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Resolve electric-underfloor overrides to their own SAP codes dragging electricity 🟥
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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Khalim Conn-Kowlessar
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# Electric underfloor is scored by its own SAP codes; the meter defers to the cert when the tariff is ambiguous
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## Status
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accepted
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## Context
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Electric-underfloor `main_heating_system` overrides had no archetype, so the LLM
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classifier funnelled them into the nearest wrong bucket (#1376):
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| description | rows | mis-mapped to |
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| --- | --- | --- |
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| Electric Underfloor Heating: In concrete slab (off-peak only) | 15 | Direct-acting electric (191), **Single** meter |
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| Electric Underfloor Heating: In screed above insulation (standard or off peak) | 63 | Direct-acting electric (191), **Single** meter |
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| Electric Underfloor Heating: Integrated (storage+direct-acting) (off peak) | 12 | Electric storage heaters, old (401) |
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The **meter is the crux**. A Landlord heating override drags an assumed meter from
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its SAP code (ADR-0035), and the `keep_existing_off_peak_meter` mitigation
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([overlay_applicator.py](../../domain/modelling/scoring/overlay_applicator.py))
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protects a cert's lodged off-peak meter — but *only when the overlay's own desired
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meter is off-peak*. Direct-acting (191) is a **Single**-meter system, so the
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mitigation never fires and the overlay **forces Single**, overwriting the cert's
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meter. For off-peak underfloor that bills overnight heat at the peak rate and
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craters SAP — the exact Class-A meter defect.
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SAP Table 4a has **dedicated electric-underfloor codes** (labels from
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`cert_to_inputs.py`): **421** in concrete slab (off-peak only), **422** integrated
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(storage+direct-acting), **424** in screed above insulation (423 integrated-low-
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off-peak and 425 timber-floor are not in the corpus). Per SAP 10.2 §12, **421/422
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are off-peak (Rule 2, 7-hour → Dual)**; **424 is not off-peak-implying (Single)**.
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A lodged-meter audit is decisive for the screed case: **51 of 63 (81%)** of the
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`"…in screed (standard or off peak)"` dwellings lodge an **off-peak** meter (24-hour
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dual or 7/10-hour), the rest single. The description itself says "standard **or**
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off peak" — so the archetype genuinely cannot know the tariff; the cert does.
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Forcing 424/Single would downgrade 81%; forcing Dual would over-credit the ~19% on
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single.
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## Decision
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Add three electric-underfloor archetypes mapping to their own SAP codes, and treat
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the meter per what the description actually asserts.
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1. **New archetypes** in `_MAIN_HEATING_CODES`:
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`"Electric underfloor, in concrete slab (off-peak)"` → **421**,
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`"Electric underfloor, integrated storage and direct-acting"` → **422**,
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`"Electric underfloor, in screed above insulation"` → **424**.
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2. **Electricity fuel (29)** drags for 421/422/424 via `_natural_fuel_for` — electric
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underfloor is unambiguously electric (ADR-0041), closing the prior no-fuel gap.
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3. **421 and 422 assert an off-peak Dual meter.** Their descriptions positively
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claim off-peak ("off-peak only" / "off peak"), and both are already in
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`_ASSUMED_DUAL_METER_CODES`; the `keep_existing_off_peak_meter` mitigation keeps
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a more-specific cert off-peak meter and applies Dual to a single-rate cert.
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4. **424 (screed) defers the meter to the cert.** Because "standard or off peak" is
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tariff-agnostic and 81% lodge off-peak, `_meter_for` returns **`None`** for 424
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(a new meter-agnostic set), so the overlay leaves `meter_type` unset and the
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lodged meter stands — each dwelling scores on its actual tariff. This is a
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**targeted exception to the "always set the meter, never let it bleed"
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invariant**: that invariant guards against a switched-*off* storage system
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leaving a stale Dual meter, but an underfloor override *confirms* underfloor —
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there is no switch-off, so deferring is faithful, not a bleed.
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5. **Deterministic `main_heating_guard` + reclassify** (TEXT-first, pgEnum-deferred),
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reusing the generalized `reclassify_main_heating.py`. All three underfloor
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descriptions are description-determined, so the guard resolves them directly (no
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fuel-awareness needed) and the reclassify follows.
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## Consequences
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- The 15 concrete-slab-off-peak rows stop being billed at the peak rate (191/Single
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→ 421/Dual); the 12 integrated rows get the correct underfloor code (401 → 422,
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meter already Dual); the 63 screed rows keep their lodged (81% off-peak) meter and
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score on code 424 with correct screed responsiveness (Table 4d R=0.75). Correct
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Rebaselining (ADR-0039).
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- **FE-owned pgEnum additions (Dan):** three new `main_heating_system` values —
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`"Electric underfloor, in concrete slab (off-peak)"`, `"Electric underfloor,
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integrated storage and direct-acting"`, `"Electric underfloor, in screed above
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insulation"`. TEXT read path fixed on merge; cache writes defer to the migration.
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- `_meter_for` becomes `Optional[str]` (a code may defer the meter). Every existing
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code keeps its current Single/Dual answer; only 424 returns `None`.
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- Extends [ADR-0041](0041-landlord-heating-classification-targets-a-complete-modellable-taxonomy.md)
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and [ADR-0035](0035-coherent-heating-system-synthesis.md); refines the
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`keep_existing_off_peak_meter` behaviour ADR-0035 introduced.
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### Alternatives rejected
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- **Force Single (424 per spec).** Rejected: downgrades the 81% lodged off-peak,
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reproducing the meter bug.
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- **Force Dual for screed.** Rejected: over-credits the ~19% genuinely on single and
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asserts a tariff the "standard or off peak" description does not claim.
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- **Map underfloor to Direct-acting (191) / old storage (401) as today.** Rejected:
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wrong code (responsiveness/meter) and the source of the mis-score.
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@ -127,6 +127,29 @@ def test_electric_cpsu_decodes_to_192_electric_on_an_off_peak_meter() -> None:
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assert simulation.heating.meter_type == "Dual"
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@pytest.mark.parametrize(
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("main_heating_value", "code"),
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[
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# Electric underfloor funnelled into Direct-acting (191) / old storage (401)
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# for want of the dedicated SAP Table 4a underfloor codes (ADR-0046).
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("Electric underfloor, in concrete slab (off-peak)", 421),
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("Electric underfloor, integrated storage and direct-acting", 422),
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("Electric underfloor, in screed above insulation", 424),
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],
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)
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def test_electric_underfloor_decodes_to_its_own_code_dragging_electricity(
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main_heating_value: str, code: int
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) -> None:
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# Act
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simulation = main_heating_overlay_for(main_heating_value, 0)
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# Assert — its own underfloor code, electric (29).
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assert simulation is not None
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assert simulation.heating is not None
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assert simulation.heating.sap_main_heating_code == code
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assert simulation.heating.main_fuel_type == 29
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@pytest.mark.parametrize(
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("main_heating_value", "code"),
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[
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