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 96227afaf..4149fe9b7 100644
--- a/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
+++ b/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
@@ -1460,7 +1460,12 @@ def test_hot_water_only_heat_network_whc_950_applies_table12c_dlf() -> None:
     # gas boiler (NOT a heat network), mirroring cert 9093 (whc 950 + a
     # warm-air main). Compare against a non-heat-network baseline at the
     # same 0.80 water efficiency (whc 901 from the same gas main): the 950
-    # HW fuel must exceed it by exactly the DLF (age E → 1.41).
+    # HW fuel must exceed it by the DLF (age E → 1.41) × the Table 4c(3)
+    # charging factor. RdSAP 10 (spec p.59, "Heat Network (HN) supplying
+    # — water heating only"): a non-PCDB water-only network defaults to
+    # "flat-rate charging" — worksheet (305a) factor 1.05 — since the
+    # scheme's own charging method is never lodged (the dwelling's
+    # heating-control code describes the SPACE network, not this one).
     part = make_building_part(construction_age_band="E")
     gas_main = MainHeatingDetail(
         has_fghrs=False,
@@ -1496,8 +1501,9 @@ def test_hot_water_only_heat_network_whc_950_applies_table12c_dlf() -> None:
     hw_network: float = cert_to_inputs(hw_network_epc).hot_water_kwh_per_yr
     baseline: float = cert_to_inputs(baseline_epc).hot_water_kwh_per_yr
 
-    # Assert — the HW-only-heat-network fuel is scaled up by the age-E DLF.
-    assert abs(hw_network / baseline - 1.41) <= 0.02
+    # Assert — the HW-only-heat-network fuel is scaled up by the age-E DLF
+    # × the flat-rate-charging default (1.41 × 1.05 = 1.4805).
+    assert abs(hw_network / baseline - 1.41 * 1.05) <= 0.02
 
 
 def test_gas_boiler_main_efficiency_unchanged_by_dlf_override() -> None: