diff --git a/applications/landlord_description_overrides/handler.py b/applications/landlord_description_overrides/handler.py
index bc5fe8ee9..6d87490e9 100644
--- a/applications/landlord_description_overrides/handler.py
+++ b/applications/landlord_description_overrides/handler.py
@@ -13,6 +13,7 @@ from domain.epc.property_overrides.glazing_type import GlazingType
 from domain.epc.property_overrides.glazing_mix_guard import glazing_mix_guard
 from domain.epc.property_overrides.main_fuel_type import MainFuelType
 from domain.epc.property_overrides.main_heating_system_type import MainHeatingSystemType
+from domain.epc.property_overrides.main_heating_guard import main_heating_guard
 from domain.epc.property_overrides.property_type import PropertyType
 from domain.epc.property_overrides.roof_type import RoofType
 from domain.epc.property_overrides.roof_party_ceiling_guard import (
@@ -193,8 +194,15 @@ def _build_columns(
         "main_heating_system": lambda src: ClassifiableColumn(
             name="main_heating_system",
             source_column=src,
-            classifier=ChatGptColumnClassifier(
-                chat_gpt, MainHeatingSystemType, MainHeatingSystemType.UNKNOWN
+            # High heat retention storage heaters have no LLM target distinct from
+            # old storage, so they were funnelled into "Electric storage heaters,
+            # old" (401); the deterministic guard resolves HHRSH to its own
+            # archetype (SAP 409) and the LLM handles the rest (#1376, ADR-0044).
+            classifier=GuardedColumnClassifier(
+                guard=main_heating_guard,
+                fallback=ChatGptColumnClassifier(
+                    chat_gpt, MainHeatingSystemType, MainHeatingSystemType.UNKNOWN
+                ),
             ),
             repo=LandlordOverridesRepository[MainHeatingSystemType](
                 session, LandlordMainHeatingSystemOverrideRow
diff --git a/docs/adr/0044-hhrsh-baseline-archetype-drags-its-intrinsic-controls.md b/docs/adr/0044-hhrsh-baseline-archetype-drags-its-intrinsic-controls.md
new file mode 100644
index 000000000..354844e20
--- /dev/null
+++ b/docs/adr/0044-hhrsh-baseline-archetype-drags-its-intrinsic-controls.md
@@ -0,0 +1,94 @@
+# A baseline HHRSH override drags its intrinsic SAP-409 controls, not the generic manual storage default
+
+## Status
+
+accepted
+
+## Context
+
+A **Landlord Override** naming *"High heat retention storage heaters"* (HHRSH) had
+no home in the heating taxonomy: `MainHeatingSystemType` carried
+`ELECTRIC_STORAGE_OLD` (SAP 401), `_SLIMLINE` (402), `_CONVECTOR` (403) and
+`_FAN` (404), but no High-Heat-Retention member. So the LLM classifier funnelled
+the **best modern storage** into **`"Electric storage heaters, old"` (401)** — the
+**worst** large-volume type. An audit found **111** `property_overrides` rows on
+that mapping (`'Electric Storage Systems: High heat retention storage heaters'` →
+`'Electric storage heaters, old'`). Same missing-archetype defect as ADR-0041.
+
+HHRSH is a distinct, modellable RdSAP system: **SAP Table 4a code 409** (high heat
+retention storage heaters), which the calculator already scores
+(`sap_efficiencies` carries `409: 1.00`). The **HHRSH *measure*** already installs
+this exact code (`_HHR_STORAGE_SAP_CODE = 409`,
+`domain/modelling/generators/heating_recommendation.py`) on a **Dual** off-peak
+meter with **control 2404**.
+
+The subtlety is the **charge control**. `main_heating_system_overlay._control_for`
+drags the **conservative manual** charge control (`2401`) for *every* storage code
+401–409, following ADR-0041's amendment ("an *unobserved* storage charge control
+defaults to the conservative manual one"). Table 4e Group 4's mean-internal-
+temperature adjustments are **manual +0.7 °C, automatic/Celect +0.4 °C, HHR 0 °C**
+— so a manual default carries a penalty. Applying it to a 409 heater would swap
+"scored as old-storage 401" for "scored as 409-but-penalised" — still under-
+crediting the 111 dwellings, and pairing 409 with manual control is not a
+combination RdSAP produces.
+
+**HHRSH has a single, intrinsic charge control (2404).** The control is therefore
+*named by the archetype*, not *unobserved* — unlike a generic storage heater whose
+control genuinely is unknown. So ADR-0041's manual-default rule does not apply to
+409.
+
+## Decision
+
+Add the baseline HHRSH archetype and let it drag its **coherent, intrinsic**
+companions — mirroring how a gas-boiler archetype drags full modern controls
+(`_control_for`) rather than a conservative default.
+
+1. **`MainHeatingSystemType.ELECTRIC_STORAGE_HIGH_HEAT_RETENTION` =
+   `"Electric storage heaters, high heat retention"`**, mapped in
+   `_MAIN_HEATING_CODES` to **SAP Table 4a code 409**.
+2. **Control 2404 (HHRSH controls), not the manual default.** `_control_for`
+   special-cases 409 → `2404` ahead of the generic storage → `2401`. HHRSH's single
+   intrinsic control is archetype-implied, an **exception to ADR-0041's
+   unobserved-storage manual default** (which stands for 401–404).
+3. **Electricity fuel (RdSAP 29).** `_natural_fuel_for` drags electricity for 409,
+   per ADR-0041's "storage → electricity" — so a mis-lodged non-electric fuel can't
+   leave an electric-storage dwelling incoherent. Scoped to 409 ("scoped per family
+   as archetypes land"); the pre-existing 401–404 fuel-drag gap is not touched here.
+4. **Dual/off-peak meter falls out of the code** — 409 is already in
+   `_ASSUMED_DUAL_METER_CODES`, so the off-peak meter drags automatically (ADR-0035
+   coherent companions).
+5. **Deterministic guard + reclassify, TEXT-first / pgEnum-deferred.** A
+   `main_heating_guard` recognises the structured `"Electric Storage Systems: High
+   heat retention storage heaters"` phrasing (wired via `GuardedColumnClassifier`);
+   `scripts/reclassify_hhrsh_storage.py` reuses the same guard as its decision
+   function, updates `property_overrides.override_value` (TEXT) unconditionally, and
+   defers the `landlord_main_heating_system_overrides.value` pgEnum-cache write for
+   the new value.
+
+## Consequences
+
+- The 111 HHRSH dwellings score as **409 + HHR control 2404 + off-peak meter +
+  electricity** — the efficient modern archetype — instead of the worst old
+  large-volume 401. Correct Rebaselining (ADR-0039).
+- **FE-owned pgEnum addition (Dan):** one new `main_heating_system` value,
+  `"Electric storage heaters, high heat retention"` (memory
+  `main-heating-system-pgenum-is-fe-owned`). TEXT read path is fixed on merge; the
+  cache write defers until the migration lands.
+- **Baseline vs measure stay distinct but consistent:** both use code 409 +
+  control 2404; the measure additionally installs a designed HW cylinder / immersion
+  end-state, which a baseline (existing-system) override does not.
+- Extends [ADR-0041](0041-landlord-heating-classification-targets-a-complete-modellable-taxonomy.md)
+  (complete taxonomy; **refines** its manual-default amendment for the single-
+  control HHRSH case) and [ADR-0035](0035-coherent-heating-system-synthesis.md)
+  (companions drag from the code).
+
+### Alternatives rejected
+
+- **409 + manual control (2401), holding to ADR-0041's default.** Rejected: HHRSH's
+  control is intrinsic (single option), not unobserved; the +0.7 °C manual penalty
+  under-credits, and 409+manual is not a real RdSAP combination.
+- **Reuse an existing storage archetype (401–404).** Rejected: none carries the
+  HHR retention/control — the whole point of the mis-score.
+- **Leave fuel unresolved (as 401–404 do).** Rejected for 409: an explicit
+  electricity drag keeps the system coherent if the lodged fuel is wrong; the older
+  archetypes' gap is a separate, out-of-scope cleanup.
diff --git a/domain/epc/property_overlays/main_heating_system_overlay.py b/domain/epc/property_overlays/main_heating_system_overlay.py
index d14f9e79c..7d34546ce 100644
--- a/domain/epc/property_overlays/main_heating_system_overlay.py
+++ b/domain/epc/property_overlays/main_heating_system_overlay.py
@@ -67,6 +67,12 @@ _ASSUMED_DUAL_METER_CODES = OFF_PEAK_IMPLYING_HEATING_CODES | _ROOM_HEATER_CODES
 # systems that take a charge control.
 _MANUAL_CHARGE_CONTROL = 2401
 _STORAGE_HEATER_CODES = frozenset(range(401, 410))
+# High heat retention storage heaters (SAP Table 4a 409) take a SINGLE intrinsic
+# charge control — Table 4e 2404 (HHR, 0 C adjustment). It is named by the
+# archetype, not unobserved, so 409 drags 2404 rather than the manual default
+# (ADR-0044) — the only storage code that overrides `_MANUAL_CHARGE_CONTROL`.
+_HHR_STORAGE_CODE = 409
+_HHR_CHARGE_CONTROL = 2404
 
 # SAP Table 4a category 10 ("Room heaters") and its conservative Table 4e Group 6
 # control. A landlord names a room heater, not its control; code 2601 ("no
@@ -162,6 +168,10 @@ _MAIN_HEATING_CODES: dict[str, int] = {
     "Electric storage heaters, slimline": 402,
     "Electric storage heaters, convector": 403,
     "Electric storage heaters, fan": 404,
+    # High heat retention storage heaters — SAP Table 4a 409. Its intrinsic HHR
+    # charge control (2404) and electricity fuel drag from the code below, not the
+    # generic manual-storage default (ADR-0044).
+    "Electric storage heaters, high heat retention": 409,
     "Direct-acting electric": 191,
     "Electric room heaters": 691,
     "Solid fuel room heater, closed": 633,
@@ -203,6 +213,8 @@ def _control_for(code: int) -> Optional[int]:
     conservative manual charge control for storage heaters, full modern controls
     for a gas boiler, None for systems that take neither (direct-acting electric).
     Overwrites a stale control inherited from the system being replaced."""
+    if code == _HHR_STORAGE_CODE:
+        return _HHR_CHARGE_CONTROL
     if code in _STORAGE_HEATER_CODES:
         return _MANUAL_CHARGE_CONTROL
     if code in _GAS_BOILER_CODES:
@@ -229,7 +241,11 @@ def _natural_fuel_for(code: int) -> Optional[int]:
     """The fuel an archetype unambiguously implies (a coherent default), or None
     where the fuel is ambiguous and must come from the `main_fuel` override. A
     present `main_fuel` override wins by last-wins composition."""
-    if code in _ELECTRIC_ROOM_HEATER_CODES or code in _HEAT_PUMP_CODES:
+    if (
+        code in _ELECTRIC_ROOM_HEATER_CODES
+        or code in _HEAT_PUMP_CODES
+        or code == _HHR_STORAGE_CODE
+    ):
         return _ELECTRICITY_FUEL
     if code in _SOLID_FUEL_ROOM_HEATER_CODES:
         return _HOUSE_COAL_FUEL
diff --git a/domain/epc/property_overrides/main_heating_guard.py b/domain/epc/property_overrides/main_heating_guard.py
new file mode 100644
index 000000000..9eb50548b
--- /dev/null
+++ b/domain/epc/property_overrides/main_heating_guard.py
@@ -0,0 +1,24 @@
+from __future__ import annotations
+
+from typing import Optional
+
+from domain.epc.property_overrides.main_heating_system_type import (
+    MainHeatingSystemType,
+)
+
+
+def main_heating_guard(description: str) -> Optional[MainHeatingSystemType]:
+    """Deterministically resolve a High Heat Retention Storage Heater (HHRSH)
+    description, which the LLM funnelled into ``"Electric storage heaters, old"``
+    (SAP 401) for want of an HHRSH archetype (#1376 / ADR-0044).
+
+    HHRSH is a distinct RdSAP system (SAP Table 4a 409) with its own intrinsic
+    charge control (2404) — the *best* modern storage, not the *worst* old large-
+    volume type. This guard claims only the structured HHRSH phrasing and returns
+    ``None`` for every other description (the remaining storage subtypes the LLM
+    already classifies correctly, and varied phrasings), so they still reach the
+    LLM classifier via ``GuardedColumnClassifier``.
+    """
+    if "high heat retention" in description.lower():
+        return MainHeatingSystemType.ELECTRIC_STORAGE_HIGH_HEAT_RETENTION
+    return None
diff --git a/domain/epc/property_overrides/main_heating_system_type.py b/domain/epc/property_overrides/main_heating_system_type.py
index d79fec1d1..3d74b93c0 100644
--- a/domain/epc/property_overrides/main_heating_system_type.py
+++ b/domain/epc/property_overrides/main_heating_system_type.py
@@ -23,6 +23,9 @@ class MainHeatingSystemType(Enum):
     ELECTRIC_STORAGE_SLIMLINE = "Electric storage heaters, slimline"
     ELECTRIC_STORAGE_CONVECTOR = "Electric storage heaters, convector"
     ELECTRIC_STORAGE_FAN = "Electric storage heaters, fan"
+    ELECTRIC_STORAGE_HIGH_HEAT_RETENTION = (
+        "Electric storage heaters, high heat retention"
+    )
     DIRECT_ELECTRIC = "Direct-acting electric"
     ELECTRIC_ROOM_HEATERS = "Electric room heaters"
     SOLID_FUEL_ROOM_HEATER_CLOSED = "Solid fuel room heater, closed"
diff --git a/scripts/reclassify_hhrsh_storage.py b/scripts/reclassify_hhrsh_storage.py
new file mode 100644
index 000000000..952b205b7
--- /dev/null
+++ b/scripts/reclassify_hhrsh_storage.py
@@ -0,0 +1,145 @@
+"""One-time re-classification of HHRSH overrides funnelled into old storage.
+
+#1376 / ADR-0044: with no High Heat Retention archetype, the LLM classified
+"Electric Storage Systems: High heat retention storage heaters" to
+``"Electric storage heaters, old"`` (SAP 401) — scoring the *best* modern storage
+as the *worst* old large-volume type. An audit found 111 such rows.
+
+The live classifier now applies ``main_heating_guard`` deterministically (so new
+intakes are correct); this fixes the rows written before it. The **same guard**
+decides the correction here, so the backfill and the live path cannot drift.
+
+Updates the TEXT ``property_overrides.override_value`` (what the modelling reads —
+the immediate fix) unconditionally. The ``landlord_main_heating_system_overrides``
+``value`` classifier cache is a ``main_heating_system`` **pgEnum**; the new HHRSH
+archetype is an FE-owned value, so its cache write is **deferred** until the
+Drizzle migration adds it (the Class-A/B pattern).
+
+DRY-RUN BY DEFAULT: prints the counts it would change and writes nothing. Pass
+``--apply`` to execute inside a transaction. Idempotent — only rows whose stored
+value differs from the target member are touched.
+"""
+
+from __future__ import annotations
+
+import argparse
+from collections.abc import Iterable
+
+from sqlalchemy import Connection, text
+
+from domain.epc.property_overrides.main_heating_guard import main_heating_guard
+from scripts.e2e_common import build_engine, load_env
+
+
+def hhrsh_storage_corrections(
+    stored: Iterable[tuple[str, str]],
+) -> dict[str, str]:
+    """``(description, stored override_value)`` → the HHRSH archetype value, for the
+    descriptions the main-heating guard resolves to HHRSH whose stored value is not
+    already it. Descriptions the guard defers and rows already on the target are
+    omitted, so re-running against corrected data is a no-op."""
+    corrections: dict[str, str] = {}
+    for description, value in stored:
+        member = main_heating_guard(description)
+        if member is not None and value != member.value:
+            corrections[description] = member.value
+    return corrections
+
+
+_DISTINCT = text(
+    """
+    SELECT DISTINCT lower(original_spreadsheet_description) AS description,
+                    override_value AS value
+    FROM property_overrides
+    WHERE override_component = 'main_heating_system'
+    """
+)
+_OVERRIDES_UPDATE = text(
+    """
+    UPDATE property_overrides
+       SET override_value = :new_value
+     WHERE override_component = 'main_heating_system'
+       AND lower(original_spreadsheet_description) = :description
+       AND override_value <> :new_value
+    """
+)
+_OVERRIDES_COUNT = text(
+    """
+    SELECT count(*) FROM property_overrides
+     WHERE override_component = 'main_heating_system'
+       AND lower(original_spreadsheet_description) = :description
+       AND override_value <> :new_value
+    """
+)
+_CACHE_UPDATE = text(
+    """
+    UPDATE landlord_main_heating_system_overrides
+       SET value = :new_value, updated_at = now()
+     WHERE lower(description) = :description
+       AND value::text <> :new_value
+    """
+)
+_ENUM_VALUES = text(
+    "SELECT e.enumlabel FROM pg_enum e JOIN pg_type t ON t.oid = e.enumtypid "
+    "WHERE t.typname = 'main_heating_system'"
+)
+
+
+def reclassify(conn: Connection, *, apply: bool) -> tuple[int, set[str]]:
+    """Re-map old-storage HHRSH overrides onto the high-heat-retention archetype.
+    Returns the number of ``property_overrides`` rows found and the set of target
+    values the live ``main_heating_system`` pgEnum does not yet carry (cache-
+    deferred until the FE migration)."""
+    stored = [(r.description, r.value) for r in conn.execute(_DISTINCT)]
+    enum_values = {r[0] for r in conn.execute(_ENUM_VALUES)}
+    total = 0
+    deferred: set[str] = set()
+    for description, new_value in hhrsh_storage_corrections(stored).items():
+        params = {"description": description, "new_value": new_value}
+        total += conn.execute(_OVERRIDES_COUNT, params).scalar() or 0
+        in_enum = new_value in enum_values
+        if not in_enum:
+            deferred.add(new_value)
+        if apply:
+            conn.execute(_OVERRIDES_UPDATE, params)
+            if in_enum:
+                conn.execute(_CACHE_UPDATE, params)
+    return total, deferred
+
+
+def main() -> None:
+    load_env()
+    parser = argparse.ArgumentParser(description=__doc__)
+    parser.add_argument(
+        "--apply",
+        action="store_true",
+        help="execute the updates (default: dry-run, writes nothing)",
+    )
+    args = parser.parse_args()
+
+    engine = build_engine()
+    with engine.begin() as conn:
+        conn.execute(text("SET statement_timeout = 120000"))
+        total, deferred = reclassify(conn, apply=args.apply)
+
+    verb = "re-classified" if args.apply else "would re-classify"
+    print(
+        f"{verb} {total} high-heat-retention storage override row(s) off "
+        "'Electric storage heaters, old' onto the HHRSH archetype (SAP 409) — "
+        "property_overrides / TEXT, what the modelling reads."
+    )
+    if deferred:
+        print(
+            f"\n{len(deferred)} target value(s) NOT yet in the main_heating_system "
+            "pgEnum — their classifier-cache rows are deferred until the FE-repo "
+            "enum migration adds these members (property_overrides was still "
+            "updated, which is what the modelling reads):"
+        )
+        for value in sorted(deferred):
+            print(f"    {value!r}")
+    if not args.apply:
+        print("\nDRY-RUN — nothing written. Re-run with --apply to execute.")
+
+
+if __name__ == "__main__":
+    main()
diff --git a/tests/domain/epc/test_main_heating_guard.py b/tests/domain/epc/test_main_heating_guard.py
new file mode 100644
index 000000000..452474d61
--- /dev/null
+++ b/tests/domain/epc/test_main_heating_guard.py
@@ -0,0 +1,40 @@
+from __future__ import annotations
+
+import pytest
+
+from domain.epc.property_overrides.main_heating_guard import main_heating_guard
+from domain.epc.property_overrides.main_heating_system_type import (
+    MainHeatingSystemType,
+)
+
+
+def test_guard_resolves_high_heat_retention_storage() -> None:
+    # Arrange — the structured phrasing the LLM funnelled into "old storage" (401).
+    description = "Electric Storage Systems: High heat retention storage heaters"
+
+    # Act
+    result = main_heating_guard(description)
+
+    # Assert — HHRSH has its own archetype (SAP 409), never old storage (ADR-0044).
+    assert result is MainHeatingSystemType.ELECTRIC_STORAGE_HIGH_HEAT_RETENTION
+
+
+@pytest.mark.parametrize(
+    "description",
+    [
+        # Other storage subtypes the LLM already classifies correctly — the guard
+        # only rescues the HHRSH gap, leaving the rest to the fallback classifier.
+        "Electric Storage Systems: Old (large volume) storage heaters",
+        "Electric Storage Systems: Modern (slimline) storage heaters",
+        "Electric Storage Systems: Fan storage heaters",
+        # Unrelated / varied phrasings are the LLM's job.
+        "Gas boiler",
+        "",
+    ],
+)
+def test_guard_defers_non_hhrsh_descriptions_to_the_llm(description: str) -> None:
+    # Act
+    result = main_heating_guard(description)
+
+    # Assert
+    assert result is None
diff --git a/tests/domain/epc/test_main_heating_system_overlay.py b/tests/domain/epc/test_main_heating_system_overlay.py
index 6e68cce03..b100f75a6 100644
--- a/tests/domain/epc/test_main_heating_system_overlay.py
+++ b/tests/domain/epc/test_main_heating_system_overlay.py
@@ -98,6 +98,35 @@ def test_storage_heater_subtypes_decode_to_their_codes(
     assert simulation.heating.sap_main_heating_code == code
 
 
+def test_high_heat_retention_storage_decodes_to_its_own_code_not_old_storage() -> None:
+    # Act
+    simulation = main_heating_overlay_for(
+        "Electric storage heaters, high heat retention", 0
+    )
+
+    # Assert — HHRSH is SAP Table 4a code 409, NOT old large-volume storage (401)
+    # the classifier funnelled it into (ADR-0044).
+    assert simulation is not None
+    assert simulation.heating is not None
+    assert simulation.heating.sap_main_heating_code == 409
+
+
+def test_high_heat_retention_storage_drags_its_intrinsic_companions() -> None:
+    # HHRSH has a single intrinsic charge control (Table 4e 2404, 0 C adjustment),
+    # so — unlike a generic storage heater whose control is unobserved — it must
+    # NOT drag the conservative manual default (2401, +0.7 C penalty), which would
+    # under-credit it. It is electric, on an off-peak (Dual) meter (ADR-0044).
+    simulation = main_heating_overlay_for(
+        "Electric storage heaters, high heat retention", 0
+    )
+
+    assert simulation is not None
+    assert simulation.heating is not None
+    assert simulation.heating.main_heating_control == 2404
+    assert simulation.heating.main_fuel_type == 29
+    assert simulation.heating.meter_type == "Dual"
+
+
 @pytest.mark.parametrize(
     "main_heating_value",
     [
diff --git a/tests/scripts/test_reclassify_hhrsh_storage.py b/tests/scripts/test_reclassify_hhrsh_storage.py
new file mode 100644
index 000000000..c3b025c53
--- /dev/null
+++ b/tests/scripts/test_reclassify_hhrsh_storage.py
@@ -0,0 +1,70 @@
+"""The HHRSH reclassify maps the "old storage" dumping ground onto the high-heat-
+retention archetype, leaves other storage subtypes alone, and is idempotent
+(#1376 / ADR-0044)."""
+
+from __future__ import annotations
+
+from domain.epc.property_overlays.main_heating_system_overlay import (
+    main_heating_overlay_for,
+)
+from domain.epc.property_overrides.main_heating_guard import main_heating_guard
+from domain.epc.property_overrides.main_heating_system_type import (
+    MainHeatingSystemType,
+)
+from scripts.reclassify_hhrsh_storage import hhrsh_storage_corrections
+
+
+def test_hhrsh_rows_are_remapped_off_old_storage() -> None:
+    # Arrange — HHRSH funnelled into old storage (the bug), plus a genuinely-old
+    # storage row and a gas row that must be left untouched.
+    stored = [
+        (
+            "electric storage systems: high heat retention storage heaters",
+            "Electric storage heaters, old",
+        ),
+        (
+            "electric storage systems: old (large volume) storage heaters",
+            "Electric storage heaters, old",
+        ),
+        ("gas boiler", "Gas boiler, regular"),
+    ]
+
+    # Act
+    corrections = hhrsh_storage_corrections(stored)
+
+    # Assert — only the HHRSH row moves, to its own archetype.
+    assert corrections == {
+        "electric storage systems: high heat retention storage heaters": (
+            "Electric storage heaters, high heat retention"
+        )
+    }
+
+
+def test_every_guard_target_round_trips_to_a_resolvable_archetype() -> None:
+    # The HHRSH archetype the guard emits must decode to a real SAP heating code —
+    # a guard/overlay drift can't silently write an unmodellable value (ADR-0044).
+    member = main_heating_guard(
+        "Electric Storage Systems: High heat retention storage heaters"
+    )
+
+    assert member is not None
+    assert member in MainHeatingSystemType
+    simulation = main_heating_overlay_for(member.value, 0)
+    assert simulation is not None
+    assert simulation.heating is not None
+    assert simulation.heating.sap_main_heating_code is not None
+
+
+def test_already_corrected_rows_need_no_change() -> None:
+    # Act / Assert — idempotent.
+    assert (
+        hhrsh_storage_corrections(
+            [
+                (
+                    "electric storage systems: high heat retention storage heaters",
+                    "Electric storage heaters, high heat retention",
+                )
+            ]
+        )
+        == {}
+    )