diff --git a/backend/documents_parser/tests/test_summary_pdf_mapper_chain.py b/backend/documents_parser/tests/test_summary_pdf_mapper_chain.py
index a9ad62b0..2b7cd2e6 100644
--- a/backend/documents_parser/tests/test_summary_pdf_mapper_chain.py
+++ b/backend/documents_parser/tests/test_summary_pdf_mapper_chain.py
@@ -64,6 +64,7 @@ _SUMMARY_000901_PDF = _FIXTURES / "Summary_000901.pdf"  # cert 3800
 _SUMMARY_000904_PDF = _FIXTURES / "Summary_000904.pdf"  # cert 9285
 _SUMMARY_000900_PDF = _FIXTURES / "Summary_000900.pdf"  # cert 2225
 _SUMMARY_000898_PDF = _FIXTURES / "Summary_000898.pdf"  # cert 2636
+_SUMMARY_000902_PDF = _FIXTURES / "Summary_000902.pdf"  # cert 9418
 
 # GOV.UK EPB API JSON for cert 001479 — the API-path counterpart of the
 # Summary_001479.pdf fixture. Together they drive the API ≡ Summary
@@ -816,6 +817,50 @@ def test_summary_2636_full_chain_sap_within_spec_floor_of_worksheet() -> None:
     assert abs(result.sap_score_continuous - worksheet_unrounded_sap) < _ASHP_COHORT_CHAIN_TOLERANCE
 
 
+def test_summary_9418_large_cylinder_routes_to_code_4() -> None:
+    # Arrange — cert 9418-3062-8205-3566-7200's Summary §15.1 lodges
+    # "Cylinder Size: Large". The dr87 worksheet lodges "Cylinder
+    # Volume 210.00" L, and the cascade lookup
+    # `_CYLINDER_SIZE_CODE_TO_LITRES = {3: 160.0, 4: 210.0}` maps code
+    # 4 → 210 L. Cert 9418 is the first cohort cert to exercise the
+    # "Large" cylinder lodging (every other cohort cert is "Medium").
+    pages = _summary_pdf_to_textract_style_pages(_SUMMARY_000902_PDF)
+    site_notes = ElmhurstSiteNotesExtractor(pages).extract()
+
+    # Act
+    epc = EpcPropertyDataMapper.from_elmhurst_site_notes(site_notes)
+
+    # Assert
+    assert epc.sap_heating.cylinder_size == 4
+
+
+def test_summary_9418_full_chain_sap_within_spec_floor_of_worksheet() -> None:
+    # Arrange — cert 9418-3062-8205-3566-7200 (Summary_000902.pdf):
+    # **Daikin EDLQ05CAV3 ASHP** (PCDB index 102421 — distinct from
+    # the rest of the cohort's Mitsubishi 104568), end-terrace house
+    # with TWO 1.64 kWp PV arrays (N + S), 210 L cylinder.
+    # `heating_duration_code='24'` per Table N4 (continuous heating).
+    # Worksheet "SAP value" lodges 84.6305.
+    #
+    # Closes the cohort: the final ASHP cert. The only Summary-mapper
+    # gap was the missing "Large" → 4 mapping in
+    # `_ELMHURST_CYLINDER_SIZE_LABEL_TO_SAP10` (Slice S0380.14, this
+    # commit) — multi-array PV + Large-cylinder were the variants
+    # cert 9418 uniquely exercises.
+    pages = _summary_pdf_to_textract_style_pages(_SUMMARY_000902_PDF)
+    site_notes = ElmhurstSiteNotesExtractor(pages).extract()
+    epc = EpcPropertyDataMapper.from_elmhurst_site_notes(site_notes)
+
+    # Act
+    result = calculate_sap_from_inputs(
+        cert_to_inputs(epc, prices=SAP_10_2_SPEC_PRICES)
+    )
+
+    # Assert — ±0.07 ASHP-cohort spec-floor tolerance.
+    worksheet_unrounded_sap = 84.6305
+    assert abs(result.sap_score_continuous - worksheet_unrounded_sap) < _ASHP_COHORT_CHAIN_TOLERANCE
+
+
 def test_summary_3800_full_chain_sap_within_spec_floor_of_worksheet() -> None:
     # Arrange — cert 3800-8515-0922-3398-3563 (Summary_000901.pdf /
     # dr87-0001-000901.pdf) is the third ASHP cohort cert to close on
diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index 743551d6..3b6869c0 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -3375,12 +3375,13 @@ _ELMHURST_GAS_BOILER_FUEL_TYPES: frozenset[str] = frozenset({
 
 # Elmhurst Summary §15.1 "Cylinder Size" labels mapped to the SAP10
 # cascade enum that `domain/sap10_calculator/rdsap/cert_to_inputs.py`
-# `_CYLINDER_SIZE_CODE_TO_LITRES` keys ({3: 160.0, 4: 210.0}). Only the
-# "Medium" lodging is exercised by the cohort (cert 0380); other size
-# labels (Small / Large / Very Large) are deferred until a fixture
-# exercises them.
+# `_CYLINDER_SIZE_CODE_TO_LITRES` keys ({3: 160.0, 4: 210.0}). Exercised
+# by the cohort: "Medium" (cert 0380 et al — 160 L) and "Large" (cert
+# 9418 — 210 L). "Small" and "Very Large" labels are deferred until a
+# fixture exercises them.
 _ELMHURST_CYLINDER_SIZE_LABEL_TO_SAP10: Dict[str, int] = {
     "Medium": 3,
+    "Large": 4,
 }