diff --git a/datatypes/epc/domain/tests/test_from_site_notes.py b/datatypes/epc/domain/tests/test_from_site_notes.py
index 0fbab02fa..77e99403c 100644
--- a/datatypes/epc/domain/tests/test_from_site_notes.py
+++ b/datatypes/epc/domain/tests/test_from_site_notes.py
@@ -122,6 +122,62 @@ class TestPhotovoltaicArrayMapping:
         assert result.sap_energy_source.photovoltaic_arrays is None
 
 
+class TestCylinderSizeCoding:
+    """The surveyed §Water-Heating "Cylinder Size" label must resolve to the
+    SAP10 cylinder-size enum (`_cylinder_volume_l_from_code` int-or-nones a raw
+    string, silently skipping the Table 28 volume convention — issue #1590
+    bug 6). "No Access" routes per RdSAP 10 Table 28 p.55: solid-fuel boiler →
+    Medium/160L, off-peak dual electric immersion → Large/210L, otherwise →
+    Normal/110L (the cohort's case: cylinder fed from a gas main).
+    """
+
+    @staticmethod
+    def _with_cylinder(label: str) -> Dict[str, Any]:
+        data = load("pashub_rdsap_site_notes_example1.json")
+        data["heating_and_hot_water"]["water_heating"]["cylinder_size"] = label
+        return data
+
+    @pytest.mark.parametrize(
+        "label, expected_code",
+        [
+            ("Normal (90-130 litres)", 2),
+            ("Large (>170 litres)", 4),
+            ("No Access", 2),  # fed from the (gas) main → Table 28 "otherwise"
+        ],
+    )
+    def test_label_maps_to_sap_code(self, label: str, expected_code: int) -> None:
+        # Arrange
+        survey = from_dict(PasHubRdSapSiteNotes, self._with_cylinder(label))
+
+        # Act
+        result = EpcPropertyDataMapper.from_site_notes(survey)
+
+        # Assert
+        assert result.sap_heating.cylinder_size == expected_code
+
+    def test_no_cylinder_stays_absent(self) -> None:
+        # Arrange — a combi dwelling: no cylinder lodged.
+        survey = from_dict(
+            PasHubRdSapSiteNotes, self._with_cylinder("No Cylinder")
+        )
+
+        # Act
+        result = EpcPropertyDataMapper.from_site_notes(survey)
+
+        # Assert
+        assert result.sap_heating.cylinder_size is None
+
+    def test_unknown_label_strict_raises(self) -> None:
+        # Arrange
+        survey = from_dict(
+            PasHubRdSapSiteNotes, self._with_cylinder("Some novel size")
+        )
+
+        # Act / Assert
+        with pytest.raises(UnmappedPasHubLabel, match="cylinder size"):
+            EpcPropertyDataMapper.from_site_notes(survey)
+
+
 class TestMechanicalVentilationKindCoding:
     """The surveyed "Ventilation type" label must resolve to the
     `MechanicalVentilationKind` enum name the §2 cascade dispatches on —