diff --git a/domain/epc/property_overrides/main_heating_guard.py b/domain/epc/property_overrides/main_heating_guard.py
index fe8e54126..9eb50548b 100644
--- a/domain/epc/property_overrides/main_heating_guard.py
+++ b/domain/epc/property_overrides/main_heating_guard.py
@@ -8,4 +8,17 @@ from domain.epc.property_overrides.main_heating_system_type import (
 
 
 def main_heating_guard(description: str) -> Optional[MainHeatingSystemType]:
-    raise NotImplementedError
+    """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