diff --git a/datatypes/epc/domain/tests/test_from_site_notes.py b/datatypes/epc/domain/tests/test_from_site_notes.py
index 48094d833..0fbab02fa 100644
--- a/datatypes/epc/domain/tests/test_from_site_notes.py
+++ b/datatypes/epc/domain/tests/test_from_site_notes.py
@@ -122,6 +122,52 @@ class TestPhotovoltaicArrayMapping:
         assert result.sap_energy_source.photovoltaic_arrays is None
 
 
+class TestMechanicalVentilationKindCoding:
+    """The surveyed "Ventilation type" label must resolve to the
+    `MechanicalVentilationKind` enum name the §2 cascade dispatches on —
+    left None, `ventilation_from_cert` silently defaults every mechanical
+    system to NATURAL (24d), dropping its ACH heat loss (issue #1590 bug 4;
+    76/205 cohort fixtures lodge a mechanical system). Targets mirror the
+    gov-API table `_API_MECHANICAL_VENTILATION_TO_KIND` (dMEV/MEV/PIV-outside
+    → EXTRACT_OR_PIV_OUTSIDE; PIV-from-loft is "as natural" → None).
+    """
+
+    @pytest.mark.parametrize(
+        "label, expected_kind",
+        [
+            ("Natural", None),
+            ("Mechanical Extract - Decentralised", "EXTRACT_OR_PIV_OUTSIDE"),
+            ("Mechanical Extract - Centralised", "EXTRACT_OR_PIV_OUTSIDE"),
+            ("Positive Input - From Outside", "EXTRACT_OR_PIV_OUTSIDE"),
+            ("Positive Input - From Loft (As Natural)", None),
+        ],
+    )
+    def test_label_maps_to_mechanical_kind(
+        self, label: str, expected_kind: Optional[str]
+    ) -> None:
+        # Arrange
+        data = load("pashub_rdsap_site_notes_example1.json")
+        data["ventilation"]["ventilation_type"] = label
+        survey = from_dict(PasHubRdSapSiteNotes, data)
+
+        # Act
+        result = EpcPropertyDataMapper.from_site_notes(survey)
+
+        # Assert
+        assert result.sap_ventilation is not None
+        assert result.sap_ventilation.mechanical_ventilation_kind == expected_kind
+
+    def test_unknown_label_strict_raises(self) -> None:
+        # Arrange
+        data = load("pashub_rdsap_site_notes_example1.json")
+        data["ventilation"]["ventilation_type"] = "Some novel system"
+        survey = from_dict(PasHubRdSapSiteNotes, data)
+
+        # Act / Assert
+        with pytest.raises(UnmappedPasHubLabel, match="ventilation type"):
+            EpcPropertyDataMapper.from_site_notes(survey)
+
+
 class TestPvConnectionCoding:
     """The surveyed "PV Connection" label must resolve to the gov-API int enum
     the calculator's credit gate keys off (`_pv_connected_to_dwelling_meter`: