# Handover — post Slices S0380.170..173

Branch: `feature/per-cert-mapper-validation`. **HEAD `e71987c2`**.
Predecessor: [`HANDOVER_POST_S0380_169.md`](HANDOVER_POST_S0380_169.md).

## TL;DR

Four community-heating slices landed. The **blocked tier emptied** for
the first time in the corpus's history (`.170`); cost cascade closed
on the CHP cluster (`.171`); CO2/PE closed on non-CHP variants
(`.172`, `.173`).

All 41 corpus variants now run end-to-end through the cascade:
**36 EXACT + 9 pinned** (oil 3/4/6 + no system + 5 community
heating). The 5 community-heating variants carry forcing-function
residuals scoped to specific Elmhurst-mirror divergences detailed
below.

| Slice | HEAD | Scope |
|---|---|---|
| S0380.170 | `9f0d23ad` | **Community heating mapper unblock.** New `CommunityHeating` dataclass on `ElmhurstSiteNotes.main_heating`; extractor `_extract_community_heating()` reads §14.1 Heat Source × Fuel Type. Mapper `_resolve_community_heating_fuel_code(heat_source, fuel)` dispatches per SAP 10.2 Table 12 (PDF p.189): Boilers+Gas→51, CHP→48, HP+Elec→41, Boilers+Oil→53, Boilers+Coal→54. All 5 variants unblocked; 5 forcing-function residuals pinned. Blocked tier tuple emptied. |
| S0380.171 | `a4b5f4e7` | **RdSAP 10 §C CHP heat-fraction cost split.** New `MainHeatingDetail.community_heating_chp_fraction` + `community_heating_boiler_fuel_type` fields populated by the mapper for SAP code 302. `_fuel_cost_gbp_per_kwh` returns `0.35 × CHP_price + 0.65 × boiler_price` when fields set. CH2/CH4 cost gap −£104 → +£0.17 (essentially exact); SAP +4.50 → −0.008. CH6 regressed (-3.52 → -8.03 SAP) — spec-correct fix exposed cert-side DLF=1.0 quirk that only CH6 lodges (in P960 input data, not in Summary). |
| S0380.172 | `36d4bf87` | **Table 4a heat-network heat-source-eff CO2/PE factor scaling.** New `_HEAT_NETWORK_HEAT_SOURCE_EFFICIENCY` dict (301→0.80, 304→3.00 per SAP 10.2 Table 4a PDF p.164). `_heat_network_heat_source_efficiency_scaling(main)` returns 1/eff. Wired into `_main_heating_co2_factor_kg_per_kwh` + `_main_heating_primary_factor` non-electric branches. CH1 CO2/PE -787/-3827 → -126/-967; CH3 CO2/PE +1614/+11879 → +473/+1749. SAP 302 excluded — converges with CHP credit in follow-up. |
| S0380.173 | `e71987c2` | **WHC=901 HW path inherits main fuel for community heating.** New `_is_community_heating_hw_from_main(epc)` predicate (WHC ∈ {901,902,914} + heat-network main + SAP code in heat-source-eff table). `_hot_water_fuel_cost_gbp_per_kwh` gains `inherit_main_for_community_heating` kwarg; HW CO2/PE get top-level branches scaled by 1/heat_source_eff. CH1 PE −967 → **−9** (essentially closed); CH3 PE +1749 → −387 (~78%); CH3 CO2 +473 → −86 (~82%). Cost/SAP signs flip on CH1/CH3 — HW matches worksheet exactly, exposing +£12 lighting/standing overage. |

Extended handover suite at HEAD: **926 pass + 1 skipped, 0 fail.**
Pyright net-zero on affected files (32 → 32 across the 4 slices).

## Current residual state at HEAD `e71987c2`

### Cascade-OK tier (41 variants — all populated corpus folders)

**36 variants EXACT (|Δ| < 1e-3) on all 4 metrics.** 9 variants
carry pinned non-zero residuals (forcing functions, ranked by
total magnitude):

| Variant | ΔSAP_c | Δcost | ΔCO2 | ΔPE | Closure driver |
|---|---:|---:|---:|---:|---|
| CH6 (CHP/Coal) | −8.03 | +£185 | −2935 | +7865 | DLF=1.0 in P960 + CHP credit |
| CH4 (CHP/Oil) | −0.008 | +£0.17 | −4397 | +495 | CHP credit (CO2) |
| CH2 (CHP/Gas) | −0.008 | +£0.17 | −1430 | +1506 | CHP credit (CO2 + PE) |
| oil 6 (B30K) | +3.05 | −£70 | −241 | −1113 | Table 4b code 126 SH+HW kWh gap |
| oil 3 (FAME) | +2.59 | −£62 | −15 | −967 | Table 4b code 128 HW kWh gap |
| oil 4 (FAME) | +2.56 | −£57 | −13 | −885 | Table 4b code 129 HW kWh gap |
| CH3 (HP/Elec) | −0.53 | +£12 | −86 | −387 | Lighting/standing + 0.8523 multiplier |
| CH1 (Boilers/Gas) | −0.53 | +£12 | +52 | **−9** | Lighting/standing (PE essentially closed) |
| no system | +1.18 | −£27 | −50 | −562 | §A.2.2 portable-electric defaults |

### Blocked tier (0 variants)

**Empty for the first time.** All previously blocked variants
(`community heating 1/2/3/4/6`, `electric 11-14`, `oil 2-6`, `no
system`, `pcdb 3`) now cascade-execute. The
`_BLOCKED_BY_MISSING_MAIN_FUEL_TYPE` tuple in
[`test_heating_systems_corpus.py`](../../../backend/documents_parser/tests/test_heating_systems_corpus.py)
is empty; the parametrized raise-test is `pytest.mark.skipif`'d
with reason `"all blocked variants have been unblocked (latest:
S0380.170)"`.

## Open fronts ranked by leverage

### 1. SAP 302 CHP CO2/PE credit cascade (3 variants — CH2, CH4, CH6)

Highest cohort leverage: closes ~−8 SAP-equivalent across CH2 / CH4
/ CH6 + their large CO2 / PE residuals simultaneously.

Per spec block 13b PE (PDF p.153) + 12b CO2:

  ```
  Space heating from CHP    (307a) × 100 ÷ (462) = ...   (463)
  less credit emissions     −(307a)×(461) ÷ (462) = ...  (464)
  Water heated by CHP       (310a) × 100 ÷ (462) = ...   (465)
  less credit emissions     −(310a)×(461) ÷ (462) = ...  (466)
  Heat from heat source 2   [(307b)+(310b)] × 100 ÷
                            (467b) = ...                  (468)
  ```

Per RdSAP 10 §C (PDF p.58) defaults: **CHP overall eff 75%,
heat-to-power ratio 2.0 → heat_eff 50% + electric_eff 25%; boiler
eff 80%**. Verified against CH2/CH4/CH6 worksheet (461)/(462) = 25%
/ 50% exactly.

**Per-line worksheet caveat.** The Elmhurst worksheet (463) energy
column = spec_formula × 0.8523 uniformly across non-CHP heat-
network rows. This 0.8523 multiplier appears in CH1 (467) too (=
spec `(307+310) × 100/80` × 0.8523 → 16717.79 instead of 19614.94).
Mechanism unidentified; not RdSAP 10 / SAP 10.2 spec-derived as
far as the spec PDFs document. **Do per-line walks before forming
hypotheses** per [[feedback-spec-floor-skepticism]]. This may need
a SAP_CALCULATOR.md §8 row.

Implementation sketch: add CHP credit factor + boiler-fuel-code
fields to MainHeatingDetail; the .172 scaling helper already keys
on `_HEAT_NETWORK_HEAT_SOURCE_EFFICIENCY` — add 302 there with
weighted overall eff once the split formula is in place. The
.173 predicate `_is_community_heating_hw_from_main` also gates on
table membership and will pick up SAP 302 automatically.

Likely 2-3 slices: (a) CHP credit + boiler-side eff for SH; (b)
mirror for HW path; (c) Elmhurst 0.8523 multiplier if it turns out
to be load-bearing.

### 2. CH1 / CH3 lighting / standing overage (+£12 cost)

Surfaced by S0380.173 closing the HW path. Cascade cost matches
worksheet exactly on SH + HW, leaves +£12 over on lighting + standing.

Probable mechanisms (in rank order):

1. Standing charge double-count. Worksheet (351) = £120 for code 51
   (heat-network). Cascade may also apply the Mains-gas standing
   even though water_heating_fuel still lodges code 26 → API code 1.
2. Lighting kWh rate mismatch. Cascade uses `other_fuel_cost_gbp_
   per_kwh = 0.1367` (18-hour high) — verify against worksheet (350)
   = 282 × 0.1367.
3. (313) electricity-for-heat-distribution kWh stream billed at
   wrong rate. Worksheet uses heat-network rate 4.24 for this; check
   cascade.

Probably 1 slice once diagnosed via per-line walk. Closes CH1 +
CH3 fully.

### 3. CH6 DLF=1.0 lodging in P960 (cert-side architecture gap)

CH6 P960 input data lodges `Distribution Loss: Two adjoining
dwellings sharing a single heating system` + `Distribution Loss
Value: 0.0`, producing worksheet (306) = 1.0000. CH4 with the
same §14 Summary shape lodges `Distribution Loss: Calculated +
Value: 1.5`, producing (306) = 1.4500.

The DLF distinguisher is NOT in the Summary PDF — only the P960
worksheet input data block. The current architecture only reads
Summary; routing through P960 inverts that.

Two paths forward:
- (a) Extend the Elmhurst Summary extractor to look for any `§17
  Additional Information` line — currently neither CH4 nor CH6
  Summaries lodge anything here, but if Elmhurst adds it the gap
  closes.
- (b) Accept CH6 as a pinned forcing function. Spec-correct
  cascade applies DLF=1.45 for age G per Table 12c; CH6's manual
  override (per spec §C3.1: "For design-stage SAP assessments, a
  DLF of >= 1 can be manually entered") is unmodelable without
  P960 access.

Recommend (b) — pin and document.

### 4. oil 3 / oil 4 (FAME) HW kWh gap

Carried over from S0380.168. ΔSAP +2.59/+2.56. Cascade HW kWh
~900 less than worksheet on FAME boilers (Table 4b codes 128/129).
Per-line walk on `_apply_water_efficiency` vs (219)m. Probably 1
slice.

### 5. oil 6 (B30K) SH + HW kWh gap

Carried over from S0380.168. ΔSAP +3.05. Likely Table 4b code-126
path differs. Probably 1 slice.

### 6. "no system" §A.2.2 portable-electric defaults

Carried over from S0380.169. ΔSAP +1.18. Cascade thinks dwelling
more efficient than worksheet. Probable §A.2.2 portable-electric
defaults gap (responsiveness/control/Table 11). Probably 1 slice.

## Critical discipline reinforced last session

**Per-line walk worksheet → spec → fix.** S0380.171 + .172 + .173
each landed via per-line worksheet dumps confirming the spec rule
before implementation. S0380.173 in particular: probing CH3 HW
factors revealed the cascade was billing HW at Mains-gas (Elmhurst
§15.0 placeholder) rather than heat-network rate; per-line walk on
worksheet (342) confirmed the fix direction.

**Spec-floor skepticism cuts BOTH ways.** S0380.171 was framed by
the prior handover as a single closure for CH2/CH4/CH6 ("biggest
leverage by spec-coherent grouping"). The actual implementation
closed CH2/CH4 exactly but REGRESSED CH6 — exposing the cert-side
DLF=1.0 quirk that was previously masked by offsetting bugs. Per
[[feedback-software-no-special-handling]] applied uniformly;
documented as forcing function rather than gated out.

**Gate carefully across SH and HW paths.** S0380.172 + .173 use the
same `_HEAT_NETWORK_HEAT_SOURCE_EFFICIENCY` table to gate the
heat-source-eff scaling. SAP 302 is intentionally absent — when the
CHP credit slice lands, ADD 302 to that table and both SH (via
.172's wiring) and HW (via .173's predicate) auto-activate.

**Cost-side and CO2/PE-side need different efficiencies for heat
networks.** Cost uses heat-network unit price × network_input
(metered at the dwelling boundary). CO2/PE uses Table 12 factor ×
fuel_input = network_input / heat_source_eff. The .172 + .173
scaling helpers express this by pre-scaling the Table 12 factor at
lookup time, leaving the cost path unaffected.

## Standard slice workflow (unchanged)

1. Read spec page + identify rule (or Elmhurst worksheet pattern)
2. Probe one 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. If mirroring Elmhurst against spec literal: add a row to
   `SAP_CALCULATOR.md §8 "Elmhurst-mirrored spec divergences"`. The
   ≥2-cert rule applies unless the new divergence shares its shape
   with an already-documented row (S0380.164 added §8.2 under this
   exception with a single-cert flag).
9. Commit with spec citation + `Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>`
10. Update `project-heating-systems-corpus` + `MEMORY.md` index

## Test baseline at HEAD `e71987c2`

```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: **926 pass + 1 skipped, 0 fail.**

## Memories to load (in order)

```
project-heating-systems-corpus            # HEAD e71987c2
feedback-sap-10-2-only-never-10-3
feedback-software-no-special-handling
feedback-spec-floor-skepticism
feedback-worksheet-not-api-reference
feedback-spec-citation-in-commits
feedback-verify-handover-claims
feedback-zero-error-strict
feedback-commit-per-slice
feedback-aaa-test-convention
feedback-e2e-validation-philosophy
feedback-abs-diff-over-pytest-approx
feedback-golden-residuals-near-zero
feedback-one-e-minus-4-across-the-board
feedback-bigger-slices-for-uniform-work
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.
- **Don't re-investigate Slices .91..173** — all settled.
- **Don't add new helpers to `domain/sap10_ml/`** — on deprecation path.
- **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 first. The Elmhurst 0.8523 multiplier on heat-network
  rows (CH1 (467), CH3 (467), CH2/CH4/CH6 (468)) is unexplained and
  may be load-bearing for the CHP credit slice.
- **Don't gate SH and HW paths separately.** The .172 + .173 wiring
  shares `_HEAT_NETWORK_HEAT_SOURCE_EFFICIENCY` membership; adding
  SAP 302 to that table auto-activates both paths.

## Master doc

The canonical architecture + API + validation doc lives at
[`domain/sap10_calculator/docs/SAP_CALCULATOR.md`](SAP_CALCULATOR.md)
(§8.1 + §8.2 documented). The next CHP-credit slice may add §8.3 if
the Elmhurst 0.8523 multiplier or block-13b PE/CO2 line formulas
turn out to diverge from spec literal.

## Good luck.