Fix HHRSH storage heaters priced at peak rate (engine 48 vs Elmhurst 71)

The high-heat-retention storage-heater (HHRSH) recommendation was computed
as LOWERING SAP and wrongly rejected by the Optimiser, leaving electric-
room-heater flats/maisonettes stuck at band E with no sensible plan.

Root cause: `_HHR_STORAGE_OVERLAY` sets `sap_main_heating_code=409` (HHR
storage) + `meter_type="Dual"` but left `main_heating_category` at the
pre-existing value (10 = electric room heaters). The SAP 10.2 Table 12a
space-heating high-rate-fraction resolver classifies by category, so it
misread the off-peak storage heating as direct-acting electric (1.00
high-rate) and priced 100% at the 7-hour peak rate (15.29p) instead of the
storage off-peak split (0.00, ~5.5p).

Verified against accredited Elmhurst RdSAP-10 on UPRN 100020942571
(8 Booth Close, SE28 8BW): engine HHRSH SAP 48 (peak-priced, cost £1087)
vs Elmhurst 71 (cost ~£611); with the fix the engine gives 73 / £576.

Two-part fix (defence in depth, so no future overlay/mapper can
reintroduce it):
- `_HHR_STORAGE_OVERLAY` now sets `main_heating_category=7`, keeping the
  overlaid EPC internally consistent.
- `_table_12a_system_for_main` now classifies electric storage heaters by
  their Table 4a SAP code (401-409) as well as `category==7`, checked
  before the room-heater branch, so a specific storage code is
  authoritative over a stale/inconsistent category.

Tests: new regression `test_storage_heater_classified_by_sap_code_over_stale_category`;
updated the HHRSH overlay pin. Full accuracy corpus still passes (no regressions).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

domain/modelling/generators/heating_recommendation.py

@@ -59,6 +59,14 @@ _HEAT_PUMP_CATEGORY = 4
 _HHR_STORAGE_OVERLAY = HeatingOverlay(
     main_fuel_type=_ELECTRICITY_FUEL,
     sap_main_heating_code=_HHR_STORAGE_SAP_CODE,
+    # RdSAP electric-storage-heater category (7). Set so the overlaid EPC stays
+    # internally consistent: without it the pre-existing system's category (e.g.
+    # 10 = electric room heaters) survives alongside the new storage code 409,
+    # and the SAP 10.2 Table 12a resolver — which classifies by category —
+    # misreads the off-peak storage heating as direct-acting electric and prices
+    # 100% at the peak rate (verified: UPRN 100020942571 HHRSH SAP 48 vs
+    # accredited Elmhurst 71; category 7 -> 73). See ADR-0024.
+    main_heating_category=7,
     main_heating_control=2404,
     water_heating_code=903,
     water_heating_fuel=_ELECTRICITY_FUEL,

domain/sap10_calculator/rdsap/cert_to_inputs.py

@@ -2574,6 +2574,17 @@ _ELECTRIC_ROOM_HEATER_SAP_CODES: Final[frozenset[int]] = frozenset(
     {691, 692, 693, 694, 699}
 )
 
+# SAP 10.2 Table 4a electric STORAGE-heater codes (PDF p.169, codes 401-409:
+# old-large-volume / slimline / fan / integrated-storage-direct 408 / high-heat-
+# retention 409, etc.). Mirrors the storage subset of `table_12a._RULE_2_STORAGE_
+# CODES` (heaters only — excludes underfloor 421/422 and storage boilers 193/195,
+# which have their own Table 12a rows). Used to classify storage heaters by SAP
+# code as well as RdSAP `main_heating_category == 7`, so a storage-coded system
+# is priced off-peak even when its category is stale or inconsistent.
+_ELECTRIC_STORAGE_HEATER_SAP_CODES: Final[frozenset[int]] = frozenset(
+    range(401, 410)
+)
+
 # SAP 10.2 Table 4a electric boilers (PDF p.170, codes 191-196) → their
 # Table 12a Grid 1 SH rows (PDF p.191). NOTE the boiler families do NOT all
 # share a row — read the spec exactly:
@@ -2623,6 +2634,26 @@ def _table_12a_system_for_main(
         main.main_heating_index_number is not None
         and heat_pump_record(main.main_heating_index_number) is not None
     )
+    # Electric STORAGE heaters — SAP 10.2 Table 12a Grid 1 (PDF p.191).
+    # Detected by RdSAP main_heating_category 7 OR, authoritatively, by a Table
+    # 4a electric-storage-heater SAP code (401-409). Keyed on the CODE as well as
+    # the category, and checked BEFORE the room-heater branch below, so a storage
+    # system whose category is stale or inconsistent — e.g. a recommendation
+    # `HeatingOverlay` that sets `sap_main_heating_code=409` (HHR storage) but
+    # leaves the pre-existing electric-room-heater `main_heating_category=10` — is
+    # still priced off-peak here instead of being misclassified as direct-acting
+    # (1.00 high-rate → 100% peak) by the category-10 gate. A specific Table 4a
+    # code is more authoritative than the coarser category. Code 408 is an
+    # "Integrated (storage + direct-acting) system" → 0.20 SH high-rate fraction
+    # at 7-hour; every other storage code → "Other storage heaters" → 0.00
+    # (charged wholly off-peak). Gated on `_is_electric_main` belt-and-braces.
+    if _is_electric_main(main) and (
+        main.main_heating_category == 7
+        or (code is not None and code in _ELECTRIC_STORAGE_HEATER_SAP_CODES)
+    ):
+        if code == 408:
+            return Table12aSystem.INTEGRATED_STORAGE_DIRECT
+        return Table12aSystem.OTHER_STORAGE_HEATERS
     # Electric room heaters are direct-acting electric → SAP 10.2 Table
     # 12a Grid 1 (PDF p.191) "Other systems including direct-acting
     # electric" row (7-hour high-rate fraction 1.00, 10-hour 0.50).
@@ -2634,9 +2665,9 @@ def _table_12a_system_for_main(
     # main_heating_category 1, NOT 10, so the category-only gate missed it
     # and it fell through to None → 100% off-peak LOW rate, billing
     # direct-acting heaters as if they charged overnight like storage
-    # (cert 2958 +32.2 SAP, the worst over-rate in the sample). Distinct
-    # from electric STORAGE heaters (category 7), which DO charge off-peak
-    # and correctly fall through to None here (→ 100% low rate). Gated on
+    # (cert 2958 +32.2 SAP, the worst over-rate in the sample). Electric
+    # STORAGE heaters (category 7 / codes 401-409) are handled by the
+    # storage-heater branch above, so they never reach here. Gated on
     # `_is_electric_main` so a non-electric room heater (gas / solid-fuel
     # cat 10) is excluded; all callers already pre-gate on electric.
     if _is_electric_main(main) and (
@@ -2665,16 +2696,6 @@ def _table_12a_system_for_main(
         211 <= code <= 217 or 221 <= code <= 227 or 521 <= code <= 524
     ):
         return Table12aSystem.ASHP_OTHER
-    # Electric STORAGE heaters (RdSAP main_heating_category 7) — SAP 10.2
-    # Table 12a Grid 1 (PDF p.191). Code 408 is an "Integrated (storage +
-    # direct-acting) system" → 0.20 SH high-rate fraction at 7-hour; every
-    # other storage code is "Other storage heaters" → 0.00 (charged wholly
-    # off-peak, the same 100%-low-rate the None fallback already gave).
-    # Gated on `_is_electric_main` belt-and-braces (all callers pre-gate).
-    if main.main_heating_category == 7 and _is_electric_main(main):
-        if code == 408:
-            return Table12aSystem.INTEGRATED_STORAGE_DIRECT
-        return Table12aSystem.OTHER_STORAGE_HEATERS
     # Electric boilers (Table 4a codes 191-196) — resolve the misleading TODO
     # that lumped them as one "direct-acting" family. They split across THREE
     # distinct Table 12a Grid 1 rows (see `_DIRECT_ACTING_ELECTRIC_BOILER_CODES`

tests/domain/modelling/test_heating_recommendation.py

@@ -76,6 +76,12 @@ def test_electric_storage_dwelling_yields_an_hhr_storage_bundle() -> None:
     ].overlay.heating == HeatingOverlay(
         main_fuel_type=30,
         sap_main_heating_code=409,
+        # Storage category (7) pins the fix for the off-peak-pricing bug: the
+        # overlay must set it so the Table 12a resolver classifies the off-peak
+        # storage heating as storage (0.00 high-rate) and not direct-acting
+        # electric (1.00) via a stale category. See test_cert_to_inputs
+        # `test_storage_heater_classified_by_sap_code_over_stale_category`.
+        main_heating_category=7,
         main_heating_control=2404,
         water_heating_code=903,
         water_heating_fuel=30,

tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py

@@ -1054,6 +1054,55 @@ def test_electric_boilers_191_195_map_to_distinct_table_12a_grid1_rows() -> None
     assert space_heating_high_rate_fraction(storage, Tariff.SEVEN_HOUR) == 0.00
 
 
+def test_storage_heater_classified_by_sap_code_over_stale_category() -> None:
+    # Regression: electric STORAGE heaters must resolve to a storage Table 12a
+    # Grid 1 row from their Table 4a SAP code (401-409) even when the RdSAP
+    # `main_heating_category` is stale/inconsistent (== 10, electric room
+    # heaters). This is exactly the state the HHRSH recommendation overlay
+    # produces — it sets `sap_main_heating_code=409` on a dwelling that was
+    # electric room heaters (category 10) — and the category-10 room-heater
+    # branch used to win, mispricing the off-peak storage heating as
+    # direct-acting electric (1.00 high-rate → 100% peak). Verified real-cert
+    # impact: UPRN 100020942571 HHRSH SAP 48 (peak-priced) vs accredited
+    # Elmhurst 71; code-authoritative classification restores it to 73.
+    from domain.sap10_calculator.tables.table_12a import (
+        Table12aSystem,
+        Tariff,
+        space_heating_high_rate_fraction,
+    )
+
+    def _storage_main(code: int, category: int) -> MainHeatingDetail:
+        return MainHeatingDetail(
+            has_fghrs=False,
+            main_fuel_type=29,  # electricity
+            heat_emitter_type=0,
+            emitter_temperature=1,
+            main_heating_control=2404,  # HHRSH controls
+            main_heating_category=category,
+            sap_main_heating_code=code,
+        )
+
+    # HHR storage (409) with a stale room-heater category (10) — the overlay case.
+    hhr_stale = _table_12a_system_for_main(_storage_main(409, category=10))
+    assert hhr_stale is Table12aSystem.OTHER_STORAGE_HEATERS
+    assert space_heating_high_rate_fraction(hhr_stale, Tariff.SEVEN_HOUR) == 0.00
+
+    # Integrated storage/direct-acting (408) is still its own row, code-driven.
+    integrated = _table_12a_system_for_main(_storage_main(408, category=10))
+    assert integrated is Table12aSystem.INTEGRATED_STORAGE_DIRECT
+    assert space_heating_high_rate_fraction(integrated, Tariff.SEVEN_HOUR) == 0.20
+
+    # The canonical consistent case (category 7) is unchanged.
+    hhr_consistent = _table_12a_system_for_main(_storage_main(409, category=7))
+    assert hhr_consistent is Table12aSystem.OTHER_STORAGE_HEATERS
+
+    # A genuine electric room heater (code 691, category 10) is NOT swept into
+    # the storage branch — it stays direct-acting (1.00 high-rate at 7-hour).
+    room_heater = _table_12a_system_for_main(_storage_main(691, category=10))
+    assert room_heater is Table12aSystem.OTHER_DIRECT_ACTING_ELECTRIC
+    assert space_heating_high_rate_fraction(room_heater, Tariff.SEVEN_HOUR) == 1.00
+
+
 def test_heat_network_distribution_electricity_per_sap_10_2_appendix_c_3_2() -> None:
     # Arrange — heat-network main (Table 4a code 301 = community heating,
     # category 6). SAP 10.2 Appendix C §C3.2 (PDF p.51): distribution