number=500140,
+        wet_rooms_count=2,
+        mechanical_vent_duct_type=2,  # rigid
+        sap_ventilation=dataclasses.replace(
+            base.sap_ventilation, mechanical_ventilation_kind="MVHR"
+        ),
+        sap_energy_source=dataclasses.replace(
+            base.sap_energy_source, meter_type="1"  # off-peak (10-hour)
+        ),
+    )
+
+    # Act
+    inputs = cert_to_inputs(epc)
+    rate = inputs.pumps_fans_fuel_cost_gbp_per_kwh
+
+    # Assert — the resolved rate is the kWh-weighted blend of the MVHR fan
+    # portion at the FANS_FOR_MECH_VENT fraction and the remaining pumps at
+    # ALL_OTHER_USES. The presence of the MVHR fan in the split makes it
+    # strictly cheaper than billing the whole bucket at ALL_OTHER_USES.
+    tariff = Tariff.TEN_HOUR
+    high, low = _tariff_high_low_rates_p_per_kwh(tariff)
+    fan_frac = other_use_high_rate_fraction(OtherUse.FANS_FOR_MECH_VENT, tariff)
+    other_frac = other_use_high_rate_fraction(OtherUse.ALL_OTHER_USES, tariff)
+    fan_blend = fan_frac * high + (1.0 - fan_frac) * low
+    other_blend = other_frac * high + (1.0 - other_frac) * low
+    fan_kwh = _mvhr_fan_kwh_per_yr_from_cert(epc)
+    total_kwh = inputs.pumps_fans_kwh_per_yr
+    non_fan_kwh = total_kwh - fan_kwh
+    expected = (
+        (fan_kwh * fan_blend + non_fan_kwh * other_blend) / total_kwh / 100.0
+    )
+    assert rate is not None
+    assert fan_kwh > 0.0
+    assert abs(rate - expected) <= 1e-12
+    assert rate < other_blend / 100.0  # regression guard: fan portion is in the split
+
+
 def test_mvhr_system_values_fall_back_to_table_4g_defaults_without_pcdb_index() -> None:
     # Arrange — an MVHR cert lodged with NO PCDF index (corpus cert
     # "Flat 1"). SAP 10.2 Table 4g (PDF p.176): default raw efficiency