diff --git a/domain/sap10_calculator/tables/table_12a.py b/domain/sap10_calculator/tables/table_12a.py
index f98e1c27..2ee7c331 100644
--- a/domain/sap10_calculator/tables/table_12a.py
+++ b/domain/sap10_calculator/tables/table_12a.py
@@ -250,13 +250,16 @@ _RULE_2_STORAGE_CODES: Final[frozenset[int]] = frozenset(
 # Rule 3: direct-acting electric + heat pumps + electric room heaters
 # → 10-hour. §12 lists "heat pump (211 to 224, 521 to 524, or
 # database)" — the "database" branch fires when the cert lodges a
-# PCDB Table 362 heat-pump index regardless of SAP code.
+# PCDB Table 362 heat-pump index regardless of SAP code. §12 also
+# names "electric room heaters" verbatim (RdSAP 10 PDF p.62) — Table 4a
+# electric room-heater codes 691 (panel/convector/radiant), 692 (fan),
+# 693 (portable), 694 (water-/oil-filled), 699 (assumed). Without these
+# a Dual-meter room-heater cert fell through to Rule 4 (7-hour default).
 _RULE_3_TEN_HOUR_CODES: Final[frozenset[int]] = frozenset(
     [191]                       # direct-acting electric boiler
     + list(range(211, 225))     # heat pumps 211-224
     + list(range(521, 525))     # warm-air heat pumps 521-524
-    # TODO: electric room heater codes (SAP Table 4a row 6xx for
-    # electric panel / radiant heaters) when a fixture surfaces them.
+    + [691, 692, 693, 694, 699]  # electric room heaters (Table 4a)
 )
 
 
diff --git a/tests/domain/sap10_calculator/test_table_12a.py b/tests/domain/sap10_calculator/test_table_12a.py
index 2b294ca2..a15cb2ed 100644
--- a/tests/domain/sap10_calculator/test_table_12a.py
+++ b/tests/domain/sap10_calculator/test_table_12a.py
@@ -17,12 +17,35 @@ from domain.sap10_calculator.tables.table_12a import (
     Table12aSystem,
     Tariff,
     other_use_high_rate_fraction,
+    rdsap_tariff_for_cert,
     space_heating_high_rate_fraction,
     tariff_from_meter_type,
     water_heating_high_rate_fraction,
 )
 
 
+def test_dual_meter_electric_room_heater_resolves_to_ten_hour_tariff() -> None:
+    # Arrange — RdSAP 10 §12 (PDF p.62) Dual-meter tariff dispatch: for a
+    # Dual meter the choice between 7-hour and 10-hour is made by the main
+    # heating type. Rule 3 verbatim: "if the main system ... is a direct-
+    # acting electric boiler (191), or electric room heaters ... it is
+    # 10-hour tariff." The electric room-heater codes are Table 4a 691
+    # (panel/convector/radiant), 692 (fan), 693 (portable), 694 (water-/
+    # oil-filled), 699 (assumed). Pre-slice these fell through to Rule 4
+    # (7-hour default), so a Dual-meter room-heater cert was billed at the
+    # 7-hour rates with Table 12a high-rate fraction 1.00 instead of the
+    # 10-hour rates with fraction 0.50 — over-charging direct-acting heat
+    # once S0380.230 routed it to OTHER_DIRECT_ACTING_ELECTRIC.
+
+    # Act / Assert — every electric room-heater code on a Dual meter → 10-hour.
+    for code in (691, 692, 693, 694, 699):
+        assert rdsap_tariff_for_cert(1, main_1_sap_code=code) is Tariff.TEN_HOUR
+    # Storage heaters (Rule 2) stay 7-hour; a gas room heater (non-Rule-3
+    # code) keeps the Rule 4 Dual default of 7-hour.
+    assert rdsap_tariff_for_cert(1, main_1_sap_code=401) is Tariff.SEVEN_HOUR
+    assert rdsap_tariff_for_cert(1, main_1_sap_code=601) is Tariff.SEVEN_HOUR
+
+
 def test_tariff_enum_has_five_members() -> None:
     """Table 12a columns: standard (no off-peak split), 7-hour, 10-hour,
     18-hour, 24-hour. Worksheet-shape fidelity: TEN_HOUR is included for