diff --git a/datatypes/epc/domain/tests/test_from_rdsap_schema.py b/datatypes/epc/domain/tests/test_from_rdsap_schema.py
index 49932030..6afea563 100644
--- a/datatypes/epc/domain/tests/test_from_rdsap_schema.py
+++ b/datatypes/epc/domain/tests/test_from_rdsap_schema.py
@@ -1671,3 +1671,37 @@ class TestRdSap18_0ReducedFieldSynthesis:
         assert result.sap_ventilation.sheltered_sides == _api_sheltered_sides(
             cert["built_form"]
         )
+
+    def test_hot_water_derives_bath_and_mixer_counts_from_room_counts(self) -> None:
+        # Arrange — ADR-0028: 18.0's instantaneous_wwhrs carries bath/shower ROOM
+        # counts (a false-friend for the WWHR device index), populated 1000/1000.
+        # Derive number_baths and mixer_shower_count so HW demand isn't pinned to
+        # the calculator's modal 1-bath default.
+        corpus = _load_18_0_corpus()
+        if not corpus:
+            pytest.skip("no RdSAP-Schema-18.0 corpus harvested")
+        cert = next(
+            (
+                c
+                for c in corpus
+                if not c.get("sap_windows")
+                and c.get("sap_heating", {}).get("instantaneous_wwhrs")
+            ),
+            None,
+        )
+        if cert is None:
+            pytest.skip("no corpus cert with instantaneous_wwhrs")
+        iw = cert["sap_heating"]["instantaneous_wwhrs"]
+        expected_baths = iw["rooms_with_bath_and_or_shower"] + iw[
+            "rooms_with_bath_and_mixer_shower"
+        ]
+        expected_mixers = iw["rooms_with_mixer_shower_no_bath"] + iw[
+            "rooms_with_bath_and_mixer_shower"
+        ]
+
+        # Act
+        result = EpcPropertyDataMapper.from_api_response(cert)
+
+        # Assert
+        assert result.sap_heating.number_baths == expected_baths
+        assert result.sap_heating.mixer_shower_count == expected_mixers