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Record the DHW-energy residual investigation: not a bug 🟪

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Root-caused the 2675-vs-1839 kWh communal-DHW delta: the engine correctly
applies the SAP §4.3 distribution-loss factor (1/(1.05×1.50)); the difference is
the DHW energy content (64) alone. The engine's 1698 kWh tracks the cert's own
lodged RHI water heating (1909) far better than Elmhurst's reduced-data 1168,
and its £113 is closer to the cert's lodged £147 than Elmhurst's £78 — so the
engine is more faithful to the cert here, not buggy. Comment accuracy only.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DX8oAGsGkBHq3U4dsxYRzz
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Jun-te Kim 2026-07-27 23:02:43 +00:00
parent 6e6c27076e
commit 2f8c086310
1 changed files with 9 additions and 7 deletions
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16
tests/domain/sap10_calculator/test_real_cert_sap_accuracy.py
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@ -1414,13 +1414,15 @@ _EXPECTATIONS: Final[tuple[RealCertExpectation, ...]] = (
# engine uses the cert's MEASURED U-values (wall 0.19, floor 0.17) + MVHR
# (PCDB 500167) → space heat 1039 kWh, where Elmhurst RdSAP uses age-band-M
# proxies + a residual build AP50 (4.88 vs the cert's 3.2) → 1676 kWh. A
# KNOWN residual pulls the OTHER way (~-2): the engine's communal-DHW energy
# (2675 kWh) exceeds the worksheet's (1839 kWh = (64) × 1.05 × 1.50). This is
# NOT a cylinder loss — the dwelling has no cylinder (has_hot_water_cylinder
# false); it is an un-root-caused SAP §4.3 community-DHW demand/distribution-
# loss reconciliation, deferred as a follow-up. PINNED TO THE OBSERVED 80 —
# mapping deliberately untuned; the Elmhurst delta is the full-SAP→RdSAP
# residual, NOT a bug.
# residual pulls the OTHER way (~-2): the engine's communal-DHW heat delivered
# (2675 kWh = (64) 1698 × 1.05 × 1.50) exceeds the worksheet's (1839 = (64)
# 1168 × 1.05 × 1.50). Investigated to root: this is NOT a bug — the engine
# correctly applies the §4.3 distribution-loss factor; the delta is purely the
# DHW energy-content demand (64), where the engine's 1698 kWh tracks the cert's
# OWN lodged RHI water-heating (1909 kWh) far better than Elmhurst's reduced-
# data 1168, and its £113 is closer to the cert's lodged £147 than Elmhurst's
# £78. PINNED TO THE OBSERVED 80 — mapping deliberately untuned; the Elmhurst
# delta is the full-SAP-measured vs RdSAP-reduced-data residual, NOT a bug.
RealCertExpectation(
schema="SAP-Schema-18.0.0",
sample="uprn_10094112770",