diff --git a/domain/epc/property_overlays/main_heating_system_overlay.py b/domain/epc/property_overlays/main_heating_system_overlay.py
index 295a7e18f..2053423b9 100644
--- a/domain/epc/property_overlays/main_heating_system_overlay.py
+++ b/domain/epc/property_overlays/main_heating_system_overlay.py
@@ -137,6 +137,15 @@ _ELECTRIC_BOILER_CODES = frozenset({191, 192})
 # HHRSH pathway, and an inherited 7 (storage) made the Table 12a resolver bill
 # direct-acting heat wholly at the off-peak low rate (ADR-0052, #1444).
 _ELECTRIC_BOILER_CATEGORY = 2
+# SAP Table 4e Group 1 boiler control: 2101 ("no time or thermostatic control
+# of room temperature", +0.6 C) is the conservative default when the landlord
+# named only the system — the largest Group 1 mean-internal-temperature
+# adjustment, so an unobserved control is never over-credited (the boiler
+# mirror of storage 2401 / room-heater 2601 / underfloor 2701). Deliberately
+# NOT the gas boilers' modern 2106: the gas archetypes assume a whole modern
+# condensing install; the electric-boiler archetypes describe an existing
+# system of unknown vintage (ADR-0052).
+_CONSERVATIVE_BOILER_CONTROL = 2101
 # Electric underfloor — SAP Table 4a 421/422 (off-peak) and 424 (screed) — also
 # unambiguously electric (ADR-0046).
 _ELECTRIC_UNDERFLOOR_CODES = frozenset({421, 422, 424})
@@ -281,8 +290,9 @@ def _meter_for(code: int) -> Optional[str]:
 def _control_for(code: int) -> Optional[int]:
     """The control to assume when the landlord named only the system: a
     conservative manual charge control for storage heaters, full modern controls
-    for a gas boiler, None for systems that take neither (direct-acting electric).
-    Overwrites a stale control inherited from the system being replaced."""
+    for a gas boiler, the conservative in-group default for the other mapped
+    families (room heaters, underfloor, electric boilers). Overwrites a stale
+    control inherited from the system being replaced."""
     if code == _HHR_STORAGE_CODE:
         return _HHR_CHARGE_CONTROL
     if code in _STORAGE_HEATER_CODES:
@@ -293,6 +303,8 @@ def _control_for(code: int) -> Optional[int]:
         return _ROOM_HEATER_CONTROL
     if code in _ELECTRIC_UNDERFLOOR_CODES:
         return _UNDERFLOOR_CONTROL
+    if code in _ELECTRIC_BOILER_CODES:
+        return _CONSERVATIVE_BOILER_CONTROL
     return None