A dwelling's heating is one conceptual system, but its fields are scattered
across EpcPropertyData (a gov-API schema mirror): the cluster on sap_heating, the
electricity tariff on sap_energy_source.meter_type, hot-water flags loose at top
level. Three places synthesise a heating system — Measure Options, Landlord
Overrides, EPC Prediction's donor — and each hand-copied a different ad-hoc
subset. The override and donor both dropped meter_type, so an electric-storage
system landed on the template's single-rate meter and billed overnight heat at
the peak rate: property 713406 scored SAP 13 (G) vs ~50 (E), inflating the HHRSH
measure to +45.8 and overshooting the plan to band A.
Establish a single Coherent Heating System boundary (CONTEXT.md) that every
synthesiser must cover, with a source-appropriate fill policy (ADR-0035):
- Override overlay *completes* the partial system the landlord named. Companion
fields are now DERIVED from the SAP code, not hand-attached per archetype: the
off-peak meter from the calculator's single off-peak classification (new
OFF_PEAK_IMPLYING_HEATING_CODES = SAP §12 Rules 1-2), and an unobserved storage
charge control defaults to the conservative manual control (Table 4e 2401). So
adding a heating archetype is just adding its code — companions can't be
forgotten. A contract test guards it (every off-peak code drags a Dual meter).
- Prediction's heating donor now *carries* the donor's meter_type alongside its
sap_heating cluster — the donor is already coherent.
Coherence is a synthesis-time obligation only; the calculator still scores a real
lodged cert exactly as lodged.
Verified on 713406: baseline 13 -> 47.8 (E), matching its recorded rating; the
phantom HHRSH recommendation is gone and the plan no longer overshoots to A.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
