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docs: ventilation mapping inconsistency — 17.1/18.0/20.0.0 fixed, 19.0/21.0.0 open

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Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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Khalim Conn-Kowlessar 2026-06-26 08:46:01 +00:00
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docs/baseline-downgrade-followups.md
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@ -3,16 +3,25 @@
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)
## 1. RdSAP ventilation mapping is inconsistent across schemas
**Severity: accuracy bug for MEV/MVHR dwellings.**
**Severity: accuracy bug for MEV/MVHR dwellings.** The mappers handle
`sap_ventilation` four different ways:
`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.)
- `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
@ -25,11 +34,14 @@ The granular **counts** (fans/flues/vents) are *not* a bug: older RdSAP open-dat
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`.
**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