# 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.