diff --git a/tests/domain/modelling/fixtures/ashp_from_gas_boiler_instant_hw_001431_after.pdf b/tests/domain/modelling/fixtures/ashp_from_gas_boiler_instant_hw_001431_after.pdf
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diff --git a/tests/domain/modelling/fixtures/ashp_from_gas_boiler_instant_hw_001431_before.pdf b/tests/domain/modelling/fixtures/ashp_from_gas_boiler_instant_hw_001431_before.pdf
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diff --git a/tests/domain/modelling/test_elmhurst_cascade_pins.py b/tests/domain/modelling/test_elmhurst_cascade_pins.py
index 50c7f7cd..e5841e37 100644
--- a/tests/domain/modelling/test_elmhurst_cascade_pins.py
+++ b/tests/domain/modelling/test_elmhurst_cascade_pins.py
@@ -641,3 +641,53 @@ def test_ashp_overlay_reproduces_the_relodged_after_from_a_gas_boiler() -> None:
 
     # Act / Assert
     _assert_overlay_reproduces_after(before, after, option.overlay)
+
+
+def test_ashp_overlay_reproduces_the_relodged_after_from_a_gas_boiler_instant_hw() -> None:
+    # Arrange — a gas boiler whose hot water is electric/instantaneous (water-
+    # heating SAP code 909, no cylinder) re-lodged as an ASHP. This exercises the
+    # overlay's water_heating_code reset (909 -> 901, "from the heat pump") that
+    # boiler-1 didn't (its HW was already 901). The relodged after lodges control
+    # 2209 vs the overlay's 2210 — SAP-equivalent zone controls, so the cascade
+    # still closes at 1e-4.
+    before: EpcPropertyData = parse_recommendation_summary(
+        "ashp_from_gas_boiler_instant_hw_001431_before.pdf"
+    )
+    after: EpcPropertyData = parse_recommendation_summary(
+        "ashp_from_gas_boiler_instant_hw_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 — overlay applied to the before reproduces the after exactly.
+    _assert_overlay_reproduces_after(before, after, option.overlay)
+
+
+_BOILER_INSTANT_HW_FUEL_REASON: Final[str] = (
+    "Blocked on the Elmhurst mapper deriving main_fuel_type for a gas boiler "
+    "lodged with EES code 'BGB' / Main Heating SAP code 102: it currently maps "
+    "to '' (empty), so Sap10Calculator raises MissingMainFuelType when "
+    "baselining the raw before. (boiler-1's 'RGE' resolves to mains gas 26; "
+    "'BGB' has no mapping.) The ASHP overlay still reproduces the after exactly "
+    "because it overwrites main_fuel_type -> 30, so the cascade pin above passes "
+    "— only baselining the unmodified before is blocked. Flips green once the "
+    "mapper derives mains gas (26) from the gas-boiler SAP code. Owner: "
+    "mapper/extractor front."
+)
+
+
+@pytest.mark.xfail(strict=True, reason=_BOILER_INSTANT_HW_FUEL_REASON)
+def test_gas_boiler_instant_hw_before_baselines() -> None:
+    # The Modelling pipeline baselines the dwelling before modelling it, so the
+    # before must be scorable on its own. This one is not yet: its main fuel is
+    # unresolved (see reason). A failing tripwire for the separate mapper fix.
+    # Arrange
+    before: EpcPropertyData = parse_recommendation_summary(
+        "ashp_from_gas_boiler_instant_hw_001431_before.pdf"
+    )
+
+    # Act / Assert — currently raises MissingMainFuelType.
+    Sap10Calculator().calculate(before)