diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index 017f3ac19..b02455c8e 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -4000,19 +4000,25 @@ def _normalize_sap_schema_16_x(data: Dict[str, Any]) -> Dict[str, Any]:
 
         # Some 16.x/15.0 certs (e.g. cert 9568-3034-6211-6089-5960, UPRN
         # 100010349400, SAP-Schema-15.0) lodge `sap_heating.cylinder_size` but
-        # omit the top-level `has_hot_water_cylinder` boolean entirely —
+        # omit the top-level `has_hot_water_cylinder` field entirely —
         # RdSapSchema17_1 requires both as independent fields. Unlike
         # `multiple_glazed_proportion` above, this is NOT a guess: RdSAP 10
         # §10.5's `cylinder_size` code 1 means "no cylinder" by definition
         # (see `_CYLINDER_SIZE_CODE_TO_LITRES` / cert_to_inputs.py:5902), the
         # exact same fact `has_hot_water_cylinder` encodes — confirmed 1:1
         # across every real 15.0/16.x fixture that lodges both fields
-        # (cylinder_size == 1 <-> has_hot_water_cylinder == false, every
-        # other code <-> true). Derive rather than leave the cert unmappable.
+        # (cylinder_size == 1 <-> has_hot_water_cylinder == "false", every
+        # other code <-> "true"). Derive rather than leave the cert
+        # unmappable. Lodged as the lowercase string "true"/"false" (RdSAP
+        # convention every real fixture uses — `from_rdsap_schema_17_1`
+        # reads it via `schema.has_hot_water_cylinder == "true"`, a bare
+        # Python bool would silently compare False either way).
         if "has_hot_water_cylinder" not in d:
             cylinder_size = heating.get("cylinder_size")
             if isinstance(cylinder_size, int):
-                d["has_hot_water_cylinder"] = cylinder_size != 1
+                d["has_hot_water_cylinder"] = (
+                    "true" if cylinder_size != 1 else "false"
+                )
 
     for bp in _dicts(d.get("sap_building_parts")):
         for fd in _dicts(bp.get("sap_floor_dimensions")):
diff --git a/datatypes/epc/domain/tests/test_from_sap_schema_15_0.py b/datatypes/epc/domain/tests/test_from_sap_schema_15_0.py
index a95345344..82be846d8 100644
--- a/datatypes/epc/domain/tests/test_from_sap_schema_15_0.py
+++ b/datatypes/epc/domain/tests/test_from_sap_schema_15_0.py
@@ -107,3 +107,18 @@ class TestFromSapSchema15_0:
 
         assert epc.has_hot_water_cylinder is False
         assert epc.sap_heating.cylinder_size == 1
+
+    def test_derives_has_hot_water_cylinder_true_from_a_sized_cylinder(
+        self,
+    ) -> None:
+        # Mirror of the above for the other branch: a lodged size code other
+        # than 1 (here 2 = Normal/110L, per sap_16_2.json) means a cylinder
+        # IS present, so an omitted `has_hot_water_cylinder` must derive
+        # True — not just default-False by omission.
+        data = load("sap_16_2.json")
+        assert data["sap_heating"]["cylinder_size"] == 2
+        del data["has_hot_water_cylinder"]
+
+        epc = EpcPropertyDataMapper.from_api_response(data)
+
+        assert epc.has_hot_water_cylinder is True