diff --git a/domain/sap10_calculator/rdsap/cert_to_inputs.py b/domain/sap10_calculator/rdsap/cert_to_inputs.py
index 8db55aa89..3daae3c28 100644
--- a/domain/sap10_calculator/rdsap/cert_to_inputs.py
+++ b/domain/sap10_calculator/rdsap/cert_to_inputs.py
@@ -1187,6 +1187,11 @@ _HEAT_NETWORK_DHW_CHARGING_FACTOR_BY_CODE: Final[dict[int, float]] = {
     2306: 1.00, 2308: 1.00, 2309: 1.00, 2310: 1.00, 2312: 1.00, 2314: 1.00,
 }
 
+# SAP 10.2 Table 4c(3) (PDF p.169) worksheet (305a) flat-rate-charging
+# factor — the RdSAP 10 (spec p.59) default for a water-ONLY heat network
+# (WHC 950/951/952), whose own charging method is never lodged.
+_TABLE_4C3_FLAT_RATE_CHARGING_FACTOR: Final[float] = 1.05
+
 
 def _heat_network_space_charging_factor(main: Optional[MainHeatingDetail]) -> float:
     """SAP 10.2 Table 4c(3) (PDF p.169) worksheet (305) — heat-network
@@ -8024,7 +8029,16 @@ def cert_to_inputs(
         # heating only ... distribution loss"). q_generated = q_useful ×
         # DLF, so delivered-per-fuel efficiency = plant_eff / DLF. Fires
         # on the WHC alone — the HW network is independent of the main.
-        water_eff = water_eff / _heat_network_dlf(primary_age)
+        # RdSAP 10 (spec p.59, "Heat Network (HN) supplying — water
+        # heating only"): a non-PCDB water-only network defaults to
+        # "flat-rate charging", so the Table 4c(3) worksheet (305a)
+        # factor 1.05 always applies — the scheme's own charging method
+        # is never lodged (the dwelling's heating-control code describes
+        # the SPACE network's charging, not this one's).
+        water_eff = water_eff / (
+            _heat_network_dlf(primary_age)
+            * _TABLE_4C3_FLAT_RATE_CHARGING_FACTOR
+        )
     is_instantaneous = epc.sap_heating.water_heating_code in _INSTANTANEOUS_WATER_CODES
     # §9a Table 11 secondary fraction — pulled forward of §4 so the
     # post-§8 Equation D1 cascade can derive Q_space = (98c)m × (204)