diff --git a/domain/modelling/scoring/overlay_applicator.py b/domain/modelling/scoring/overlay_applicator.py
index 73934779..11d6e562 100644
--- a/domain/modelling/scoring/overlay_applicator.py
+++ b/domain/modelling/scoring/overlay_applicator.py
@@ -90,6 +90,16 @@ def _fold_heating(epc: EpcPropertyData, overlay: HeatingOverlay) -> None:
         value = getattr(overlay, field_name)
         if value is not None:
             setattr(main, field_name, value)
+    # `main_heating_index_number` (PCDB-resolved, e.g. a heat pump) and
+    # `sap_main_heating_code` (Table 4a-resolved, e.g. storage heaters) are
+    # mutually-exclusive efficiency anchors: a whole-system replacement to one
+    # must clear the other, else a stale code from the old system wins the
+    # calculator's dispatch (e.g. a gas-boiler code 104 left beside a heat-pump
+    # index makes hot water use boiler efficiency, not the HP SCOP).
+    if overlay.main_heating_index_number is not None:
+        main.sap_main_heating_code = None
+    elif overlay.sap_main_heating_code is not None:
+        main.main_heating_index_number = None
     for field_name in _SAP_HEATING_FIELDS:
         value = getattr(overlay, field_name)
         if value is not None:
diff --git a/tests/domain/modelling/fixtures/ashp_001431_after.pdf b/tests/domain/modelling/fixtures/ashp_001431_after.pdf
new file mode 100644
index 00000000..538a8ed2
Binary files /dev/null and b/tests/domain/modelling/fixtures/ashp_001431_after.pdf differ
diff --git a/tests/domain/modelling/fixtures/ashp_from_gas_boiler_001431_before.pdf b/tests/domain/modelling/fixtures/ashp_from_gas_boiler_001431_before.pdf
new file mode 100644
index 00000000..f67db819
Binary files /dev/null and b/tests/domain/modelling/fixtures/ashp_from_gas_boiler_001431_before.pdf differ
diff --git a/tests/domain/modelling/test_elmhurst_cascade_pins.py b/tests/domain/modelling/test_elmhurst_cascade_pins.py
index 7410d969..50c7f7cd 100644
--- a/tests/domain/modelling/test_elmhurst_cascade_pins.py
+++ b/tests/domain/modelling/test_elmhurst_cascade_pins.py
@@ -620,3 +620,24 @@ def test_hhr_storage_overlay_reproduces_the_relodged_after_from_no_system() -> N
 
     # Act / Assert
     _assert_overlay_reproduces_after(before, after, option.overlay)
+
+
+def test_ashp_overlay_reproduces_the_relodged_after_from_a_gas_boiler() -> None:
+    # Arrange — a typical mains-gas combi house re-lodged as an air-source heat
+    # pump (fuel 26 -> 30, SAP code 104 -> PCDB index 101413 + category 4,
+    # control 2106 -> 2210), off mains gas, gaining a heat-pump cylinder
+    # (ADR-0024).
+    before: EpcPropertyData = parse_recommendation_summary(
+        "ashp_from_gas_boiler_001431_before.pdf"
+    )
+    after: EpcPropertyData = parse_recommendation_summary(
+        "ashp_001431_after.pdf"
+    )
+    recommendation: Recommendation | None = recommend_heating(before, _AnyProduct())
+    assert recommendation is not None
+    option = next(
+        o for o in recommendation.options if o.measure_type == "air_source_heat_pump"
+    )
+
+    # Act / Assert
+    _assert_overlay_reproduces_after(before, after, option.overlay)
diff --git a/tests/domain/modelling/test_overlay_applicator.py b/tests/domain/modelling/test_overlay_applicator.py
index 16228ec5..75d2c901 100644
--- a/tests/domain/modelling/test_overlay_applicator.py
+++ b/tests/domain/modelling/test_overlay_applicator.py
@@ -327,11 +327,54 @@ def test_baseline_heating_is_not_mutated_by_a_heating_overlay() -> None:
     assert baseline.sap_energy_source.mains_gas == original_mains_gas
 
 
+def test_heating_index_overlay_clears_a_stale_sap_main_heating_code() -> None:
+    # Arrange — 000490's gas combi lodges a Table 4a code; an ASHP bundle sets a
+    # PCDB index instead. The two are mutually-exclusive efficiency anchors, so
+    # the stale code must be cleared or it wins the calculator's dispatch.
+    baseline: EpcPropertyData = build_epc()
+    baseline.sap_heating.main_heating_details[0].sap_main_heating_code = 104
+
+    # Act
+    result: EpcPropertyData = apply_simulations(
+        baseline,
+        [
+            EpcSimulation(
+                heating=HeatingOverlay(
+                    main_heating_index_number=101413, main_heating_category=4
+                )
+            )
+        ],
+    )
+
+    # Assert — the index is set and the old SAP code is gone.
+    main = result.sap_heating.main_heating_details[0]
+    assert main.main_heating_index_number == 101413
+    assert main.sap_main_heating_code is None
+
+
+def test_heating_sap_code_overlay_clears_a_stale_index() -> None:
+    # Arrange — a dwelling with a PCDB-indexed system; an HHR storage bundle sets
+    # a Table 4a code instead, so the stale index must be cleared.
+    baseline: EpcPropertyData = build_epc()
+    baseline.sap_heating.main_heating_details[0].main_heating_index_number = 8262
+
+    # Act
+    result: EpcPropertyData = apply_simulations(
+        baseline,
+        [EpcSimulation(heating=HeatingOverlay(sap_main_heating_code=409))],
+    )
+
+    # Assert
+    main = result.sap_heating.main_heating_details[0]
+    assert main.sap_main_heating_code == 409
+    assert main.main_heating_index_number is None
+
+
 def test_baseline_lighting_is_not_mutated_by_a_lighting_overlay() -> None:
     # Arrange — 000490 lodges 8 low-energy-unknown bulbs, 0 LED.
     baseline: EpcPropertyData = build_epc()
     original_led: int = baseline.led_fixed_lighting_bulbs_count
-    original_lel: int = baseline.low_energy_fixed_lighting_bulbs_count
+    original_lel: int | None = baseline.low_energy_fixed_lighting_bulbs_count
 
     # Act — fold an all-LED overlay (led = the 8 total).
     _: EpcPropertyData = apply_simulations(