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SAP 10.2 §12.4.4 (PDF p.36-37): "With open fire back boilers or closed
room heaters with boilers, an alternative system (electric immersion)
may be provided for heating water in summer. In that case water
heating is provided by the boiler for months October to May and by the
alternative system for months June to September."
The spec-literal CO2 / PE formula multiplies summer immersion fuel by
the Table 12d / 12e monthly cascade (per Table 12 footnotes (s) and
(t): "monthly factors in Table 12d/12e should be used in the SAP
worksheet"). The BRE-approved Elmhurst engine adds an extra
`summer_fuel × Table 12 annual electric` term ON TOP of the monthly
cascade for dual-rate tariffs — same Elmhurst-mirror shape as S0380.163
(§8.1) but additive rather than substitutive. Cost is computed
cleanly per spec — the double-count quirk only affects the (264) HW
CO2 and (278) HW PE factor lines.
Worksheet evidence (heating-systems corpus property 001431,
`solid fuel 2` — Table 4a code 158 closed-room-heater + back boiler,
65 % winter η + 100 % summer η, anthracite, 18-hour off-peak tariff):
(62)m heat 303.12 .. 168.95 .. 175.91 .. 300.40 kWh
winter fuel (W) = 2205.80 / 0.65 = 3393.51 kWh anthracite
summer fuel (S) = 684.55 / 1.00 = 684.55 kWh immersion
total fuel = (219) = 4078.06 kWh
(264) HW CO2 = 4078.06 × 0.3710 = 1513.15 kg/yr
= W × 0.395 + S × (0.116 monthly_summer + 0.136 annual)
= 1340.43 + 79.61 + 93.10 = 1513.14 ✓ within rounding
(278) HW PE = 4078.06 × 1.3771 = 5616.04 kWh/yr
= W × 1.064 + S × (1.429 monthly_summer + 1.501 annual)
= 3610.69 + 977.84 + 1027.51 = 5616.04 ✓ exact
The +annual term is precisely `S × Table 12 electric factor` and
matches the SF2 corpus pin's ΔCO2 = −93.10 and ΔPE = −1027.51 exactly.
Per [[feedback-software-no-special-handling]] mirror the engine.
Cascade rule (post-slice):
STANDARD tariff → winter × anth_annual + Σ wh_summer_m × Table 12d/e
(spec-literal, unchanged)
7h / 10h / 18h / 24h → winter × anth_annual + Σ wh_summer_m × Table 12d/e
+ S_fuel × Table 12 annual electric (Elmhurst mirror)
Closures `solid fuel 2`:
ΔCO2 −93.10 → +0.0000 EXACT
ΔPE −1027.51 → +0.0000 EXACT
ΔSAP and Δcost remain EXACT (cascade cost path was already correct).
The 41-variant heating-systems corpus is now closed on its 25-variant
cascade-OK tier: all 25 SAP / cost / CO2 / PE EXACT (|Δ| < 1e-3) vs
the Elmhurst worksheet. Only `pcdb 1` carries a sub-tolerance gap
(−0.011 SAP / +5.7 PE — PCDB Eq D1 cascade gap on PCDF index 716, a
separate small slice).
⚠ Single-cert evidence
SF2 is the only §12.4.4 fixture in the corpus (`solid fuel 1` =
code 156 is an empty folder; no other variant exercises a back-boiler
combo with summer immersion). Per the handover ≥2-cert rule for new
§8 divergence rows, this slice was admitted under an explicit
exception: the divergence shares its shape with §8.1 (S0380.163's
Table 12 annual mirror for dual-rate HW), and the math matches the
worksheet to within rounding. The new §8.2 row is tagged with a
"⚠ Single-cert evidence" subsection so future agents know to revisit
if a second §12.4.4 cert worksheet ever diverges from this rule.
Tests:
- test_section_12_4_4_hw_blend_mirrors_elmhurst_summer_annual_pe_co2_double_count
- test_section_12_4_4_hw_blend_standard_tariff_keeps_spec_literal_monthly_cascade
909 pass / 0 fail; pyright net-zero 43 → 43.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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|---|---|---|
| .. | ||
| fixtures | ||
| __init__.py | ||
| test_elmhurst_end_to_end.py | ||
| test_elmhurst_extractor.py | ||
| test_end_to_end.py | ||
| test_extractor.py | ||
| test_heating_systems_corpus.py | ||
| test_pdf.py | ||
| test_summary_pdf_mapper_chain.py | ||