diff --git a/domain/sap10_calculator/rdsap/cert_to_inputs.py b/domain/sap10_calculator/rdsap/cert_to_inputs.py
index d6e2db76..efc2d20a 100644
--- a/domain/sap10_calculator/rdsap/cert_to_inputs.py
+++ b/domain/sap10_calculator/rdsap/cert_to_inputs.py
@@ -3638,6 +3638,24 @@ def cert_to_inputs(
             main_category=water_main.main_heating_category if water_main is not None else None,
             main_fuel=_main_fuel_code(water_main),
         )
+    # SAP 10.2 Table 4c row "No boiler interlock — regular boiler:
+    # DHW −5%" (PDF p.169). RdSAP §3 (PDF p.57) defines boiler
+    # interlock as "Assumed present if there is a room thermostat and
+    # (for stored hot water systems heated by the boiler) a cylinder
+    # thermostat. Otherwise not interlocked." A combi-fed cylinder
+    # routes the boiler as a regular boiler for the DHW circuit (the
+    # combi's instantaneous-DHW capability is bypassed), so the
+    # regular-boiler row applies. Note c): the adjustment caps at
+    # −5pp (no thermostatic control and no boiler interlock do not
+    # accumulate). Cert 000565 (cylinder lodged + cyl-stat absent +
+    # WHC 914 to PCDB combi Main 2) closes 79% → 74% — matches
+    # worksheet (217)m exactly.
+    if (
+        epc.has_hot_water_cylinder
+        and epc.sap_heating.cylinder_thermostat != "Y"
+        and water_pcdb_main is not None
+    ):
+        water_eff -= 0.05
     # SAP 10.2 Appendix N3.6 + N3.7(a) — when an HP cert lodges a PCDB
     # Table 362 record, the cascade replaces the Table 4a defaults with
     # APM-interpolated η_space and η_water at the dwelling's PSR.
diff --git a/domain/sap10_calculator/rdsap/tests/test_cert_to_inputs.py b/domain/sap10_calculator/rdsap/tests/test_cert_to_inputs.py
index be6ca923..21aa00ef 100644
--- a/domain/sap10_calculator/rdsap/tests/test_cert_to_inputs.py
+++ b/domain/sap10_calculator/rdsap/tests/test_cert_to_inputs.py
@@ -1802,6 +1802,65 @@ def test_cert_with_hot_water_cylinder_computes_primary_loss_59m_from_sap_table_3
     )
 
 
+def test_table_4c_no_boiler_interlock_applies_minus_5_dhw_adjustment_when_cylinder_lodged_without_thermostat() -> None:
+    """SAP 10.2 Table 4c (PDF p.169-170) "Efficiency adjustments":
+
+        (2) Efficiency adjustment due to control system     Space   DHW
+            No boiler interlock - regular boiler            -5      -5
+            No boiler interlock - combi                     -5       0
+
+    Note c): "These do not accumulate as no thermostatic control or
+    presence of a bypass means that there is no boiler interlock."
+
+    RdSAP 10 §3 (PDF p.57) "Boiler interlock" definition:
+        Assumed present if there is a room thermostat and (for stored
+        hot water systems heated by the boiler) a cylinder thermostat.
+        Otherwise not interlocked.
+
+    A boiler feeding a hot-water cylinder is in the "stored hot water
+    systems heated by the boiler" category — when no cylinder
+    thermostat is lodged, boiler interlock is absent and Table 4c
+    applies -5 percentage points to the DHW seasonal efficiency. This
+    holds regardless of whether the boiler is a combi or regular type;
+    in a combi-fed-cylinder configuration the combi acts as a regular
+    boiler for the DHW circuit (instantaneous DHW capability is
+    bypassed by the cylinder routing), so the regular-boiler row of
+    Table 4c (-5%) applies.
+
+    Cert 000565 (ASHP Main 1 + Vaillant PCDB 15100 combi Main 2 + WHC
+    914 + 160 L cylinder + cylinder thermostat absent) reproduces the
+    pattern. PCDB summer η = 79.0; worksheet (217)m = 74.0; the
+    cascade was returning 79% (PCDB summer unchanged), driving the
+    -238 kWh HW pin over-shoot that survived S0380.79.
+
+    Predicted HW closure: 2778.72 / 0.74 = 3754.99 vs worksheet
+    3755.03 — within 0.04 kWh.
+    """
+    # Arrange — use the real cert 000565 fixture (Elmhurst extractor +
+    # mapper) so the (62)m demand cascade is the worksheet-pinned
+    # tuple and only the (217)m efficiency step is under test.
+    from domain.sap10_calculator.worksheet.tests._elmhurst_worksheet_000565 import (
+        build_epc as build_cert_000565,
+    )
+    from domain.sap10_calculator.calculator import calculate_sap_from_inputs
+    epc = build_cert_000565()
+
+    # Act
+    result = calculate_sap_from_inputs(cert_to_inputs(epc))
+
+    # Assert — cert 000565 worksheet line (219) sum = 3755.0288 (HW
+    # fuel kWh/yr). Pre-fix: 3517.37 (off by −238 from (64)/0.79).
+    # With Table 4c -5% applied: (64)/0.74 = 2778.72/0.74 = 3754.99,
+    # closing to worksheet 3755.03 within 0.04 kWh.
+    expected_hw_kwh = 3755.0288
+    assert abs(result.hot_water_kwh_per_yr - expected_hw_kwh) < 0.1, (
+        f"cert 000565 hot_water_kwh_per_yr: got "
+        f"{result.hot_water_kwh_per_yr!r}, want {expected_hw_kwh!r} per "
+        f"SAP 10.2 Table 4c (No boiler interlock — regular boiler: "
+        f"DHW −5%); RdSAP §3 (no cylinder thermostat → no boiler interlock)"
+    )
+
+
 def test_cylinder_storage_loss_applies_57m_solar_adjustment_per_sap_4_line_7693() -> None:
     """SAP 10.2 §4 line 7693 (PDF p.137):