diff --git a/domain/epc/property_overlays/main_heating_system_overlay.py b/domain/epc/property_overlays/main_heating_system_overlay.py
index 9782174e3..d0c498320 100644
--- a/domain/epc/property_overlays/main_heating_system_overlay.py
+++ b/domain/epc/property_overlays/main_heating_system_overlay.py
@@ -28,6 +28,7 @@ are left UNKNOWN until modelled. Unresolvable values produce no overlay.
 
 from __future__ import annotations
 
+import logging
 from typing import Optional
 
 from domain.modelling.simulation import EpcSimulation, HeatingOverlay
@@ -35,6 +36,8 @@ from domain.sap10_calculator.tables.table_12a import (
     OFF_PEAK_IMPLYING_HEATING_CODES,
 )
 
+logger = logging.getLogger(__name__)
+
 # Off-peak (Economy 7) meter. Electric storage / CPSU systems charge overnight at
 # the low rate and cannot run economically on a single-rate meter; "Dual" lets
 # the §12 dispatch resolve the specific tariff (storage 7-hour, CPSU 10-hour).
@@ -351,13 +354,32 @@ def main_heating_overlay_for(
         return None
     if code in _GAS_BOILER_CODES:
         return EpcSimulation(heating=_gas_boiler_overlay(code))
+    category = _category_for(code)
+    control = _control_for(code)
+    if category is None or control is None:
+        # A system-replacing override should stamp a complete, coherent companion
+        # set; an unmapped category or control leaves the field unset, so the
+        # effective cert inherits the REPLACED system's value (a silent mis-rating
+        # — e.g. a room heater keeping a storage heater's category/control). We do
+        # not yet have confident defaults for every archetype, so log-and-continue
+        # (matching `flag_fuel_mismatch`) to make the gap visible for review
+        # rather than raise mid-run.
+        logger.error(
+            "Landlord main_heating_system %r (SAP code %d) overlaid with an "
+            "incomplete companion set (category=%s, control=%s); the effective "
+            "cert may inherit the replaced system's value — known gap, review",
+            main_heating_value,
+            code,
+            category,
+            control,
+        )
     return EpcSimulation(
         heating=HeatingOverlay(
             sap_main_heating_code=code,
-            main_heating_category=_category_for(code),
+            main_heating_category=category,
             main_fuel_type=_natural_fuel_for(code),
             meter_type=_meter_for(code),
-            main_heating_control=_control_for(code),
+            main_heating_control=control,
             # A landlord override describes the existing dwelling, so its assumed
             # off-peak meter must not downgrade a more-off-peak cert meter
             # (e.g. a 24-hour all-low tariff). Measures, which re-meter for real,