diff --git a/packages/domain/src/domain/ml/tests/_fixtures.py b/packages/domain/src/domain/ml/tests/_fixtures.py
index 0d139839..37fccde4 100644
--- a/packages/domain/src/domain/ml/tests/_fixtures.py
+++ b/packages/domain/src/domain/ml/tests/_fixtures.py
@@ -85,6 +85,7 @@ def make_sap_heating(
     cylinder_size: Optional[Union[int, str]] = None,
     cylinder_insulation_thickness_mm: Optional[int] = None,
     secondary_fuel_type: Optional[int] = None,
+    secondary_heating_type: Optional[int] = None,
 ) -> SapHeating:
     """Build a SapHeating with SAP10 API defaults."""
     return SapHeating(
@@ -98,6 +99,7 @@ def make_sap_heating(
         cylinder_size=cylinder_size,
         cylinder_insulation_thickness_mm=cylinder_insulation_thickness_mm,
         secondary_fuel_type=secondary_fuel_type,
+        secondary_heating_type=secondary_heating_type,
     )
 
 
diff --git a/packages/domain/src/domain/sap/rdsap/cert_to_inputs.py b/packages/domain/src/domain/sap/rdsap/cert_to_inputs.py
index d5bc6383..5b0b63d7 100644
--- a/packages/domain/src/domain/sap/rdsap/cert_to_inputs.py
+++ b/packages/domain/src/domain/sap/rdsap/cert_to_inputs.py
@@ -533,7 +533,17 @@ def _secondary_fraction(
     when (a) the cert has a secondary system lodged OR (b) the main
     heating code is in the §A.2.2 forced-secondary set (electric storage
     heaters). Returns 0.0 when neither applies — the most common case
-    for gas/oil main systems whose cert doesn't lodge a secondary."""
+    for gas/oil main systems whose cert doesn't lodge a secondary.
+
+    `main_heating_fraction` on the cert is NOT consulted here: empirical
+    probe shows it tracks main-system-1 vs main-system-2 allocation in
+    multi-main configurations (99% of corpus has =1, meaning "single
+    main, 100% allocation"), not main-vs-secondary. Elmhurst applies
+    Table 11's 10% secondary even when main_heating_fraction=1; the
+    spec is silent on overriding (only the §A.2.2 forced-secondary rule
+    is explicit), and an S-B30 attempt to override yielded SAP MAE
+    +0.16 — the wrong direction.
+    """
     if main is None:
         return 0.0
     code = main.sap_main_heating_code
diff --git a/packages/domain/src/domain/sap/rdsap/tests/test_cert_to_inputs.py b/packages/domain/src/domain/sap/rdsap/tests/test_cert_to_inputs.py
index 2915dde0..2d13e2f5 100644
--- a/packages/domain/src/domain/sap/rdsap/tests/test_cert_to_inputs.py
+++ b/packages/domain/src/domain/sap/rdsap/tests/test_cert_to_inputs.py
@@ -16,6 +16,8 @@ from __future__ import annotations
 
 from typing import Final
 
+import pytest
+
 from datatypes.epc.domain.epc_property_data import MainHeatingDetail
 
 from domain.ml.tests._fixtures import (
@@ -78,6 +80,75 @@ def _typical_semi_detached_epc():
     )
 
 
+def test_main_heating_fraction_does_not_override_table11_secondary_default() -> None:
+    # Arrange — the S-B30 attempt assumed `main_heating_fraction=1` meant
+    # "no secondary heating" and dropped the Table 11 default in that
+    # case. Empirically that gave SAP MAE +0.16 (worse). The cert
+    # software (Elmhurst) DOES apply Table 11's 10% secondary even when
+    # main_heating_fraction=1 — the field tracks multi-main allocation
+    # not main-vs-secondary. This test pins the spec-aligned behaviour
+    # to prevent regressions.
+    main = MainHeatingDetail(
+        has_fghrs=False,
+        main_fuel_type=26,
+        heat_emitter_type=1,
+        emitter_temperature=1,
+        main_heating_control=2106,
+        main_heating_category=2,
+        sap_main_heating_code=102,
+        main_heating_fraction=1,
+    )
+    epc = make_minimal_sap10_epc(
+        total_floor_area_m2=_TYPICAL_TFA_M2,
+        habitable_rooms_count=4,
+        country_code="ENG",
+        sap_building_parts=[make_building_part()],
+        sap_heating=make_sap_heating(
+            main_heating_details=[main],
+            secondary_heating_type=691,
+        ),
+    )
+
+    # Act
+    inputs = cert_to_inputs(epc)
+
+    # Assert — Table 11's 0.10 applies regardless of main_heating_fraction.
+    assert inputs.secondary_heating_fraction == pytest.approx(0.1, abs=0.001)
+
+
+def test_main_heating_fraction_missing_falls_back_to_table11_default() -> None:
+    # Arrange — when main_heating_fraction isn't lodged AND the cert
+    # has a secondary system lodged, Table 11's 0.10 default still
+    # applies (the pre-S-B30 behaviour for ~340 of 30 000 corpus certs).
+
+    main = MainHeatingDetail(
+        has_fghrs=False,
+        main_fuel_type=26,
+        heat_emitter_type=1,
+        emitter_temperature=1,
+        main_heating_control=2106,
+        main_heating_category=2,
+        sap_main_heating_code=102,
+        main_heating_fraction=None,
+    )
+    epc = make_minimal_sap10_epc(
+        total_floor_area_m2=_TYPICAL_TFA_M2,
+        habitable_rooms_count=4,
+        country_code="ENG",
+        sap_building_parts=[make_building_part()],
+        sap_heating=make_sap_heating(
+            main_heating_details=[main],
+            secondary_heating_type=691,
+        ),
+    )
+
+    # Act
+    inputs = cert_to_inputs(epc)
+
+    # Assert
+    assert inputs.secondary_heating_fraction == pytest.approx(0.1, abs=0.001)
+
+
 def test_minimal_cert_round_trips_through_calculator_and_returns_sap_result() -> None:
     # Arrange — a complete-enough RdSAP cert (envelope + heating + windows
     # + region) that the mapper can populate every CalculatorInputs field.