diff --git a/domain/epc/property_overrides/water_heating_guard.py b/domain/epc/property_overrides/water_heating_guard.py
index 4b447ff6e..1970f6bfd 100644
--- a/domain/epc/property_overrides/water_heating_guard.py
+++ b/domain/epc/property_overrides/water_heating_guard.py
@@ -6,4 +6,36 @@ from domain.epc.property_overrides.water_heating_type import WaterHeatingType
 
 
 def water_heating_guard(description: str) -> Optional[WaterHeatingType]:
-    raise NotImplementedError
+    """Deterministically resolve the structured water-heating descriptions the LLM
+    funnelled into the ``"From main system, house coal"`` dumping ground (#1376).
+
+    Landlord water-heating descriptions arrive as ``"<system>: <fuel>"`` (e.g.
+    ``"From main heating system: Wood Logs"``). Where the taxonomy had no biomass /
+    wood / dual-fuel / biodiesel water fuel, the classifier force-picked house coal,
+    scoring a biomass dwelling's DHW at ~14x its carbon (ADR-0043). This guard keys
+    on the **fuel token** and returns the faithful archetype — the ``"Hot-water only
+    community scheme"`` system flattens to ``"From main system, …"`` exactly as the
+    community mains-gas description already does. ``"No Heating or Hot Water"`` under
+    an ``"electric immersion assumed"`` system is the assessment's own immersion
+    assumption, so it resolves to ``ELECTRIC_IMMERSION``.
+
+    Returns ``None`` for fuels the LLM already classifies correctly (mains gas,
+    electricity, oil, …) and for varied / unstructured phrasings, so those still
+    reach the LLM classifier via ``GuardedColumnClassifier``. Order matters:
+    biodiesel and dual fuel are tested before the bare biomass / wood tokens they
+    contain.
+    """
+    text = description.lower()
+    if "biodiesel" in text:
+        return WaterHeatingType.FROM_MAIN_BIODIESEL_COMMUNITY
+    if "biomass" in text:
+        return WaterHeatingType.FROM_MAIN_BIOMASS_COMMUNITY
+    if "dual fuel" in text:
+        return WaterHeatingType.FROM_MAIN_DUAL_FUEL_MINERAL_WOOD
+    if "wood log" in text:
+        return WaterHeatingType.FROM_MAIN_WOOD_LOGS
+    if "house coal" in text:
+        return WaterHeatingType.FROM_MAIN_HOUSE_COAL
+    if "no heating or hot water" in text:
+        return WaterHeatingType.ELECTRIC_IMMERSION
+    return None