Model/docs/sap-spec/SPEC_COVERAGE.md

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 2026-05-20 after §3 close (slices 344a9c9d…cf244762).

The canonical SAP10.2 algorithm lives in 2026-05-19-17-18 RdSap10Worksheet.xlsx at the repo root — each line ref (1)..(486) maps to a cell. The worksheet sub-modules under packages/domain/src/domain/sap/worksheet/ implement those line refs directly; Elmhurst worksheets validate end-to-end via tests/_elmhurst_worksheet_*.py.

Sections §§1–13 (the SAP worksheet)

§	xlsx rows	Topic	Module	Status	Notes / gaps
1	1–120 (approx)	Dimensions	worksheet/dimensions.py	Full	Porches, conservatories, RIR deferred per ADR-0009
2	121–206 (approx)	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	121–207	Heat transmission	worksheet/heat_transmission.py	Full (non-RR)	LINE_31/33/36/37 exact for both non-RR Elmhurst fixtures (000474, 000490). Suspended-timber + Table 20 exposed-floor routes wired. RR sub-areas (gable/slope/stud-wall) deferred until SapRoomInRoof carries them. Global y-factor (Table R2 per-junction deferred).
4	207–304	Hot water + Appendix J	worksheet/water_heating.py (new) + legacy domain.ml.demand.predicted_hot_water_kwh (calculator still calls this)	Happy-path done — see slice progress below. Worksheet-driven module lands line refs (42)..(65) for the combi-gas-boiler population (~70% of corpus). Cylinder + solar + renewables branches deferred. Calculator.py not yet wired to new module — next step.
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.

§4 — slice progress (xlsx rows 207–304)

Line ref	Description	Status	Commit
(42)	Assumed occupancy N from Appendix J	✅	aff678e8
(42a)m	Mixer showers monthly	✅	1dcbdb28
(42b)m	Baths monthly	✅	dad7fbf3
(42c)m	Other uses monthly	✅	5cc68ab3
(43)	Annual avg	✅	702b1c6c
(44)m	Daily monthly total	✅	702b1c6c
(45)m	Energy content	✅	a3c687f1
(46)m	Distribution loss	✅	a3c687f1
(47)–(56)	Storage / HIU loss	Combi zero-branch only; cylinder paths deferred	—
(57)m	Dedicated solar storage	Combi zero-branch only; solar HW deferred	—
(59)m	Primary loss	Combi zero-branch only; boiler+cylinder deferred	—
(61)m	Combi loss	Table 3a row "time-clock keep-hot" only	bfba610b
(62)m	Total demand	✅	bfba610b
(63a-d)	WWHRS/PV/Solar/FGHRS reductions	Zero-only — non-zero paths deferred	feef8198
(64)m	Output from water heater	✅ (with max-clamp)	feef8198
(64a)m	Electric shower energy	Zero-only — non-zero path deferred	—
(65)m	Heat gains	✅	43da3ea0

Happy path closes both non-RR Elmhurst fixtures to <1e-3 kWh end-to-end on lines (42)..(65) (000474's PCDB-backed Table 3b combi loss is the one hand-lodged value).

Remaining §4 work

1. Orchestrator water_heating_from_cert(epc, ...) that wires the leaf functions, mirroring heat_transmission_from_cert for §3.
2. Integrate with calculator.py — replace the legacy domain.ml.demand.predicted_hot_water_kwh call.
3. End-to-end SAP score validation against the worksheets — the real e2e test the user has been asking for.
4. Cylinder + solar + renewables paths for full population coverage (cylinders, FGHRS, WWHRS, PV-diverter).
5. PCDB-backed Table 3b/3c combi loss for tested boilers (000474 sits here).