diff --git a/domain/sap10_calculator/rdsap/cert_to_inputs.py b/domain/sap10_calculator/rdsap/cert_to_inputs.py
index 283b9f8c6..1fc568e03 100644
--- a/domain/sap10_calculator/rdsap/cert_to_inputs.py
+++ b/domain/sap10_calculator/rdsap/cert_to_inputs.py
@@ -1995,16 +1995,15 @@ def _is_community_heating_hw_from_main(epc: EpcPropertyData) -> bool:
 def _heat_network_standing_charge_gbp(
     epc: EpcPropertyData, main: Optional[MainHeatingDetail]
 ) -> Optional[float]:
-    """SAP 10.2 Table 12 note (l) + §C3.2 heat-network standing charge, or
-    None when the dwelling is not on a heat network (caller then falls back
-    to the fuel-based `additional_standing_charges_gbp`).
+    """SAP 10.2 Table 12 note (l) + §C6 heat-network standing charge that
+    REPLACES the dwelling's fuel-based standing, or None when no replacement
+    applies (`_standing_charges_total_gbp` then composes the fuel-based
+    `additional_standing_charges_gbp` itself).
 
     A heat network carries the Table 12 £120/yr standing charge regardless
     of the network fuel — full when the SPACE heating is on the network
-    (§C3.2 "the total standing charge is the normal heat network standing
-    charge"), halved to £60 when ONLY DHW is provided by the heat network
-    (note (l)). This REPLACES the fuel-based gas/off-peak standing for a
-    heat-network main, so it must not be added on top of
+    (§C6 p.55 "the total standing charge is the normal heat network standing
+    charge"). Being a TOTAL, it must not be added on top of
     `additional_standing_charges_gbp` (which would double-count: a
     Summary-path community-gas main lodges Table-32 code 1 and already
     draws the £120 gas standing). Worksheet-validated: simulated case 14
@@ -2013,6 +2012,17 @@ def _heat_network_standing_charge_gbp(
     The API path under-counted this: an EPC community fuel (e.g. 20 = mains
     gas community) is not a Table-32 gas code, so `_is_gas_code` returned
     False and the standing came out £0 — cert 9390 lost the whole £120.
+
+    The £60 branch is reachable ONLY for WHC 914 (`_water_heating_main`
+    returns Main 2 for 914 alone, and 901/902's Main 1 would already have
+    hit the branch above). Main 2 is a MAIN heating system, so it carries a
+    space-heating fraction — meaning the space heating IS also a heat
+    network, which is precisely C6's "unless", pointing at the full £120.
+    #1608 tracks that; it is deliberately left as-is here.
+
+    NOTE: this is NOT the C6 half charge for water-only networks (WHC
+    950/951/952) — that one is additive and lives in
+    `_standing_charges_total_gbp`.
     """
     if is_heat_network_main(main):
         return _HEAT_NETWORK_STANDING_CHARGE_GBP