diff --git a/domain/epc/property_overlays/main_heating_system_overlay.py b/domain/epc/property_overlays/main_heating_system_overlay.py
index c1c9f2cc..ded78999 100644
--- a/domain/epc/property_overlays/main_heating_system_overlay.py
+++ b/domain/epc/property_overlays/main_heating_system_overlay.py
@@ -12,10 +12,11 @@ field-wise with the main_fuel / water_heating overlays.
 electricity tariff (meter) and, for storage heaters, its charge control. Rather
 than hand-attach those per archetype (easy to forget when a new system is
 added), they are **derived from the SAP code**: the off-peak meter from the
-calculator's single off-peak classification (`OFF_PEAK_IMPLYING_HEATING_CODES`,
-SAP §12), and the conservative manual charge control for storage heaters. So
-adding a heating archetype is just adding its code — coherent companions fall
-out. Synthesis owns coherence; the calculator never normalises a lodged cert.
+overlay's assumed-Dual classification (`_ASSUMED_DUAL_METER_CODES` — the §12
+off-peak systems plus all-electric room-heater dwellings), and the conservative
+manual charge control for storage heaters. So adding a heating archetype is just
+adding its code — coherent companions fall out. Synthesis owns coherence; the
+calculator never normalises a lodged cert.
 
 The SEDBUK A-G efficiency band the Hyde "Heating" column carries is NOT honoured
 yet (no efficiency slot on the overlay/MainHeatingDetail) -- archetypes map to
@@ -45,6 +46,19 @@ _OFF_PEAK_METER = "Dual"
 # split (the mirror of the storage→Dual drag, ADR-0035).
 _SINGLE_RATE_METER = "Single"
 
+# Electric room heaters (SAP Table 4a 691). They don't *require* off-peak the way
+# storage/CPSU do, so they're absent from the calculator's §12
+# `OFF_PEAK_IMPLYING_HEATING_CODES` (Rules 1-2). But a dwelling heated by them is
+# all-electric and realistically billed on Economy 7 — its immersion hot water
+# charges overnight and §12 Rule 3 gives the room heaters a 10-hour off-peak
+# window. So when the landlord names only the system, the coherent meter to
+# assume is Dual; the §12 dispatch then applies the realistic high/low split
+# (not a single-rate over-penalty, nor an all-low over-credit).
+_ROOM_HEATER_CODES = frozenset({691})
+# Codes for which the overlay assumes a Dual (off-peak) meter: the §12-mandated
+# off-peak systems plus the all-electric room-heater dwellings above.
+_ASSUMED_DUAL_METER_CODES = OFF_PEAK_IMPLYING_HEATING_CODES | _ROOM_HEATER_CODES
+
 # SAP Table 4e Group 4 storage charge-control code. Manual charge control is the
 # *conservative* assumption when the landlord didn't tell us the control: its
 # +0.7 C mean-internal-temperature adjustment is the largest of the storage
@@ -100,9 +114,10 @@ _MAIN_HEATING_CODES: dict[str, int] = {
 
 def _meter_for(code: int) -> str:
     """The coherent meter a heating code implies: an off-peak ("Dual") meter for
-    the calculator's §12 off-peak systems, an explicit single-rate ("Single")
-    meter for every other system. Always set — never left to bleed."""
-    return _OFF_PEAK_METER if code in OFF_PEAK_IMPLYING_HEATING_CODES else _SINGLE_RATE_METER
+    the §12 off-peak systems and all-electric room-heater dwellings
+    (`_ASSUMED_DUAL_METER_CODES`), an explicit single-rate ("Single") meter for
+    every other system. Always set — never left to bleed."""
+    return _OFF_PEAK_METER if code in _ASSUMED_DUAL_METER_CODES else _SINGLE_RATE_METER
 
 
 def _control_for(code: int) -> Optional[int]:
diff --git a/tests/domain/epc/test_main_heating_system_overlay.py b/tests/domain/epc/test_main_heating_system_overlay.py
index c85f1f38..7abccefd 100644
--- a/tests/domain/epc/test_main_heating_system_overlay.py
+++ b/tests/domain/epc/test_main_heating_system_overlay.py
@@ -11,12 +11,10 @@ import pytest
 from domain.epc.property_overrides.main_heating_system_type import MainHeatingSystemType
 from domain.epc.property_overlays.main_fuel_overlay import fuel_overlay_for
 from domain.epc.property_overlays.main_heating_system_overlay import (
+    _ASSUMED_DUAL_METER_CODES,
     _MAIN_HEATING_CODES,
     main_heating_overlay_for,
 )
-from domain.sap10_calculator.tables.table_12a import (
-    OFF_PEAK_IMPLYING_HEATING_CODES,
-)
 from domain.epc.property_overlays.water_heating_overlay import (
     water_heating_overlay_for,
 )
@@ -241,7 +239,7 @@ def test_off_peak_archetypes_drag_dual_others_drag_single() -> None:
     for value, code in _MAIN_HEATING_CODES.items():
         simulation = main_heating_overlay_for(value, 0)
         assert simulation is not None and simulation.heating is not None
-        expected = "Dual" if code in OFF_PEAK_IMPLYING_HEATING_CODES else "Single"
+        expected = "Dual" if code in _ASSUMED_DUAL_METER_CODES else "Single"
         assert simulation.heating.meter_type == expected, value