diff --git a/domain/epc/property_overlays/main_heating_system_overlay.py b/domain/epc/property_overlays/main_heating_system_overlay.py
index d14f9e79c..d93790a54 100644
--- a/domain/epc/property_overlays/main_heating_system_overlay.py
+++ b/domain/epc/property_overlays/main_heating_system_overlay.py
@@ -162,6 +162,10 @@ _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,
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"