diff --git a/backend/documents_parser/tests/test_heating_systems_corpus.py b/backend/documents_parser/tests/test_heating_systems_corpus.py
index 60f363f8..ab7889e4 100644
--- a/backend/documents_parser/tests/test_heating_systems_corpus.py
+++ b/backend/documents_parser/tests/test_heating_systems_corpus.py
@@ -740,6 +740,24 @@ _EXPECTATIONS: tuple[_CorpusExpectation, ...] = (
     # and PE −249.32→−0.0000 (× (1.5569−1.501)) now EXACT. Non-electric
     # heat networks (CH1 gas 51, CH6 coal 54) have no monthly factor set
     # → unchanged.
+    #
+    # CH6 — PROVEN PIN-FOREVER (Summary-export gap, not a mapper miss).
+    # CH6's P960 *worksheet input* lodges Distribution Loss = "Two
+    # adjoining dwellings sharing a single heating system" → Value 0.0 →
+    # (306) DLF = 1.0000, whereas CH4 lodges "Calculated" → 1.5 → (306) =
+    # 1.4500. That DLF choice swings SAP / cost / CO2 / PE materially.
+    # But it is NOT in the Summary PDF: a controlled pair differing ONLY
+    # by the adjoining-dwellings setting (`CH adjoined dwellings/Summary_
+    # 001431 (1) vs (2).pdf`) is byte-identical across every RdSAP INPUT
+    # field — the two Summaries differ solely in the derived header
+    # (SAP 80 vs 75, bill £954 vs £1237, emissions 5.407 vs 7.394 t). A
+    # case-insensitive scan of the CH6 Summary for "distribution"/"adjoin"
+    # returns 0 hits. Since CH4 and CH6 Summaries are themselves identical
+    # bar fuel type, no Summary-derivable rule can yield CH4=1.45 AND
+    # CH6=1.0. Closing CH6 would require the P960 worksheet as a mapper
+    # input or an Elmhurst Summary-export change — neither is available.
+    # Pin held; do not re-litigate (verified 2026-06-02 with the
+    # user-supplied adjoining-dwellings pair).
     _CorpusExpectation(variant='community heating 1', block='11b', expected_sap_resid=+0.0000, expected_cost_resid_gbp=-0.0000, expected_co2_resid_kg=-0.0000, expected_pe_resid_kwh=+0.0000),
     _CorpusExpectation(variant='community heating 2', block='11b', expected_sap_resid=-0.0000, expected_cost_resid_gbp=-0.0000, expected_co2_resid_kg=+0.0000, expected_pe_resid_kwh=+0.0000),
     _CorpusExpectation(variant='community heating 3', block='11b', expected_sap_resid=+0.0000, expected_cost_resid_gbp=-0.0000, expected_co2_resid_kg=+0.0000, expected_pe_resid_kwh=-0.0000),