diff --git a/domain/sap10_calculator/rdsap/cert_to_inputs.py b/domain/sap10_calculator/rdsap/cert_to_inputs.py
index 21b05af9b..a1a8af635 100644
--- a/domain/sap10_calculator/rdsap/cert_to_inputs.py
+++ b/domain/sap10_calculator/rdsap/cert_to_inputs.py
@@ -7108,13 +7108,21 @@ def _primary_loss_override(
         water_heating_code=epc.sap_heating.water_heating_code,
     ):
         return None
+    # Heat-network DHW: either the space main is a network (WHC 901/902/914
+    # inherit its DHW) or the DHW is its own water-only community scheme
+    # (WHC 950/951/952). SAP 10.2 §4.3 (PDF p.24) and Table 3's heat-network
+    # row both scope on the DHW source — "Where hot water is provided by a
+    # heat network" — not on the space main.
+    heat_network_dhw = is_heat_network_main(main) or (
+        epc.sap_heating.water_heating_code in _WATER_HEAT_NETWORK_ONLY_CODES
+    )
     # SAP 10.2 §4 "Heat networks" (PDF p.17 line 1482) pins community-
     # heating primary pipework to "insulated" (p=1.0), overriding the
     # RdSAP §3 age-band default which would otherwise return 0 for
     # pre-2007 stock. See `_HEAT_NETWORK_PIPEWORK_INSULATION_FRACTION`.
     pipework_p = (
         _HEAT_NETWORK_PIPEWORK_INSULATION_FRACTION
-        if is_heat_network_main(main)
+        if heat_network_dhw
         else _pipework_insulation_fraction_table_3(primary_age)
     )
     base = primary_loss_monthly_kwh(
@@ -7126,7 +7134,7 @@ def _primary_loss_override(
         # row applies regardless of the thermostat / separate-timing
         # lodgement (and so is robust to the community-fuel-as-electric
         # collision that would otherwise route DHW to the h=5 row).
-        heat_network=is_heat_network_main(main),
+        heat_network=heat_network_dhw,
     )
     # SAP 10.2 §12.4.4 (PDF p.36-37): for back-boiler combos summer DHW
     # comes from an electric immersion, not from the boiler — the boiler