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An instantaneous point-of-use electric water heater (WHC 907/909, no cylinder) on an off-peak tariff was billed 100% at the off-peak LOW rate by the generic electric-off-peak else-branch. But SAP 10.2 §12 (PDF line 2680) computes the off-peak on-peak HW proportion via Table 13, "dependent on the total floor area and the CYLINDER size" — it presumes a stored-water cylinder charged overnight. An instantaneous heater has NO cylinder, heats on demand, and cannot shift to the off-peak window, so 100% of its consumption bills at the HIGH rate (Table 12a WH high-rate fraction 1.0). Both the SAP-cost rate (`_hot_water_fuel_cost_gbp_per_kwh`) and the ADR-0014 / CO2-PE fraction (`_hot_water_high_rate_fraction`) fixed consistently (low-rate scalar -> 7-hour high rate; fraction 0.0 -> 1.0). Localised by deep-diving corpus cert 74061136 (HHR-storage mid-floor flat, WHC 909): PE matched lodged (+1.6, roof+floor zero-loss) while SAP over-rated +7.72 — the cost-only signature. Its DHW was ours £59 vs lodged £344 (5.8x); the tariff was the whole gap. Fix: +7.72 -> -1.25 (residual is separate small fabric). 7 corpus certs carry electric-instantaneous DHW on off-peak; the 3 outside 0.5 all move sharply inward (MAE the win, not within-0.5 crossings). RdSAP 10 §12 tariff routing confirmed spec-correct (Unknown meter + storage 409 -> off-peak 7-hour, Rule 2) — the bug was the DHW rate, not the tariff. Unit-pinned in test_cert_to_inputs; RealCertExpectation 74061136 = 72. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com> |
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| .. | ||
| addresses | ||
| billing | ||
| data_transformation | ||
| epc | ||
| epc_prediction | ||
| fuel_rates | ||
| magicplan | ||
| modelling | ||
| property | ||
| property_baseline | ||
| sap10_calculator | ||
| tasks | ||
| __init__.py | ||
| test_building_geometry.py | ||
| test_postcode.py | ||