# 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. Older RdSAP mappers drop `mechanical_ventilation_kind` (calc-facing)

**Severity: accuracy bug for MEV/MVHR dwellings.**

`mechanical_ventilation_kind` (the natural / MEV / MVHR ventilation **type**) is
mapped in **only `from_rdsap_schema_21_0_1`** ([mapper.py:2650], via
`_api_mechanical_ventilation_kind`). The other six API mappers —
`from_rdsap_schema_17_0/17_1/18_0/19_0/20_0_0/21_0_0` — build
`SapVentilation(sheltered_sides=…)` and **drop** the cert's `mechanical_ventilation`
field. (The full-SAP path maps it via `_sap_17_1_ventilation` — not affected.)

- 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.

**Fix:** mirror `mechanical_ventilation_kind=_api_mechanical_ventilation_kind(schema.mechanical_ventilation)`
into the six older RdSAP mappers. **Calc-facing → validate** with the RdSAP-21.0.1
corpus (must hold 73.3% / MAE 0.774) plus an Elmhurst-anchored MEV/MVHR cert (the
corpus is natural-vent-dominated, so add a mechanical-vent `RealCertExpectation`).
Quantify blast radius: count older-RdSAP certs with `mechanical_ventilation ≠ 0`.

## 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.