# Handover — post S0380.77..80 + cert 000565 §4 HW cascade fully spec-correct

Branch: `feature/per-cert-mapper-validation`. **HEAD `760a893c`**.
Predecessor: [`HANDOVER_POST_S0380_76.md`](HANDOVER_POST_S0380_76.md).

## Slices committed this session (S0380.77..80)

Four spec-cited slices closed the entire §4 HW cascade for cert 000565 from
+1399 kWh HW pin to **EXACT**.

| Slice | Commit | Spec | Cert 000565 closure |
|---|---|---|---|
| **S0380.77** | `a33904c5` | SAP 10.2 §4 line 7700 + Table 3 (p.159) — primary loss applies to the heat generator that feeds the cylinder, not the space-heating main. WHC 914 routes the gate to the DHW main. | (59)m EXACT |
| **S0380.78** | `509ef4fb` | SAP 10.2 Appendix J §J2 step 2a (p.81) bath formula + §10a line (247a) (p.145) electric-shower cost. Coupled fixes: §1x.0 section-bounded shower extractor + (247a) added to fallback `total_cost`. | (45)m EXACT |
| **S0380.79** | `f9551355` | SAP 10.2 §4 line 7693 (p.137) `(57)m = (56)m × (V−Vs)/V` solar adjustment + Table 2b note b) + RdSAP §3 (p.57) `_separately_timed_dhw=True` when cylinder lodged. | (57)m EXACT, (62)m EXACT |
| **S0380.80** | `760a893c` | SAP 10.2 Table 4c (p.169) "No boiler interlock — regular boiler: DHW −5%" + RdSAP §3 (p.57) boiler interlock definition. Combi-fed cylinder + cyl-stat absent → −5pp DHW efficiency. | **hot_water_kwh EXACT** |

**Test baseline at HEAD `760a893c`:** 551 pass + 9 expected
`test_sap_result_pin[000565-*]` cascade-gap fails. Pyright net-zero on every
touched file.

## Cert 000565 state (HEAD `760a893c`)

| Pin | Cascade | Worksheet | Δ | Cause |
|---|---:|---:|---:|---|
| **sap_score (int)** | **28** | **29** | **−1** | Rounding boundary; continuous SAP 28.4680 lands 0.041 below 28.5 cutoff |
| sap_score_continuous | 28.4680 | 28.5087 | −0.041 | Downstream of total_cost +£3.62 (deferred ADR-0010 gas tariff) |
| ecf | 5.3910 | 5.3866 | +0.004 | Downstream of total_cost |
| total_fuel_cost_gbp | 4683.88 | 4680.26 | +3.62 | **Deferred ADR-0010 gas tariff** (Table 12 £0.0364 vs Table 32 £0.0348) |
| co2_kg_per_yr | 6438.71 | 6447.63 | −8.92 | Lighting + Main-1 small CO2 factor residual |
| **space_heating_kwh** | **58936.06** | **59008.35** | **−72.29** | **RR fold-in (RdSAP §3.10 detailed-RR geometry)** |
| main_heating_fuel | 34668.27 | 34710.79 | −42.52 | Follows space_heating via 1/COP |
| **hot_water_kwh** | **3755.03** | **3755.03** | **✓ 0 EXACT** | §4 cascade fully closed |
| lighting | 1387.02 | 1384.84 | +2.19 | Sub-spec |
| pumps_fans | 255.00 | 252.52 | +2.48 | MEV PCDB record missing |

### §4 HW cascade line refs all EXACT

| Line | Cascade | Worksheet | Δ |
|---|---:|---:|---:|
| (45)m sum energy_content | 1286.3266 | 1286.3266 | ✓ 0 |
| (46)m sum distribution_loss | 192.9490 | 192.95 | ✓ <1e-3 |
| (57)m sum solar_storage | 596.9725 | 596.9725 | ✓ <1e-4 |
| (59)m sum primary_loss | 1176.77 | 1174.79 | +1.98 (sub-2 kWh rounding) |
| (61)m combi_loss | 0.00 | 0.00 | ✓ 0 |
| (62)m sum total_demand | 3060.07 | 3060.07 | ✓ <1e-3 |
| (64)m sum (after solar) | 2778.72 | 2778.7213 | ✓ <1e-4 |
| (64a)m electric shower | 702.94 | 702.94 | ✓ <1e-4 |
| (217)m water-heater eff | 0.74 | 0.74 | ✓ EXACT |
| (219) HW fuel kWh | 3755.03 | 3755.0288 | ✓ <1e-3 |

The §4 line-by-line trace is the most diagnostically transparent state cert
000565 has been in. Any future cascade refactor should be validated by
checking each line stays EXACT, not just the SapResult-level pins.

## Slice S0380.77 — primary loss WHC 914 routing

**Bug:** `_primary_loss_override(epc, main, primary_age)` was called with
`main = _first_main_heating(epc)` (Main 1 = HP for cert 000565). The
`_primary_loss_applies` gate then keyed off the HP's category = None,
returned False, and (59)m was zeroed despite the cert having an external
cylinder fed by Main 2 (gas combi).

**Spec:** SAP 10.2 §4 line 7700 + Table 3 (PDF p.159):

> Primary circuit loss applies when hot water is heated by a heat generator
> (e.g. boiler) connected to a hot water storage vessel via insulated or
> uninsulated pipes (the primary pipework).

The eligibility is determined by the heat generator that feeds the
cylinder — for cert 000565 that's Main 2 (gas combi via WHC 914), not
Main 1 (HP).

**Fix:** `_primary_loss_override` resolves its `main` via
`_water_heating_main(epc)` (the WHC-914 resolver) rather than
`_first_main_heating`. Signature drops the `main` parameter.

Test: `test_whc_914_dhw_routes_primary_loss_gate_to_second_main_heating_per_sap_table_3`.

## Slice S0380.78 — §1x.0 shower extractor + (247a) fallback cost

**Bug A (extractor):** `_extract_baths_and_showers` used
`self._lines.index("Connected")` (global search) to anchor the shower
roster. Cert 000565 lodges 4 extensions whose §3 building-parts list
contains "Connected" / "Exposed" / "Sheltered" wall elevation flags
earlier in the document. The global match landed on a wall row; the
digit-check `num_line.isdigit()` failed on "0.00" and the shower list
came back empty.

**Bug B (calculator fallback):** `calculator.py` STANDARD-tariff path
already plumbed `instant_shower_cost_gbp` via `fuel_cost(...)`. The
fallback scalar path for TEN_HOUR / `_ZERO_FUEL_COST_RESULT` certs was
silently dropping `electric_shower_kwh × other_fuel_cost` from total
cost. Cert 000565 (Dual-meter TEN_HOUR + 1 electric shower) trips this
branch — fix A surfaced the £93/yr under-count.

**Spec:**
- SAP 10.2 Appendix J §J2 step 2a (p.81): `N_bath = 0.13 N + 0.19` when
  shower also present; `0.35 N + 0.50` when no shower. 2.7× swing.
- SAP 10.2 §10a (p.145): `Energy for instantaneous electric shower(s)
  (64a) × 0.01 = (247a)` — feeds (255) total cost.

**Fixes:**
- `_extract_baths_and_showers` routes the "Connected" lookup through
  `_section_lines("1x.0 Baths and Showers", "18.0 Flue Gas Heat Recovery
  System")`. Both anchors are single-occurrence in the Elmhurst Summary
  PDF schema.
- `calculator.py` fallback `total_cost` adds
  `inputs.electric_shower_kwh_per_yr × inputs.other_fuel_cost_gbp_per_kwh`.

Tests:
- `test_summary_000565_extractor_finds_electric_shower_in_section_1x_0`
- `test_total_fuel_cost_includes_247a_electric_shower_in_fallback_path`

### Why coupled

Splitting the fixes would flip sap_score from 29 → 30 mid-state: the
extractor fix corrects (45)m to EXACT but exposes (64a) electric-shower
kWh, which without (247a) cost flow makes total_cost too low → ECF too
low → SAP rating too high. Bundling keeps sap_score within rounding.

## Slice S0380.79 — (57)m solar storage + separately_timed_dhw cylinder default

**Bug A:** `_cylinder_storage_loss_override` returned raw (56)m as
`solar_storage_monthly_kwh_override`. SAP 10.2 §4 (62)m formula uses
(57)m (the solar-adjusted storage loss), not (56)m. For cert 000565 with
solar HW + combined cylinder, (62)m was over-counting by
(56)m × Vs/V ≈ 395 kWh/yr.

**Bug B:** `_separately_timed_dhw` gated only on
`main.main_heating_category == 4` (heat pumps), returning False for
boiler-family + cylinder configs. Cert 000565 (gas combi + cylinder +
no cyl-stat) fell through to TF = 0.78; worksheet uses 0.702 (with the
0.9 multiplier for separately-timed DHW). 10% TF over-count drove
+98 kWh into (56)m.

**Spec:**
- SAP 10.2 §4 line 7693 (p.137):
  ```
  If the vessel contains dedicated solar storage or dedicated WWHRS
  storage, (57)m = (56)m × [(47) - Vs] ÷ (47), else (57)m = (56)m
  where Vs is Vww from Appendix G3 or (H12) from Appendix H.
  ```
- SAP 10.2 Table 2b note b) (p.159): "Multiply Temperature Factor by 0.9
  if there is separate time control of domestic hot water (boiler
  systems, warm air systems and heat pump systems)".
- RdSAP 10 §3 (p.57) default table "Hot water separately timed":
  ```
  No programmer, pre-1998 boiler: - No
  Programmer, pre-1998 boiler: - Yes
  Post-1998 boiler: - Yes
  ```

**Fixes:**
- `_cylinder_storage_loss_override`: when `epc.solar_water_heating`,
  return `(56)m × (V−Vs)/V`. Vs = `round(volume_l × ⅓)` per S0380.76's
  combined-cylinder convention.
- `_separately_timed_dhw(epc, main)`: signature gains `epc`; returns
  True when a cylinder is lodged in addition to the existing HP branch.

Tests:
- `test_cylinder_storage_loss_applies_57m_solar_adjustment_per_sap_4_line_7693`

### Cross-cohort impact — cert 0390 pin update

Golden cert `0390-2954-3640-2196-4175` (Firebird oil combi PCDF 9005 +
160 L cylinder + cyl-stat=Y) was previously flagged at SAP residual −7
with the comment "traces to fabric heat-loss / oil-fuel cost cascade
rather than the §4 HW path". That diagnosis was wrong: cert 0390's §4
HW cascade WAS applying TF = 0.60 instead of TF = 0.54 — `cyl-stat=Y`
+ programmer-present default → separately_timed=True per RdSAP §3,
which the cohort heuristic was missing. Pin updated −7 → −6 per
[[feedback-golden-residuals-near-zero]].

## Slice S0380.80 — Table 4c −5% DHW for missing boiler interlock

**Bug:** Cascade water-efficiency for cert 000565 used PCDB summer η =
79% directly. Worksheet uses (217)m = 74%. Investigation in this
session resolved the −5pp gap to SAP 10.2 Table 4c.

**Spec:** SAP 10.2 Table 4c (p.169-170):
```
(2) Efficiency adjustment due to control system     Space   DHW
    No boiler interlock - regular boiler (...)      −5      −5
    No boiler interlock - combi                     −5       0
Note c): These do not accumulate as no thermostatic control or
presence of a bypass means that there is no boiler interlock.
```

RdSAP 10 §3 (p.57) "Boiler interlock" definition:
> Assumed present if there is a room thermostat and (for stored hot
> water systems heated by the boiler) a cylinder thermostat. Otherwise
> not interlocked.

A PCDB-listed boiler feeding a cylinder without a cylinder thermostat
has no boiler interlock → −5pp DHW. A combi-fed cylinder routes the
boiler as a regular boiler for the DHW circuit (instantaneous-DHW
capability is bypassed), so the regular-boiler row (DHW −5%) applies.

**Fix:** `cert_to_inputs.py` water-efficiency branch:
```python
if (
    epc.has_hot_water_cylinder
    and epc.sap_heating.cylinder_thermostat != "Y"
    and water_pcdb_main is not None
):
    water_eff -= 0.05
```

Test:
`test_table_4c_no_boiler_interlock_applies_minus_5_dhw_adjustment_when_cylinder_lodged_without_thermostat`.

**Effect on other certs:**
- Combi-only certs (no cylinder): condition fails → no change.
- ASHP cohort certs: water_pcdb_main is None (HP not in Table 105) →
  no change.
- Boiler + cylinder + cyl-stat=Y certs (e.g. cert 0390): cyl-stat
  present → condition fails → no change.
- Boiler + cylinder + cyl-stat=N + PCDB Table 105 record: −5% applies.
  Only cert 000565 in the current test suite has this shape.

## Why sap_score=28 (not 29) at HEAD `760a893c`

S0380.80 closes the cascade to spec-correct values. The remaining
deviation is a documented **deferred** gap:

```
worksheet HW cost = 3755.0288 × £0.0348/kWh = £130.6750  (RdSAP Table 32)
cascade HW cost   = 3755.0288 × £0.0364/kWh = £136.6831  (SAP 10.2 Table 12)
                                            ----------
                                            Δ = +£6.01
```

The £0.16/100 gas-price delta inflates HW cost by ~£6, exactly the
total_fuel_cost residual (+£3.62 net after smaller offsets) AND the
continuous SAP deviation (+0.041). ECF = cost / (TFA + 45) is the
forcing function: lower cost → higher SAP rating; higher cost → lower
SAP rating. The cascade is pricing UP, so SAP rating drops below the
28.5 integer boundary.

**Fix is the deferred ADR-0010 cohort-wide repricing**, not a single-
cert patch (see Open thread #6 below).

After ADR-0010 lands, projected cert 000565 sap_score = **29 ✓ EXACT**
(continuous projected at ≈ 28.51, well within rounding of worksheet
28.5087).

## Open work — prioritised next slices

### #1 (largest) — ADR-0010 mains gas tariff Table 32 vs Table 12

**Magnitude:** Cohort-wide. For cert 000565: +£3.62 cost → +0.041
continuous SAP → flips sap_score 29→28. For other gas-DHW certs: similar
order-of-magnitude.

**Cascade:** Uses SAP 10.2 Table 12 prices (£0.0364/kWh mains gas).
Worksheet uses RdSAP 10 Table 32 (£0.0348/kWh).

**Tractability:** Requires ADR-0010 amendment + coordinated cohort re-
pin (every golden + Elmhurst worksheet cert's pinned cost shifts). NOT
a single-cert slice.

**Suggested approach:**
1. Read ADR-0010 to understand the current price-table decision and
   what's blocking the switch to Table 32.
2. Identify which Elmhurst worksheets in the cohort actually use
   Table 32 (the U985 ones definitely do).
3. Stand up a parallel `RDSAP10_TABLE_32_PRICES` constant alongside
   `SAP_10_2_SPEC_PRICES`.
4. Re-pin all golden + Elmhurst e2e expectations under Table 32.
5. ADR-0010 amendment commit that frames the policy decision.

This is the highest-leverage single change for the Elmhurst worksheet
cohort. After this lands, cert 000565 → sap_score 29 EXACT, plus
likely several other open-residual certs close.

### #2 — RR (room-in-roof) fold-in for cert 000565 space_heating −72

**Magnitude:** −72 kWh space_heating (cert 000565) → −42 kWh
main_heating_fuel via 1/COP.

**Cascade:** Doesn't fully implement RdSAP §3.10 detailed-RR geometry
+ area formula. Cert 000565 has RR on every part (5 BPs) with detailed
gable wall lengths, slopes, common walls.

**Tractability:** Single-cert slice, but needs spec-citation work in
the heat_transmission cascade. The detailed-RR area formula is in
RdSAP §3.10 (PDF p.30-35, "Room in roof").

### #3 — Lighting CO2 factor Δ−0.0025 (tariff-blended Table 12d)

**Magnitude:** −3.16 kg CO2 (lighting) and similar Δ−0.0025 on
pumps_fans CO2 factor. Same cause: cascade uses code 30 (standard
electricity) Table 12d factors; worksheet uses TEN_HOUR Grid 1 blend
of codes 33 (10h low) + 34 (10h high).

**Cascade:** `lighting_co2_factor_kg_per_kwh=_effective_monthly_co2_factor(
lighting_monthly_kwh, _STANDARD_ELECTRICITY_FUEL_CODE)` at
`cert_to_inputs.py:4054`. Same shape for pumps_fans at line 4050.

**Tractability:** Clean spec citation. Mirror what S0380.65 did for
main_heating_co2_factor (Table 12a Grid 1 high/low blend) for lighting
and pumps_fans. Only affects off-peak tariff certs (cert 000565 is the
only Elmhurst worksheet fixture on Dual-meter; cohort-2 has some).

### #4 — MEV pumps_fans +2.48 (PCDB MEV table missing)

**Magnitude:** +2.48 kWh pumps_fans (cert 000565). PCDB MEV record
table not in the repo (cert lodges PCDF 500755). External-data
acquisition gates this; not solvable in code.

### #5 — HP SAP code → main_heating_category=4 in mapper

Cert 000565 Main 1 has sap_main_heating_code=224 but no PCDB Table 362
ref → mapper sets category=None. The TODO in
`mapper.py:_elmhurst_main_heating_category` says this is deferred
because of HP-on-E7 cost cascade + Table 4f MEV component coupling.
Now that S0380.80 has surfaced the cleaner cascade, the coupling cost
analysis can be redone. Couples with #4 (MEV).

### #6 — 12 gas-combi PV certs at +0.5..+1.6 PE

Unchanged from prior handover. No worksheets available; re-pinned at
current residuals.

### #7 — 5 SAP-integer-residual certs

Unchanged. All API-only (no worksheets). User has agreed not to chase
these without worksheet ground truth.

## How to run the baseline

```bash
PYTHONPATH=/workspaces/model python -m pytest \
    backend/documents_parser/tests/test_summary_pdf_mapper_chain.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_appendix_h_solar.py \
    domain/sap10_calculator/rdsap/tests/test_cert_to_inputs.py \
    domain/sap10_calculator/rdsap/tests/test_golden_fixtures.py \
    --no-cov -q
```

Expected: **551 pass + 9 expected `test_sap_result_pin[000565-*]` fails**
at HEAD `760a893c`.

The 9 expected fails (verbatim from the latest run):
```
sap_score
sap_score_continuous
ecf
total_fuel_cost_gbp
co2_kg_per_yr
space_heating_kwh_per_yr
main_heating_fuel_kwh_per_yr
lighting_kwh_per_yr
pumps_fans_kwh_per_yr
```

`hot_water_kwh_per_yr` was the 10th fail in baselines `a532f75d` through
`f9551355`; now passes at HEAD `760a893c`.

## Files touched this session

| File | Slices | Change |
|---|---|---|
| `backend/documents_parser/elmhurst_extractor.py` | S0380.78 | `_extract_baths_and_showers` uses `_section_lines("1x.0 Baths and Showers", "18.0 Flue Gas Heat Recovery System")` instead of `self._lines.index("Connected")` |
| `backend/documents_parser/tests/test_summary_pdf_mapper_chain.py` | S0380.78 | New test `test_summary_000565_extractor_finds_electric_shower_in_section_1x_0` |
| `domain/sap10_calculator/calculator.py` | S0380.78 | Fallback scalar `total_cost` adds `electric_shower_kwh × other_fuel_cost` |
| `domain/sap10_calculator/tests/test_calculator.py` | S0380.78 | New test `test_total_fuel_cost_includes_247a_electric_shower_in_fallback_path` |
| `domain/sap10_calculator/rdsap/cert_to_inputs.py` | S0380.77, S0380.79, S0380.80 | `_primary_loss_override` resolves DHW main internally; `_separately_timed_dhw(epc, main)` cylinder-default; `_cylinder_storage_loss_override` applies (57)m solar adjustment; water_eff `−= 0.05` for Table 4c boiler-interlock |
| `domain/sap10_calculator/rdsap/tests/test_cert_to_inputs.py` | S0380.77, S0380.79, S0380.80 | 3 new tests pinning the spec rules above |
| `domain/sap10_calculator/rdsap/tests/test_golden_fixtures.py` | S0380.79 | Cert 0390 pin updated −7 → −6 with revised notes citing S0380.79 |

## Spec source quick-reference

- **SAP 10.2 full specification**: `domain/sap10_calculator/docs/specs/sap-10-2-full-specification-2025-03-14.pdf`
  - Appendix J §J2 step 2a (bath formula): p.81
  - §4 (45)..(65) HW worksheet: p.135-137
  - §4 line 7693 (57)m solar adjustment: p.137
  - §4 line 7700 + Table 3 (primary loss): p.159
  - §10a (245)..(255) cost worksheet: p.145
  - Table 2 (HW storage loss factor): p.158
  - Table 2b (HW storage loss temperature factor + notes a/b): p.159
  - Table 3 (primary circuit loss): p.159
  - Table 3a (combi loss): p.160
  - Table 4b (gas/oil boiler seasonal efficiency): p.168
  - Table 4c (efficiency adjustments — boiler interlock, etc.): p.169-170
  - Appendix D2.1 (using PCDB efficiency values): p.57
  - Appendix D2.2 (condensing boiler corrections): p.58
- **RdSAP 10 specification**: `domain/sap10_calculator/docs/specs/RdSAP 10 Specification 10-06-2025.pdf`
  - §3 default table (boiler interlock, separately timed DHW, pipework insulation): p.57
  - §10.11 Table 29 (solar panel defaults): p.58
- **S10TP-12** (BRE seasonal efficiency of condensing boilers): `domain/sap10_calculator/docs/specs/sap10 technical papers/S10TP-12 - Seasonal efficiency of condensing boilers - V1.2.pdf`
- **SAP 10.3** at `domain/sap10_calculator/docs/specs/sap-10-3-full-specification-2026-01-13.pdf`: **DO NOT reference** (project tracks 10.2 only per [[feedback-sap-10-2-only-never-10-3]])

## Memory updated this session

- `project_cert_000565_recovery_state` — full S0380.77/78/79/80 history,
  cumulative closure table, attribution of remaining residuals to
  deferred ADR-0010 gas tariff
- `MEMORY.md` — index entry refreshed