diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index 978711c5..b9de5feb 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -1877,6 +1877,14 @@ class EpcPropertyDataMapper:
                 # default 2 → over-counted shelter factor → -2.42
                 # ACH/month infiltration shortfall).
                 sheltered_sides=_api_sheltered_sides(schema.built_form),
+                # RdSAP-Schema-21 `mechanical_ventilation` enum → §2 (24a..d)
+                # MV-kind dispatch. Without this an MEV / PIV-from-outside
+                # dwelling defaulted to NATURAL and under-stated its
+                # ventilation heat loss (+1.90 SAP over-rate on the n=20
+                # MEV cohort).
+                mechanical_ventilation_kind=_api_mechanical_ventilation_kind(
+                    schema.mechanical_ventilation
+                ),
             ),
         )
 
@@ -2780,6 +2788,55 @@ def _api_sheltered_sides(built_form: object) -> Optional[int]:
     return _API_BUILT_FORM_TO_SHELTERED_SIDES[built_form]
 
 
+# GOV.UK API `mechanical_ventilation` integer (RdSAP-Schema-21 enum) →
+# `MechanicalVentilationKind` enum name picking the SAP 10.2 §2 (24a..d)
+# effective-air-change formula. Mirrors the Elmhurst-path
+# `_ELMHURST_MV_TYPE_TO_KIND` so both source paths converge on the same
+# cascade dispatch.
+#   0 natural                                 → None  (NATURAL, 24d)
+#   1 mechanical ventilation, no HR (MV)      → MV    (24b)
+#   2 mechanical extract, decentralised (MEVdc)→ EXTRACT_OR_PIV_OUTSIDE (24c)
+#   3 mechanical extract, centralised (MEV c) → EXTRACT_OR_PIV_OUTSIDE (24c)
+#   5 positive input from loft                → None  — RdSAP 10 §2.6 treats
+#                                               loft-sourced PIV as natural
+#                                               (no added system air change)
+#   6 positive input from outside             → EXTRACT_OR_PIV_OUTSIDE (24c)
+# Code 4 (MVHR, 24a) is DEFERRED: its (24a)m formula needs the lodged
+# heat-recovery efficiency (`mvhr_efficiency_pct`, PCDB Table 326) which the
+# API→cascade path does not yet plumb; mapping it to MVHR with a null
+# efficiency would mis-model it as MV (no recovery), so it stays NATURAL
+# until the efficiency is wired. The extract systems (2/3/6) carry no
+# efficiency, so this slice closes the clean +1.90 SAP over-rate on the
+# MEV/PIV-outside cohort (n=20, 5% within 0.5) — they were silently
+# defaulting to NATURAL, under-stating ventilation heat loss.
+_API_MECHANICAL_VENTILATION_TO_KIND: Final[dict[int, Optional[str]]] = {
+    0: None,
+    1: "MV",
+    2: "EXTRACT_OR_PIV_OUTSIDE",
+    3: "EXTRACT_OR_PIV_OUTSIDE",
+    4: None,  # MVHR — efficiency plumbing deferred; treat as natural
+    5: None,
+    6: "EXTRACT_OR_PIV_OUTSIDE",
+}
+
+
+def _api_mechanical_ventilation_kind(mechanical_ventilation: object) -> Optional[str]:
+    """Translate the API `mechanical_ventilation` integer to a
+    `MechanicalVentilationKind` enum name for the §2 cascade dispatch.
+
+    Strict-coverage: a lodged integer outside the mapped set raises
+    `UnmappedApiCode` rather than silently defaulting to NATURAL (which
+    under-states ventilation heat loss for any mechanical system).
+    Non-int / None lodging stays as no-lodging (NATURAL)."""
+    if isinstance(mechanical_ventilation, str) and mechanical_ventilation.isdigit():
+        mechanical_ventilation = int(mechanical_ventilation)
+    if not isinstance(mechanical_ventilation, int):
+        return None
+    if mechanical_ventilation not in _API_MECHANICAL_VENTILATION_TO_KIND:
+        raise UnmappedApiCode("mechanical_ventilation", mechanical_ventilation)
+    return _API_MECHANICAL_VENTILATION_TO_KIND[mechanical_ventilation]
+
+
 # GOV.UK API `glazing_type` integer → (u_value W/m²K, g_perpendicular,
 # frame_factor) lookup the cascade reads via `window_transmission_
 # details` for per-window cascade fidelity. The cascade defaults to a
diff --git a/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py b/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
index 3c961919..b10fb12f 100644
--- a/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
+++ b/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
@@ -2715,6 +2715,45 @@ def test_api_type_1_gable_kind_maps_sheltered_and_connected_codes() -> None:
     assert _api_type_1_gable_kind(3) == "connected_wall"
 
 
+def test_api_mechanical_ventilation_maps_extract_systems_to_cascade_kind() -> None:
+    # Arrange — RdSAP-Schema-21 `mechanical_ventilation` enum → the SAP
+    # 10.2 §2 (24a..d) MechanicalVentilationKind dispatch. The mapper
+    # previously dropped this field on the API path, so every mechanical
+    # system defaulted to NATURAL and under-stated its ventilation heat
+    # loss (the MEV-dc cohort, code 2, over-rated +1.90 SAP, n=20).
+    from datatypes.epc.domain.mapper import (
+        _api_mechanical_ventilation_kind,  # pyright: ignore[reportPrivateUsage]
+    )
+
+    # Act / Assert — extract / positive-input-from-outside systems carry
+    # the (24c) kind; MV-no-HR carries (24b); natural and PIV-from-loft
+    # stay NATURAL; MVHR (4) is deferred (efficiency not yet plumbed) and
+    # stays NATURAL rather than mis-modelling as MV.
+    assert _api_mechanical_ventilation_kind(0) is None          # natural
+    assert _api_mechanical_ventilation_kind(1) == "MV"          # MV, no HR
+    assert _api_mechanical_ventilation_kind(2) == "EXTRACT_OR_PIV_OUTSIDE"  # MEV dc
+    assert _api_mechanical_ventilation_kind(3) == "EXTRACT_OR_PIV_OUTSIDE"  # MEV c
+    assert _api_mechanical_ventilation_kind(4) is None          # MVHR (deferred)
+    assert _api_mechanical_ventilation_kind(5) is None          # PIV from loft
+    assert _api_mechanical_ventilation_kind(6) == "EXTRACT_OR_PIV_OUTSIDE"  # PIV outside
+    assert _api_mechanical_ventilation_kind(None) is None
+
+
+def test_api_mechanical_ventilation_unmapped_code_raises() -> None:
+    # Arrange — an out-of-range `mechanical_ventilation` integer is a
+    # spec-coverage gap: raise rather than silently default to NATURAL
+    # (which would under-state ventilation heat loss). Mirror of the
+    # `_api_sheltered_sides` / `_api_type_1_gable_kind` strict-raise.
+    from datatypes.epc.domain.mapper import (
+        UnmappedApiCode,  # pyright: ignore[reportPrivateUsage]
+        _api_mechanical_ventilation_kind,  # pyright: ignore[reportPrivateUsage]
+    )
+
+    # Act / Assert
+    with pytest.raises(UnmappedApiCode):
+        _api_mechanical_ventilation_kind(7)
+
+
 def test_elmhurst_detailed_rir_keeps_roof_surfaces() -> None:
     # Arrange — a Detailed (§3.10) assessment DOES measure slope / flat
     # ceiling, so they must be retained (regression guard so the