diff --git a/domain/epc/property_overlays/main_heating_system_overlay.py b/domain/epc/property_overlays/main_heating_system_overlay.py
index 94d4947c..c1c9f2cc 100644
--- a/domain/epc/property_overlays/main_heating_system_overlay.py
+++ b/domain/epc/property_overlays/main_heating_system_overlay.py
@@ -80,9 +80,11 @@ _FROM_MAIN_WATER_HEATING_CODE = 901
 # Canonical system archetype → representative SAP `sap_main_heating_code`. Codes
 # map to the modern/condensing variant (A-G efficiency deferred): 102 regular
 # condensing, 104 condensing combi, 120 CPSU, 401-404 storage heaters, 191
-# direct-acting electric. Companion fields (meter / control / fuel / hot water)
-# are NOT listed here — they are derived from the code below, so a new archetype
-# is just a code (ADR-0035 drag-along).
+# direct-acting electric, 691 panel/convector/radiant electric room heaters
+# (Table 4a — direct-acting, so a single-rate meter, NOT off-peak storage).
+# Companion fields (meter / control / fuel / hot water) are NOT listed here —
+# they are derived from the code below, so a new archetype is just a code
+# (ADR-0035 drag-along).
 _MAIN_HEATING_CODES: dict[str, int] = {
     "Gas boiler, combi": 104,
     "Gas boiler, regular": 102,
@@ -92,6 +94,7 @@ _MAIN_HEATING_CODES: dict[str, int] = {
     "Electric storage heaters, convector": 403,
     "Electric storage heaters, fan": 404,
     "Direct-acting electric": 191,
+    "Electric room heaters": 691,
 }