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 #1333** (mirror `_api_mechanical_ventilation_kind`).
- **`19_0` / `21_0_0`** — set **no `sap_ventilation` at all** → the
  `EpcPropertyData` default (`None`). They dropped the **entire** ventilation
  block (sheltered sides + kind), not just the kind. **Fixed in
  `feat/rdsap-19-21-ventilation`**: both now construct `SapVentilation(...)`
  mirroring 21.0.1, but only for the fields each schema lodges — the top-level
  `mechanical_ventilation` enum (→ kind) and `sheltered_sides` (from
  `built_form`). Neither lodges the fan/flue/vent counts, so those stay unset
  (RdSAP Table-5 age defaults — see counts note below).

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.

**19.0 / 21.0.0 fix (done, `feat/rdsap-19-21-ventilation`):** both build
`SapVentilation(sheltered_sides=…, mechanical_ventilation_kind=…)` mirroring
21.0.1. Guarded by the extended mapper-level MVHR test + a `sheltered_sides`
regression, with the corpus (held 73.3% / MAE 0.774 — it's 21.0.1-only so the
change isn't exercised by it) and the 6002-case mapper-corpus staying green.
**Outstanding SAP-accuracy fast-follow:** an **Elmhurst-anchored MEV/MVHR
`RealCertExpectation`** (the corpus is natural-vent-dominated, so the kind
change isn't exercised by it). **Blast radius (still to quantify):** count
portfolio-796 certs on schema 19.0 / 21.0.0 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 synthesis path
(`domain/epc_prediction/epc_prediction.py`), **not** the lodged-cert mappers and
**not** the landlord-override overlay (`overlays_from` is simulation-time only,
ADR-0032 — it never writes the persisted/displayed EPC). Investigated and fixed
on `fix/predicted-property-display-unknowns`:

- **Heating Control — FIXED (Model).** `_apply_heating_donor` copied the donor's
  *calc* `sap_heating` cluster but left the *display* rows (`main_heating`,
  `main_heating_controls`) on the size-representative structural template —
  incoherent with the donated system, and "Unknown" whenever the template lodged
  no control row (exactly 721167: no `main_heating_controls` energy element
  persisted). Now carries the donor's display heating + control alongside the
  calc cluster.
- **Ventilation — FIXED (Model).** The prediction never synthesised ventilation
  — it kept the template's `sap_ventilation`, so a predicted dwelling in an
  MEV/MVHR neighbourhood was scored + displayed as natural. New
  `_apply_ventilation_mode` modes `mechanical_ventilation_kind` across the cohort
  (recency/geo-weighted, mirroring glazing). Natural-vent cohorts mode to None
  and stay natural (§2 default) — so a genuinely natural dwelling like 721167
  correctly stays natural; the fix moves only real MEV/MVHR neighbourhoods.
- **Main Fuel — NOT a prediction gap.** `epc_main_heating_detail.main_fuel_type`
  *is* persisted for predicted properties (721167 → `26` = mains gas, carried
  from the heating donor). The "Unknown" is the **same FE code→name gap as item
  2** (the Drizzle/Next repo not mapping the fuel code), not a Model issue.

Both fixes are display-only for the calculator (component-accuracy gate, corpus
harness, and prediction e2e all green); persistence already carries
`main_heating_controls` (energy element) and `ventilation_mechanical_ventilation_kind`,
so the synthesised values reach the passport on the next prediction run.