Model/docs/baseline-downgrade-followups.md

Follow-ups from the full-SAP baseline-downgrade work (ADR-0037)

Open items surfaced while fixing the full-SAP mapper / portfolio-796 downgrades. Each is separate from the full-SAP fix (fix/baseline-downgrades).

1. RdSAP ventilation mapping is inconsistent across schemas

Severity: accuracy bug for MEV/MVHR dwellings. The mappers handle sap_ventilation four different ways:

• 17_0 + full-SAP — build via _sap_17_1_ventilation, which maps mechanical_ventilation_kind. ✅
• 21_0_1 — rich inline SapVentilation(...) incl. the kind. ✅
• 17_1 / 18_0 / 20_0_0 — built SapVentilation(sheltered_sides=…) and dropped the kind. Fixed in this PR (mirror _api_mechanical_ventilation_kind).
• 19_0 / 21_0_0 — set no sap_ventilation at all → the dataclass default empty object. They drop the entire ventilation block (sheltered sides + kind + everything), not just the kind. Still open — a bigger, separate consistency fix (give them a proper sap_ventilation construction, mirroring 21.0.1), not a one-liner.

Either way, an MEV/MVHR cert (mechanical_ventilation ≠ 0) is treated as natural by the affected mappers — wrong §2 ventilation cascade (and heat recovery). Natural certs (code 0/5 → None) are unaffected.

• For natural-ventilation certs (mechanical_ventilation = 0, e.g. UPRN 100020603823 / property 726605) it's benign — unmapped → None defaults to natural in the §2 cascade.
• For MEV/MVHR certs (mechanical_ventilation ≠ 0) the calculator treats the dwelling as naturally ventilated, getting the SAP §2 ventilation cascade (and any heat recovery) wrong → mis-scored SAP.

The granular counts (fans/flues/vents) are not a bug: older RdSAP open-data certs don't lodge them, and the calc correctly uses RdSAP Table-5 age defaults. percent_draughtproofed is mapped (top-level) and read by the calc.

Remaining fix (19.0 / 21.0.0): give them a proper sap_ventilation construction mirroring 21.0.1. Calc-facing → validate with the RdSAP-21.0.1 corpus (must hold 73.3% / MAE 0.774) plus an Elmhurst-anchored MEV/MVHR RealCertExpectation (the corpus is natural-vent-dominated, so the kind change isn't exercised by it). Quantify blast radius: count older-RdSAP certs with mechanical_ventilation ≠ 0. The 17.1/18.0/20.0.0 fix in this PR is guarded by a mapper-level MVHR test + the corpus/mapper-corpus staying green, with the Elmhurst MEV/MVHR anchor as the SAP-accuracy fast-follow.

2. FE "Main Fuel: Unknown" is FE-side, not a Model mapper gap

main_fuel_type (the gov fuel code) is populated Model-side — UPRN 10093412452 (709791) → 1, UPRN 100020603823 (726605) → 26 — and is persisted (epc_main_heating_detail.main_fuel_type). So the panel's "Main Fuel: Unknown" is the front-end either not mapping the code → fuel name or reading a field we don't populate. Needs an FE-repo (Drizzle/Next) check, not a Model change.

3. Survivor clusters from the post-fix population sweep

scripts/hyde/case_a_population_sweep.py: the representative sample rebaselined cleanly (0 survivors), but the worst-old-drop sample held 28 survivors (lodged A/B → new C/D, 15–25 SAP) in tight UPRN clusters (new-build blocks), spanning multiple schemas (16.0/16.1/17.1/18.0.0) and heating types. No single mapper-gap signature → likely genuine SAP-2012→10.2 drops for very-high-lodged new-builds, but triage one in Elmhurst to confirm genuine vs a residual calc issue before trusting the cohort.

4. Predicted-property display path (e.g. property 721167)

721167 has no lodged EPC (predicted). Its Heating-Control / Main-Fuel / Ventilation Unknowns come from the prediction + landlord-override overlay not populating the display fields — a separate path from the lodged-cert mappers.