fix(ventilation): map API mechanical_ventilation_index_number for MEV fan electricity

Follow-up to the §2 MV-kind slice. Once MEV dwellings stopped
under-stating their ventilation HEAT loss, a +0.9 SAP over-rate residual
remained — the MEV FAN ELECTRICITY (§5 Table 4f line (230a),
`SFPav × 1.22 × V`, PCDB Tables 322 decentralised-MEV + 329 in-use
factors). `_mev_decentralised_kwh_per_yr_from_cert` already composes it,
but reads `epc.mechanical_ventilation_index_number` +
`epc.mechanical_vent_duct_type`, and the API builder
(`from_rdsap_schema_21_0_1`) never set either — so `pcdf_id is None`
short-circuited the fan energy to 0 on every API cert (the Summary/
Elmhurst path set them, so cert 000565 already billed it).

Wire both schema fields through the 21.0.1 API construction (the corpus
schema). Eval: the 9 MEV certs carrying a PCDB index closed +0.90 ->
+0.13 signed (fan electricity now billed); headline within-0.5 55.01% ->
55.12%, mean|err| 1.233 -> 1.232, 909 computed / 0 raises. Only those 9
certs move (clean diff). The 11 index-less MEV certs still sit at +1.36 —
they need the SAP Table 4h DEFAULT specific fan power (no PCDB record), a
separate slice.

New end-to-end test + fixture (cert 1300, Titon-class dMEV index 500777,
Flexible duct): from_api_response preserves the index + duct type and
(230a) resolves to a positive fan-energy contribution. Goldens + full
calc/epc regression green; pyright net-zero.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

datatypes/epc/domain/mapper.py

@@ -1559,6 +1559,18 @@ class EpcPropertyDataMapper:
             open_chimneys_count=schema.open_chimneys_count,
             insulated_door_count=schema.insulated_door_count,
             draughtproofed_door_count=schema.draughtproofed_door_count,
+            # Mechanical ventilation PCDB plumbing — feeds the §5 Table 4f
+            # line (230a) decentralised-MEV fan electricity
+            # (`SFPav × 1.22 × V`, PCDB Tables 322/329). Without the index
+            # the cascade's `_mev_decentralised_kwh_per_yr_from_cert`
+            # short-circuits to 0, leaving the MEV fan running cost off the
+            # bill (the +0.9 SAP over-rate residual on the MEV cohort once
+            # the §2 ventilation heat-loss was fixed). Duct type selects the
+            # Table 329 in-use factor (1=Flexible / 2=Rigid).
+            mechanical_ventilation_index_number=(
+                schema.mechanical_ventilation_index_number
+            ),
+            mechanical_vent_duct_type=schema.mechanical_vent_duct_type,
             # Lighting
             led_fixed_lighting_bulbs_count=schema.led_fixed_lighting_bulbs_count,
             cfl_fixed_lighting_bulbs_count=schema.cfl_fixed_lighting_bulbs_count,

tests/domain/sap10_calculator/rdsap/test_golden_fixtures.py

@@ -983,6 +983,31 @@ def test_api_to_domain_mapper_preserves_main_heating_index_number(
         assert abs(inputs.main_heating_efficiency - expected_winter_eff) <= 1e-3
 
 
+def test_api_mapper_preserves_mechanical_ventilation_pcdb_index_for_mev_fan_energy() -> None:
+    # Arrange — cert 1300 is a decentralised-MEV dwelling
+    # (mechanical_ventilation=2) lodging a PCDB index (500777) + duct type
+    # (1=Flexible). The API path previously DROPPED
+    # `mechanical_ventilation_index_number`, so the §5 Table 4f line (230a)
+    # MEV fan electricity (`SFPav × 1.22 × V`, PCDB Tables 322/329)
+    # short-circuited to 0 in `_mev_decentralised_kwh_per_yr_from_cert` —
+    # leaving the fan running cost off the bill (+0.9 SAP over-rate residual
+    # on the MEV cohort once the §2 ventilation heat loss was fixed).
+    from domain.sap10_calculator.rdsap.cert_to_inputs import (
+        _mev_decentralised_kwh_per_yr_from_cert,  # pyright: ignore[reportPrivateUsage]
+    )
+
+    doc = _load_cert("1300-7634-0922-3203-3563")
+
+    # Act
+    epc = EpcPropertyDataMapper.from_api_response(doc)
+
+    # Assert — the PCDB pointer + duct type survive the API mapping, and the
+    # (230a) MEV fan electricity resolves to a positive contribution.
+    assert epc.mechanical_ventilation_index_number == 500777
+    assert epc.mechanical_vent_duct_type == 1
+    assert _mev_decentralised_kwh_per_yr_from_cert(epc) > 0.0
+
+
 def test_0240_api_wall_type_4_windows_map_to_roof_windows() -> None:
     """Cert 0240 lodges 6 windows with `window_wall_type=4` — the RdSAP
     API code for a roof window ("Roof of Room" rooflight / inclined