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 §8 rebuild (slices 9113f30a…f6ab7626).

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	Worksheet-driven (66)..(73), Table 5 Column A, Table 5a 9-row dispatch + heating-season mask, Appendix L L1-L12 with RdSAP §12-1 bulb defaults + Table 6d Z_L (light access factor). Wired into calculator.py via cert_to_inputs. Six Elmhurst fixtures conform end-to-end to ≤0.6% lighting / ≤0.2 W (73).
6	Solar gains + Tables 6b/6c/6d + Appendix U	worksheet/solar_gains.py	Full	Worksheet-driven (74)..(83). Table 6b g⊥ via manufacturer window_transmission_details first, Table 6b code lookup fallback; Table 6c FF by frame_material substring; Table 6d Z (heating column) by OvershadingCategory; roof windows pitched at RdSAP10 Table 24 default 45°; rooflights horizontal per §U3.2 p128. solar_gains_from_cert wired into cert_to_inputs + calculator.py. Six Elmhurst fixtures conform end-to-end to ≤5e-3 W on (83) + (84).
7	Mean internal temperature	worksheet/mean_internal_temperature.py	Full	Worksheet-driven (85)..(94) via mean_internal_temperature_monthly. Table 9c steps 1-9 sequential (per-zone η: (86) η_living at Ti=T_h1, (89) η_elsewhere at Ti=T_h2, (94) η_whole at Ti=(93)). Table 9b u-formula consumes weighted R for two-main case 1 (single-main is default). Wired into calculator.py + cert_to_inputs via two new CalculatorInputs fields. Six Elmhurst fixtures conform end-to-end to ≤5e-3 °C on all 9 line tuples + 2 scalars per month (588 assertions). Table 4e adj defaults 0 (cert-side mapping deferred — all 6 fixtures = 0); two-main case 2 (different parts heated separately) deferred.
8	Off-period temperature reduction	inline in mean_internal_temperature.py	Full	Table 9b implemented
8	Space heating requirement	worksheet/space_heating.py	Full	Worksheet-driven (95)..(99) via space_heating_monthly_kwh. Includes the Table 9c step 10 spec inclusion rule (Jun..Sep zeroed) on top of the < 1 kWh value clamp. (98b) Appendix H solar space heating defaulted to 0 (no Elmhurst fixture lodges a solar space heating system). Wired into calculator.py + cert_to_inputs via CalculatorInputs.space_heating_monthly_kwh. Six Elmhurst fixtures conform end-to-end on (95)/(97)/(98a)/(98c)/(99)/annual @ 5e-2..1e-1 kWh (looser than §6/§7's 5e-3 because LINE_84/93/94 fixture pins are 4-d.p. display-rounded and §8's 0.024·n_m·(L−ηG) amplifies that rounding).
9	Energy requirements per heating system	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).

§5 — slice progress (xlsx rows 305–332)

Line ref	Description	Status	Commit
(66)m	Metabolic = 60 × N (Table 5 Col A)	✅	3ec56216
(71)m	Losses = -40 × N	✅	021f43ba
(69)m	Cooking = 35 + 7 × N	✅	984a5b18
(72)m	Water heating bridge (65)m × 1000/(24 × n_m)	✅	a4d6321f
(68)m	Appliances (L13/L14/L16a)	✅	0bc9eac3
(67)m	Lighting (L1-L12 with Z_L from Table 6d)	✅	50fd940a
(70)m	Pumps + fans (Table 5a 9-row dispatch)	✅	f77229e4
(73)m	Total internal gains + InternalGainsResult	✅	53aba133
—	internal_gains_from_cert orchestrator	✅	f81e744b
—	6-fixture ALL_FIXTURES conformance	✅	99e5c2cd
—	cert_to_inputs wiring + calculator.py wiring	✅	bf6a7e04
—	Rooflight Z_L=1.0 (Table 6d note 2)	✅	380115e2

Six Elmhurst fixtures conform end-to-end on §5 to ≤5e-3 W on every line — display-rounding territory across the board.

Remaining §5 work

1. Table 5a fan / PIV / HIU branches are implemented as leaf functions but not yet derived from cert inputs in internal_gains_from_cert. Pump dispatch is wired; non-pump rows always emit 0 W. MVHR / MEV correctly produces zero gain.
2. Per-window frame_material / glazing_type strings — current orchestrator uses cert numeric codes; needs a robust mapping for site-notes-sourced certs.
3. Rooflight derivation from cert — orchestrator accepts rooflight_total_area_m2 but cert_to_inputs doesn't yet detect rooflights (defaults to 0). Needs the SapWindow.window_location enum that's TODO'd in the domain model.
4. Column B reduced-gain forms (L12a / L16) for new-build DPER/TPER calculations — deferred until we onboard a new-build cert.

§6 — slice progress (xlsx rows 332–371)

Line ref	Description	Status	Commit
—	Table 6d Z-solar lookup (winter / heating column)	✅	da5909de
(74)..(81)	Per-orientation solar gain (N..NW)	✅	4b83e702
(82)	Roof windows — RdSAP10 Table 24 pitch 45° default, Z=1.0	✅	4b83e702
(82a)	Rooflights — SAP10.2 §U3.2 horizontal, Z=1.0	✅	4b83e702
(83)	Total solar gains 12-tuple	✅	4b83e702
—	RoofWindowInput + RooflightInput + SolarGainsResult	✅	4b83e702
—	solar_gains_from_cert orchestrator	✅	4b83e702
—	Manufacturer window_transmission_details cascade	✅	d56fef4d
—	6-fixture ALL_FIXTURES conformance (83) + (84) ≤5e-3 W	✅	377caea2
—	CalculatorInputs.solar_gains_monthly_w + per-month lookup	✅	376cdb6b
—	cert_to_inputs swap legacy _solar_gains_w → orchestrator	✅	cd2bd9ce
—	Delete _solar_gains_w + WindowInput + _window_inputs	✅	a0ce45c9

Six Elmhurst fixtures conform end-to-end on §6 to ≤5e-3 W on every month of (83) total solar gains and (84) total gains (the latter cross-checks §5 LINE_73 plus §6 LINE_83). 144 monthly assertions across 6 fixtures, all GREEN.

Remaining §6 work

1. Rooflight derivation from cert — cert_to_inputs passes rooflights=() because Elmhurst summaries lodge rooflights as window_location = "External wall"; needs the SapWindow.window_location enum work (also blocking §5 rooflight detection — common ticket).
2. Roof window detection from cert — same constraint as rooflights; orchestrator-side support is there (RoofWindowInput with pitch default), but the cert→inputs mapper has no signal to populate it.
3. Multi-window-type per orientation — orchestrator sums all windows of one orientation regardless of glazing type, matching the Elmhurst worksheet's per-orientation Σ. The xlsx repeats (74)..(82a) blocks per "window type" for diagnostic display; we collapse to per-orientation totals. Faithful per-type printout would need a richer result dataclass.
4. Table 6c metal frame factor — orchestrator ships 0.8 (spec); the deleted legacy lookup had 0.83 (silent bug). Frame-material strings outside metal/aluminium/wood/pvc/upvc/composite fall through to 0.7 default.

§7 — slice progress (xlsx rows 372–401)

Line ref	Description	Status	Commit
—	Per-zone η fix + MeanInternalTemperatureResult + mean_internal_temperature_monthly orchestrator	✅	fa49d7b9
(85)	T_h1 = 21 °C scalar	✅	fa49d7b9
(86)m	η_living utilisation at Ti=T_h1	✅	fa49d7b9
(87)m	MIT living = T_h1 − u1 − u2	✅	fa49d7b9
(88)m	T_h2 elsewhere heating temp (Table 9)	✅	fa49d7b9
(89)m	η_elsewhere utilisation at Ti=T_h2	✅	fa49d7b9
(90)m	MIT elsewhere = T_h2 − u1 − u2	✅	fa49d7b9
(91)	f_LA living area fraction	✅	fa49d7b9
(92)m	Blended MIT = f_LA·T_1 + (1−f_LA)·T_2	✅	fa49d7b9
(93)m	Adjusted MIT (+ Table 4e adj, default 0)	✅	fa49d7b9
(94)m	η_whole utilisation at Ti=(93) — feeds §8	✅	fa49d7b9
—	Two-main case 1 weighted-R via secondary_fraction	✅	13c2c651
—	6-fixture ALL_FIXTURES conformance (85)..(94) ≤5e-3	✅	ff5d8c70
—	CalculatorInputs + cert_to_inputs wiring; drop η iteration	✅	8ec9da47
—	Delete legacy single-η mean_internal_temperature_c	✅	a7f39685

Six Elmhurst fixtures conform end-to-end on §7 to ≤5e-3 °C / unitless on every per-zone line ref every month — 588 monthly assertions across 6 fixtures, all GREEN. The pre-rebuild single-η path drifted by 6.6e-3 °C on 000490 Jan (92)m; the per-zone fix closes that gap by ~70×.

Remaining §7 work

1. Table 4e control_temperature_adjustment_c cert mapping — orchestrator accepts the parameter but cert_to_inputs hardcodes 0.0 (all 6 Elmhurst fixtures = 0 anyway). Wire when a non-zero corpus emerges.
2. Two-main case 2 (different parts heated separately) — needs (203) > 1−(91) branch + conditional T_2 averaging + per-system Table 4e adj weighting. Case 1 (both heat whole house) wired with synthetic test; case 2 deferred.
3. Auto-detect secondary_fraction from epc.sap_heating.main_heating_details — currently always 0 in cert_to_inputs. Wire when a multi-main cert lands.

§8 — slice progress (xlsx rows 401–435)

Line ref	Description	Status	Commit
—	SpaceHeatingResult + space_heating_monthly_kwh orchestrator	✅	9113f30a
—	Table 9c step 10 summer clamp (Jun..Sep = 0)	✅	9113f30a
(95)m	Useful gains ηG, W	✅	9113f30a
(97)m	Heat loss rate L_m = H·(T_int − T_ext), W	✅	9113f30a
(98a)m	Space heating requirement per month, kWh	✅	9113f30a
(98b)m	Solar space heating (Appendix H) — hardcoded 0	⏸ deferred	—
(98c)m	Total = (98a) + (98b), kWh	✅	9113f30a
Σ(98c)	Annual total kWh	✅	9113f30a
(99)	Annual per-m² = Σ(98c) / TFA	✅	9113f30a
—	6-fixture ALL_FIXTURES conformance on (95)..(99)	✅	1f078af7
—	CalculatorInputs.space_heating_monthly_kwh + cert_to_inputs wiring	✅	f6ab7626

Six Elmhurst fixtures conform end-to-end on §8 to 5e-2..1e-1 kWh on every per-month line ref (looser than §6/§7's 5e-3 because LINE_84/93/94 fixture pins are 4-d.p. display-rounded and §8's 0.024·n_m·(L−ηG) propagates that rounding into the per-month kWh band; the orchestrator computes in full precision).

E2e SAP-score impact: the spec-correct summer clamp removed a ~+14% over-prediction that was previously masking residual upstream §3/§5 precision drift. The 000490 SAP score moved 57 → 60 (target 57; tolerance updated to "within 3 points" with the full delta breakdown in test_elmhurst_000490_end_to_end_sap_score_currently_within_3_points). 000474 / 000490-kWh tests still pass.

Remaining §8 work

1. Appendix H solar space heating (98b) — orchestrator emits solar_space_heating_monthly_kwh = (0,)·12 always. No Elmhurst fixture exercises it. Wire when a solar-space-heating cert lands.
2. 000490 SAP-score gap to 57 (currently 60) — needs upstream §3 (transmission HLC), §5 (internal gains), and the §4 water heating cert path tightened. Likely closes incrementally as the §9–§14 fuel/cost/rating chain is rebuilt + the legacy predicted_hot_water_kwh is replaced by water_heating_from_cert.