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Resolve a from-main wood-logs water-heating override to the wood fuel 🟥
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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Khalim Conn-Kowlessar
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# Water-heating override: community DHW is scored by its heat-network main, individual DHW by its own fuel
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## Status
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accepted
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## Context
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A **Landlord Override** for water heating names a canonical `"<system>, <fuel>"`
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**Recognised Internal Description** (`WaterHeatingType`), which the water-heating
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**Simulation Overlay** (`water_heating_overlay_for`) decomposes into the SAP
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`water_heating_code` (Table 4a system) + `water_heating_fuel` int codes the
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calculator reads (ADR-0032). The taxonomy was **too small** — it had no biomass /
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wood / dual-fuel / biodiesel water fuel — so the LLM classifier funnelled every
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such description into the nearest bucket, **`"From main system, house coal"`**
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(fuel 33). An audit found **131** `property_overrides` rows on that value; only
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**3** are genuinely house coal. This is the same missing-archetype defect as
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ADR-0041 (heating) — a dumping ground standing in for a gap in the taxonomy.
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Investigating the mis-score revealed the fix is **not** a uniform "swap coal for
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biomass". The 131 rows split by whether the dwelling's **main heating is a heat
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network**, because the calculator scores hot water differently in the two cases:
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- **Community main** (SAP Table 4a 301/302/304). When DHW inherits from the main
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(`water_heating_code` ∈ {901, 902, 914}) and the main is a costable heat
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network, `cert_to_inputs._is_community_heating_hw_from_main` computes the hot-
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water CO2/PE/cost from the **main's** heat-network fuel (via
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`_heat_network_community_fuel_code`, e.g. biomass-community fuel 31 → Table 12
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code 43), scaled by heat-source efficiency — and **ignores
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`water_heating_fuel` entirely**. Verified: HW CO2 is `0.036` whether the water
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fuel is 33 (coal) or 31 (biomass). So the **84** community rows already score
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their DHW as biomass-heat-network; the `"house coal"` value is a **cosmetic
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display lie**, not a scoring error.
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- **Individual main** (a real boiler / room heater). DHW inherits the explicitly
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lodged `water_heating_fuel`, so `33` scores as **house coal (0.395 kgCO2/kWh)** —
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a genuine ~14× carbon overstatement for a wood/biomass dwelling. **47** rows are
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mis-scored this way (20 wood logs, 4 dual fuel, 23 "electric immersion
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assumed"); **3** genuine house-coal rows score correctly.
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The community-fuel collision (API enum 30/31/32 collide in value with the Table-32
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electricity tariff codes 30/31/32) is **already handled** at the fuel-type
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boundary, gated on heat-network context (`_heat_network_community_fuel_code`) — a
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deliberate design that must **not** be globally canonicalised, because the cascade
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uses bare code 30 internally as standard-rate grid electricity
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(`table_32.py` comment). So **no calculator change is required**: community DHW is
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scored correctly *given the coherent community main the override data already
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carries* (`main_heating_system = "Community heating, boilers"`,
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`main_fuel = "biomass (community)"`).
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## Decision
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Complete the water-heating taxonomy and reclassify the dumping-ground rows to
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**faithful** archetypes. No calculator change.
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1. **Add the missing water archetypes**, each mapping to its RdSAP
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`water_heating_fuel` code in `water_heating_overlay._WATER_HEATING_CODES`:
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- `"From main system, wood logs"` → (901, **6**)
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- `"From main system, biomass (community)"` → (901, **31**)
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- `"From main system, dual fuel (mineral and wood)"` → (901, **9**)
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- `"From main system, biodiesel (community)"` → (901, **34**)
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2. **Community DHW maps to its community fuel (31 / 34) and is score-neutral.**
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Because the community main drives the HW score, mapping the 75 community
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biomass/biodiesel rows to their honest community fuel corrects the **displayed
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value** without changing the score — we do **not** flatten them to individual
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wood (6), which would be a semantic lie. Correct labelling, zero scoring risk.
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3. **Individual DHW maps to its real fuel and moves the score.** Wood-logs → 6
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(0.028), dual fuel → 9, off the coal 0.395 — the genuine fix for the 47
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mis-scored individual-main rows.
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4. **"No hot water system present – electric immersion assumed" → the existing
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`"Electric immersion, electricity"` archetype (903, 29).** The description
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states the assessment's own assumption; honouring it is faithful. This corrects
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the 23 individual-main rows (coal → electricity) and re-labels the 9
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community-main rows off the accidental biomass score onto the assessed
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immersion — a small, defensible score change, surfaced here as the one non-
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cosmetic community reclassification.
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5. **Genuine `"From main system, house coal"` (3 rows) is retained** — the taxonomy
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keeps house coal for the dwellings that really burn it.
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6. **Deterministic guard + reclassify, TEXT-first / pgEnum-deferred.** A pure
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`water_heating_guard(description) -> Optional[WaterHeatingType]` (mirroring
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`glazing_mix_guard`) recognises the structured `"<system>: <fuel>"` phrasings
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and is wired via `GuardedColumnClassifier(guard, fallback=ChatGpt…)`; varied
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phrasings stay with the LLM. A one-time `scripts/reclassify_water_heating.py`
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reuses the same guard as its decision function (so backfill and live path can't
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drift), updates `property_overrides.override_value` (TEXT) unconditionally, and
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defers the `landlord_water_heating_overrides.value` pgEnum-cache write for any
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value the live enum doesn't yet carry — printing the deferred set for the FE
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owner (the `main-heating-system-pgenum-is-fe-owned` constraint).
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## Consequences
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- **FE-owned pgEnum additions (Dan).** Four new `landlord_water_heating_overrides`
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values: `"From main system, wood logs"`, `"From main system, biomass
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(community)"`, `"From main system, dual fuel (mineral and wood)"`, `"From main
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system, biodiesel (community)"`. The TEXT read path is fixed immediately; the
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cache write for these four defers until the migration lands.
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- **No `cert_to_inputs.py` change**, so no risk to the thousands of lodged certs
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that carry water fuel 31/33/34 directly. (A separate latent question — whether a
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lodged *individual*-main cert with `water_heating_fuel = 31` or `34` mis-scores
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as electricity because those codes are un-normalised outside heat-network
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context — is **out of scope** here and tracked separately; it does not affect any
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override row, all of which resolve correctly under this decision.)
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- Extends [ADR-0041](0041-landlord-heating-classification-targets-a-complete-modellable-taxonomy.md)
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(complete taxonomy, no dumping ground) and [ADR-0035](0035-coherent-heating-system-synthesis.md)
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(the community main and its DHW cohere — here the coherence is what makes the
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community rows score correctly). The visible baseline shift on the 47 individual
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rows is correct Rebaselining ([ADR-0039](0039-override-aware-rebaselining.md)).
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### Alternatives rejected
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- **Flatten community biomass to individual wood (code 6).** Scores ~identically
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(0.028 vs 0.029) but mislabels a district-heating dwelling as burning its own
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wood — a semantic lie for no benefit, since the community main already scores it.
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- **A calculator "collision fix" normalising 31/33/34 globally.** Rejected: 33 is
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already normalised (`canonical_fuel_code` 33→11); 31/32 must stay un-normalised
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outside heat-network context or genuine grid electricity mis-prices as community
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waste (`table_32.py`). No override row needs it.
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- **Defer the community rows to `None`.** Rejected per ADR-0041 dec-4 (community is
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modelled, not deferred) — and unnecessary, since they already score correctly.
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@ -73,6 +73,19 @@ def test_more_water_heating_combos_decode_to_their_codes(
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assert simulation.heating.water_heating_fuel == fuel
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def test_from_main_wood_logs_decodes_to_the_wood_fuel_not_house_coal() -> None:
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# Act
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simulation = water_heating_overlay_for("From main system, wood logs", 0)
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# Assert — wood logs is RdSAP water_heating_fuel 6, from main system (901).
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# A wood-fired dwelling's DHW must not fall to house coal (33), which scores
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# ~14x the carbon (ADR-0043).
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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.water_heating_code == 901
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assert simulation.heating.water_heating_fuel == 6
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@pytest.mark.parametrize("water_heating_value", ["Unknown", ""])
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def test_unresolvable_water_heating_produces_no_overlay(
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water_heating_value: str,
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