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docs: SAP 10.2 / RdSAP 10 spec coverage map

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Per user suggestion (switch from probe-driven to worksheet-driven
iteration), enumerates the §§1-15 worksheet + Appendices A-U state in
the calculator with a status grade and a prioritised gap list. Becomes
the roadmap for Session B remaining slices.

Next slice from this list: Table 11 secondary heating allocation —
10% fraction on most boiler-main certs that we currently model as 0.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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# SAP 10.2 / RdSAP 10 Coverage Map
Tracks which sections of the SAP 10.2 specification are implemented in `packages/domain/src/domain/sap/`. Per ADR-0009 the calculator is built from the spec, not reverse-engineered from cert data. This doc is the worksheet-driven roadmap for what remains.
Updated 2026-05-18 after S-B19.
## Sections §§1–13 (the SAP worksheet)
| § | Topic | Module | Status | Notes / gaps |
|---|---|---|---|---|
| 1 | Dimensions | `worksheet/dimensions.py` | **Full** | Porches, conservatories, RIR deferred per ADR-0009 |
| 2 | Ventilation | `worksheet/ventilation.py` | Partial | No mechanical ventilation (MVHR/MEV), no wind-shelter factor, no pressure-test override (worksheet lines 17-18), no AP4 override (worksheet line 19) |
| 3 | Heat transmission | `worksheet/heat_transmission.py` | Partial | DwellingExposure for flats ✓, but uses global y-factor (Table R2 per-junction deferred); no roof-light separate U-value |
| 4 | Hot water + Appendix J | uses `domain.ml.demand.predicted_hot_water_kwh` | Partial | Instantaneous-system flag ✓; no shower/bath count adjustments, no FGHRS/WWHRS, no PV-diverter |
| 5 | Internal gains + Appendix L | `worksheet/internal_gains.py` | Full | Default occupancy + Appendix L lighting fallback |
| 6 | Solar gains + Tables 6b/6c/6d + Appendix U | `worksheet/solar_gains.py` | Partial | Per-orientation/pitch ✓; Tables 6b/6c lookups ✓; Z (overshading) hardcoded to 0.77 average; roof-lights treated as vertical |
| 7 | Mean internal temperature | `worksheet/mean_internal_temperature.py` | Partial | Living area fraction from Table 27 ✓ (per S-A7b); control_type from main_heating_control code ✓ (S-B5); control_temperature_adjustment_c always 0 |
| 8 | Off-period temperature reduction | inline in `mean_internal_temperature.py` | Full | Table 9b implemented |
| 9 | Space heating | `worksheet/space_heating.py` | Partial | Single main system only — **no Table 11 secondary heating allocation** (10% fraction on most boilers — likely big MAE) |
| 10 | Cooling | — | Not implemented | Rare in UK dwellings; defer |
| 11 | FEE | — | Not implemented | Only for new-build; not required for ratings |
| 12 | Total energy + fuel costs | `calculator.py` | Partial | Per-end-use cost split ✓; meter_type tariff routing ✓ (S-B15); PV cost credit ✓ (S-B19); **standing charges not included** (Table 12 note (a) says rating omits standing charge for std electricity tariff) |
| 13 | SAP rating | `worksheet/rating.py` | Full | Equations 7-9 verified against SAP 10.2 §13 |
| 14 | CO2 + primary energy | `calculator.py` SapResult.co2_kg_per_yr | Partial | Single CO2 factor on main fuel; no per-end-use CO2 mixing; **no primary energy calculation** |
| 15 | Building regs | n/a | n/a | Not relevant to ratings |
## Appendices
| Appendix | Topic | Status | Notes |
|---|---|---|---|
| A | Main + secondary heating identification | **Not implemented** | First main only; Table 11 secondary fraction missing — high-value gap |
| B | Gas/oil boilers + boiler interlock | Partial | Via Table 4b in `domain.ml.sap_efficiencies.seasonal_efficiency` |
| C | Community heating | Not implemented | Heat network certs handled by Table 4a codes 301-304 (efficiency only) |
| D | PCDB | Stubbed | Per ADR-0009 grill: `NoOpPcdbLookup` returning None; Session C bundles a CSV PCDB extract |
| E | Heat pumps | Partial | Category fallback to 2.30 for cat=4; no MCS installation factor (×1.39 GSHP); no flow temp adjustment |
| F | Electric CPSU | Not implemented | Rare |
| G | FGHRS / WWHRS / PV-diverter | Not implemented | Rare |
| H | Solar water heating | Partial | Boolean `has_solar_water_heating` reduces HW by 250 kWh; no actual collector calc |
| J | Hot water demand | Partial | See §4 row above |
| K | Thermal bridging | Partial | Using global y per age band (per ADR-0009 grill: per-junction Table R2 deferred) |
| L | Lighting | Full | Existing-dwelling fallback ✓ |
| M | PV / wind / hydro generation | Partial | PV ✓ (S-B19); wind / hydro / micro-CHP not implemented |
| N | Micro-CHP | Not implemented | Rare |
| P | Electric storage heaters detail | Partial | Identified via codes 401-409; Table 12a high-rate fractions not exact (we use 100% off-peak per cert-calibration heuristic) |
| Q | Special features | Not implemented | One-off energy-use additions; rare |
| R | Reference dwelling | Not implemented | Only needed for compliance, not ratings |
| S | RdSAP procedures | Separate PDF (114pp) | Cert→inputs mapper in `domain.sap.rdsap.cert_to_inputs` ports key rules; many sections of RdSAP 10 not yet read |
| T | Improvement measures | Not relevant | Recommendation engine, not rating |
| U | Climate data | Full | Tables U1/U2/U3 + solar declination + Table U5 k1-k9 |
## Prioritised gap list (by likely MAE impact)
1. **Table 11 Secondary heating allocation** — most boiler-main certs allocate 10% of space heating to a secondary system (often a less-efficient room heater on a different fuel). We model 0%. Likely +1-2 SAP-point bias on affected certs.
2. **Wind-shelter factor on infiltration** (§2 worksheet lines 19-21) — multiplies infiltration by `1 - 0.075 × sheltered_sides`. We have no shelter input; assume 2 sheltered sides default. Net effect on infiltration ACH probably ~10%.
3. **Table 12a high-rate fraction for off-peak dwellings** — we currently bill 100% of E7 space heating at the low rate. Real spec says e.g. heat pumps on 7h tariff at 80% high-rate. Affects ~5% of certs.
4. **Cylinder-loss factor cascade** — currently uses simplified buckets in `domain.ml.demand._STORAGE_LOSS_FACTOR`. Spec has more precise interpolation rules from cylinder volume + insulation thickness.
5. **Standing charges in cost** — Table 12 note (a) gives the rule for when standing charges are included (energy use vs rating). May affect bias.
6. **Per-junction thermal bridging (Table R2)** — only relevant when assessor lodged junction-count data, otherwise global y is the spec answer for RdSAP-driven assessments.
Status now: 100-cert MAE 4.49, 300-cert MAE 5.45, bias near zero (±0.2). Worksheet-driven phase begins with **Secondary heating Table 11** as the next slice.