# Handover — post Slices S0380.150..152

Branch: `feature/per-cert-mapper-validation`. **HEAD `d4f6ff0f`**.
Predecessor: [`HANDOVER_POST_S0380_149.md`](HANDOVER_POST_S0380_149.md).

## TL;DR

Three slices landed. The session pivoted partway through from
incremental fixes to a **spec-led cluster audit** (the user pushed
back that we were spinning wheels). The audit identified three
distinct clusters; two were closed.

| Slice | Commit | Spec rule closed |
|---|---|---|
| S0380.150 | `a658f736` | SAP 10.2 §12 / Appendix F2 — 18-hour tariff: pumps + lighting bill at 18-hour HIGH rate (13.67 p/kWh) not standard (13.19) |
| S0380.151 | `fb173cdf` | RdSAP 10 §4.1 Table 5 — extract-fans age-band default (`max(lodged, table_5_default)`) |
| S0380.152 | `d4f6ff0f` | SAP 10.2 Table 3 — primary loss for ANY wet boiler + cylinder + WHC=901 (not just Table 4b gas/oil) |

Extended handover suite at HEAD: **896 pass, 0 fail.** Pyright
net-zero (43 → 43).

## The mid-session pivot — read this before doing anything

The user explicitly called out "spinning wheels" partway through.
I'd shipped S0380.150 (18-hour tariff fix) which closed ~£2/variant
uniformly across the cohort, but several variants got *worse*. The
user asked for a **spec-led picture** of where the actual gaps were
across the open variants, not more incremental fixes.

The audit produced this categorisation:

**Cluster A** — cohort-wide systematic ~-1.2% SH USEFUL kWh deficit
across 18 of 25 variants. Same property, same magnitude on every
variant. Root cause: RdSAP 10 Table 5 extract-fans default missing
(lodged 0 was being trusted verbatim instead of `max(lodged, default)`).
**Closed in S0380.151.**

**Cluster B** — three variants overshoot by +2.3% (solid fuel 2/3,
electric 5). My audit hypothesised this was a Table 9c step 12 sign
convention for low-R systems. **This was wrong.** When I probed
solid fuel 2's monthly MIT, it was actually 0.035°C LOWER than the
worksheet (not higher), yet had MORE SH demand. The decomposition
showed the entire 73 W gain gap was in (72) water-heating gains —
because cascade (59) primary loss was 0 while worksheet was ~505
kWh/yr. **Partially closed in S0380.152** — SF3 fully (+1.31 →
+0.30), SF2 partially (+2.77 → +2.06).

**Cluster C** — HW kWh mismatch on 4 specific variants (gshp,
electric 2, solid fuel 2/3). Different spec rules per variant.

The audit doc lives at the top of the conversation. The key
discipline: don't form a spec hypothesis from headline residuals;
walk the per-line cascade against the worksheet PDF, find which
line ref diverges, then look up the spec rule that produces that
line. My Cluster B hypothesis didn't survive contact with the
data — see [[feedback-spec-floor-skepticism]] for the discipline
that cuts both ways.

## Current residual state at HEAD `d4f6ff0f`

### Cascade-OK tier (25 variants on pin grid)

Sorted by |ΔSAP_c|:

| Variant | ΔSAP_c | Δcost | ΔPE | Cluster | Notes |
|---|---:|---:|---:|:--|---|
| oil 1 | **+0.0000** | **+0.0000** | **+0.0000** | — | EXACT |
| oil pcdb 1/2 | **+0.0000** | **+0.0000** | **+0.0000** | — | EXACT |
| oil pcdb 3 | **+0.0000** | **+0.0000** | **-0.0000** | — | EXACT |
| electric 1 | **-0.0000** | **-0.0000** | +48.66 | — | SAP exact, PE +49 kWh follow-up |
| solid fuel 5 | **+0.0000** | **+0.0000** | +48.66 | — | SAP exact |
| solid fuel 6 | **+0.0000** | **+0.0000** | +48.66 | — | SAP exact |
| solid fuel 7 | **-0.0000** | **+0.0000** | +48.66 | — | SAP exact |
| solid fuel 8 | **-0.0000** | **+0.0000** | +48.66 | — | SAP exact |
| pcdb 1 | -0.0108 | +£0.24 | +5.70 | — | basically exact |
| ashp | -0.024 | +£0.55 | +36.34 | — | basically exact |
| solid fuel 4 | +0.085 | -£1.96 | -5.78 | — | close |
| solid fuel 11 | +0.0912 | -£2.10 | -0.74 | — | close |
| electric 8 | +0.0941 | -£2.17 | +6.58 | — | close |
| electric 7 | +0.1017 | -£2.34 | +3.10 | — | close |
| electric 6 | +0.1081 | -£2.49 | +0.16 | — | close |
| solid fuel 9 | +0.1072 | -£2.47 | -5.07 | — | close |
| solid fuel 10 | +0.1134 | -£2.61 | -13.91 | — | close |
| electric 9 | +0.1199 | -£2.76 | -4.51 | — | close |
| electric 3 | +0.1215 | -£2.80 | -5.99 | — | close |
| **solid fuel 3** | **+0.2968** | **-£6.84** | **-214.25** | B (~done) | **closed by .152** |
| **electric 2** | **-0.4584** | **+£10.56** | **+443.13** | C | warm-air ASHP HW cascade |
| **gshp** | **+0.9373** | **-£21.60** | **-418.92** | C | HP DHW Appendix N3 |
| **electric 5** | **-1.1759** | **+£27.09** | **+438.03** | B (open) | storage code 402, R=0.40 — distinct cause |
| **solid fuel 2** | **+2.0649** | **-£47.58** | **-754.09** | B (partial) | needs `_separately_timed_dhw=False` |

Σ |ΔSAP_c| across 25 variants ≈ **6.4 SAP points** (was ~14.5 pre-
session, ~6.4 now = ~55% reduction across 3 slices).

### Blocked tier (16 variants — `MissingMainFuelType`)

Unchanged. Community heating × 5, electric storage 11-14, no
system, oil 2-6, pcdb 3.

## Open fronts ranked by leverage

### 1. **SF2 separately-timed-DHW for solid-fuel back-boilers** — +2.06 SAP

The cascade post-S0380.152 applies primary loss year-round (h=3
winter / h=3 summer via `_separately_timed_dhw=True`). Worksheet
applies winter-only (h=5 winter / 0 summer). Daily-rate diff = the
ENTIRE remaining SF2 residual.

Spec hint: `_separately_timed_dhw` at line 3765 currently returns
True for cylinder + non-electric HW fuel. For solid-fuel back-
boilers the HW timing is *tied to the room fire* (no separate
programmer) — the cascade should return False here, switching the
formula to (h=5, h=3). And then there's still the summer-zero
question — possibly a separate rule for "back-boiler doesn't run in
summer".

Compare SF2 to SF3 (both code 158/160 + WHC=901): SF3 has Jun-Sep
non-zero (~42 kWh/month) while SF2 has Jun-Sep = 0. Same property,
same boiler type. Probably a lodging difference (cylinder thermostat
or DHW timing). Worth a 30-min probe before coding.

### 2. **Cluster C — gshp HW cascade** — +0.94 SAP / -419 PE

Cascade HW = 841 kWh vs worksheet 1138 kWh — under by 26%.
Spec: SAP 10.2 Appendix N3.6 / N3.7 (PDF p.107-109) — HP DHW
efficiency cascade. The current cascade may be applying the wrong
in-use factor (Table N8) or PSR interpolation. Cohort-1 ASHP closed
via Appendix N N3.6 reciprocal interpolation in S0380.28 — the gshp
fix may share a path.

### 3. **Cluster C — electric 2 (warm-air HP) HW cascade** — -0.46 SAP / +443 PE

Cascade HW = 2849 kWh vs worksheet 2384 = OVER by 19%. Different
direction from gshp. Code 524 (warm-air ASHP). Probably wrong
water_heating efficiency dispatch.

### 4. **electric 5** — -1.18 SAP / +438 PE

Storage heater code 402 (R=0.40, +0.4 K Table 4e adjustment).
Worsened by S0380.145 (then was net-zero from offsetting bugs)
and by S0380.151 (lighting now correctly billed). Cascade SH
USEFUL was +196 kWh OVER worksheet pre-cluster-A. After Cluster A
and now the secondary cascade fixes, the residual is the *real*
spec gap. Need to probe MIT cascade for electric 5 specifically.

### 5. **Lighting-only PE +48.66 cohort cluster** — 5 variants

Variants where SAP / cost are EXACT but PE is +48.66 kWh/yr (and
CO2 +11.94 kg/yr). Identical offset across electric 1, solid fuel
5/6/7/8. This is suspicious — same exact value. Probably a Table
12e PE factor mismatch on the added extract fan kWh.

Diagnostic: 48.66 / (10 m³/h × something) = ? — back-solve for the
per-kWh PE factor diff. Then check `_pumps_fans_pe_factor`.

## Slice history (this session)

| Slice | HEAD | Scope |
|---|---|---|
| S0380.150 | `a658f736` | SAP 10.2 §12 (p.45) + Appendix F2 (p.63) — 18-hour tariff non-heating uses bill at 18-hour high rate (13.67 not 13.19 p/kWh). New `_other_fuel_cost_gbp_per_kwh` branch for `Tariff.EIGHTEEN_HOUR` returning the Table 32 code 38 high rate. Closures: oil 1 -£9.31→-£6.69, all 25 variants shift £1.35-£2.62. |
| S0380.151 | `fb173cdf` | RdSAP 10 §4.1 Table 5 (PDF p.28) — extract-fans default when lodged is unknown/zero. New `_rdsap_extract_fans_default(age_band, habitable_rooms, *, is_park_home)` helper + `max(lodged, default)` wiring in `ventilation_from_cert`. Cohort: 8 variants → EXACT, 11 → ±0.02-0.12. Golden cert 0240 PE +2.18→+5.80, cert 0390-2954 PE -28.27→-27.97. |
| S0380.152 | `d4f6ff0f` | SAP 10.2 Table 3 (PDF p.160) — primary circuit loss applies to ANY heat generator + cylinder via primary pipework, not just Table 4b. `_primary_loss_applies(...)` gains optional `water_heating_code` parameter + new branch using `_is_wet_boiler_main(main)` + WHC ∈ {901, 902, 914}. Closures: solid fuel 3 +1.31→+0.30, solid fuel 2 +2.77→+2.06 (partial; needs separately-timed-DHW fix). |

## Standard slice workflow (unchanged)

1. Read spec page + identify rule
2. Probe one cluster variant; verify diagnosis via monkey-patch / direct walk
3. Write failing AAA test (literal `# Arrange / # Act / # Assert`)
4. Implement helper / dispatch entry / mapper extension
5. Re-pin affected variants (DO NOT widen tolerance)
6. Run extended handover suite (command below)
7. Pyright net-zero check (`git stash` → pyright → `git stash pop` → pyright)
8. Commit with spec citation + `Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>`
9. Update `project-heating-systems-corpus` + `MEMORY.md` index

**Bonus discipline from this session**: when forming a spec
hypothesis, dump the per-line worksheet values for the variant and
walk them against the cascade output BEFORE writing the slice. My
Cluster B narrative had the wrong spec section entirely — what
looked like Table 9c was Table 3. The data caught it; the audit
narrative didn't.

## Test baseline at HEAD `d4f6ff0f`

```bash
PYTHONPATH=/workspaces/model python -m pytest \
    backend/documents_parser/tests/test_summary_pdf_mapper_chain.py \
    backend/documents_parser/tests/test_heating_systems_corpus.py \
    backend/documents_parser/tests/test_elmhurst_extractor.py \
    backend/documents_parser/tests/test_elmhurst_end_to_end.py \
    domain/sap10_calculator/worksheet/tests/test_e2e_elmhurst_sap_score.py \
    domain/sap10_calculator/worksheet/tests/test_heat_transmission.py \
    domain/sap10_calculator/worksheet/tests/test_internal_gains.py \
    domain/sap10_calculator/worksheet/tests/test_solar_gains.py \
    domain/sap10_calculator/worksheet/tests/test_dimensions.py \
    domain/sap10_calculator/worksheet/tests/test_rating.py \
    domain/sap10_calculator/worksheet/tests/test_ventilation.py \
    domain/sap10_calculator/worksheet/tests/test_appendix_h_solar.py \
    domain/sap10_calculator/worksheet/tests/test_mev.py \
    domain/sap10_calculator/rdsap/tests/test_cert_to_inputs.py \
    domain/sap10_calculator/rdsap/tests/test_golden_fixtures.py \
    domain/sap10_calculator/tests/test_pcdb_table_322_lookup.py \
    domain/sap10_calculator/tests/test_pcdb_table_329_lookup.py \
    domain/sap10_calculator/tests/test_table_12a.py \
    --no-cov -q
```

Expected: **896 pass, 0 fail.**

## Memories to load (in order)

```
project-heating-systems-corpus            # HEAD d4f6ff0f
feedback-sap-10-2-only-never-10-3         # CRITICAL — never reference SAP 10.3
feedback-software-no-special-handling     # CRITICAL — apply spec uniformly, no empirical gates
feedback-worksheet-not-api-reference
feedback-spec-citation-in-commits
feedback-verify-handover-claims
feedback-zero-error-strict                # TARGET: ΔSAP_c < 1e-4 vs worksheet
feedback-commit-per-slice
feedback-aaa-test-convention
feedback-e2e-validation-philosophy
feedback-abs-diff-over-pytest-approx
feedback-spec-floor-skepticism            # CUTS BOTH WAYS — be skeptical of your own narrative
feedback-golden-residuals-near-zero
feedback-one-e-minus-4-across-the-board
reference-unmapped-sap-code
reference-unmapped-api-code
project-oil-price-spec-divergence
```

## What NOT to do

- **Don't reference SAP 10.3** — track 10.2 deliberately
- **Don't widen pin tolerances** — re-pin smaller or find the spec gap
- **Don't add empirical gates** to keep cohort pins stable when a
  spec rule clearly applies
- **Don't re-investigate Slices .91..152** — all settled
- **Don't add new helpers to `domain/sap10_ml/`** — on deprecation
  path; `domain/sap10_calculator/tables/` is the canonical home
- **Don't treat ΔSAP=0.07 as "closed"** — target is <1e-4 vs worksheet
- **Don't form a spec hypothesis without per-line data** — walk the
  worksheet line-by-line for the failing variant first, then look up
  the spec rule. Headline residuals tell you a gap exists; only the
  per-line walk tells you which section of the spec it lives in.

## Spec source quick-reference

All under `domain/sap10_calculator/docs/specs/`:

- **SAP 10.2 full spec**: `sap-10-2-full-specification-2025-03-14.pdf`
  - **§4** (p.135-137) — water heating worksheet (45..65)
  - **§9** (p.155+) — MIT calc, Tables 9/9a/9b/9c
  - **§9.4.11** (p.30) — Boiler interlock: -5pp to BOTH SH and DHW
  - **§12** (p.45) — Electricity tariff types (7/10/18/24-hour rules)
  - **§A.2.2** (~p.189) — Forced-secondary set
  - **Appendix D §D2.1 (2)** (p.57) — Eq D1 monthly water eff cascade
  - **Appendix F2** (p.63) — 18-hour CPSU: high rate for all other uses
  - **Appendix N3** (p.107-109) — Heat pump DHW efficiency cascade
  - **Table 3** (p.160) — Primary circuit loss; zero-loss list. **Slice .152**
    extended this to all wet boilers + cylinder + WHC=901.
  - **Table 4a** (p.163-170) — heating systems incl. R column
  - **Table 4b** (p.168) — gas/liquid boilers seasonal efficiency
  - **Table 4f** (p.174) — pumps + fans
  - **Table 9c** (p.184) — MIT cascade (step 8 = Table 4e adj wired)
  - **Table 11** (p.188) — secondary heating fraction
  - **Table 12** (p.191) — SAP rating fuel prices + standing charges
  - **Table 12a** (p.191) — high/low-rate fraction by system × tariff
- **RdSAP 10 spec**: `RdSAP 10 Specification 10-06-2025.pdf`
  - **§4.1 Table 5** (p.28) — Ventilation parameters incl. **extract fans
    age-band default** (slice .151)
  - **§5** (p.29) — Floor infiltration spec rule
  - **§10.11 Table 29** (p.56) — Heating/HW parameters; inaccessible cylinder
  - **§19 Table 32** (p.95) — RdSAP10 fuel prices / CO2 / PE

## Good luck.