diff --git a/tests/datatypes/epc/domain/test_mapper_ventilation_piv_loft.py b/tests/datatypes/epc/domain/test_mapper_ventilation_piv_loft.py
new file mode 100644
index 000000000..60dc529e7
--- /dev/null
+++ b/tests/datatypes/epc/domain/test_mapper_ventilation_piv_loft.py
@@ -0,0 +1,24 @@
+"""Mapper boundary: the Elmhurst §12.1 "Positive input from loft" mechanical-
+ventilation label.
+
+SAP 10.2 §2 (24d) treats positive input ventilation sourced from the LOFT as
+natural ventilation — unlike PIV from OUTSIDE, which is whole-house extract and
+resolves to EXTRACT_OR_PIV_OUTSIDE / (24c). The MechanicalVentilationKind.NATURAL
+docstring states the loft case explicitly. Before this mapping existed the label
+raised UnmappedElmhurstLabel, blocking the whole Summary (surfaced across 8 WCHG
+Elmhurst certs).
+"""
+
+from datatypes.epc.domain.mapper import (
+    _elmhurst_mv_kind,  # pyright: ignore[reportPrivateUsage]
+)
+
+
+def test_positive_input_from_loft_maps_to_natural() -> None:
+    # Arrange — the Elmhurst §12.1 PIV-from-loft ventilation label.
+
+    # Act
+    kind = _elmhurst_mv_kind("Positive input from loft")
+
+    # Assert — SAP 10.2 §2 (24d): loft-sourced PIV is treated as natural vent.
+    assert kind == "NATURAL"