diff --git a/domain/epc/property_overlays/main_heating_system_overlay.py b/domain/epc/property_overlays/main_heating_system_overlay.py
index b8ea16c0d..957ce94da 100644
--- a/domain/epc/property_overlays/main_heating_system_overlay.py
+++ b/domain/epc/property_overlays/main_heating_system_overlay.py
@@ -98,6 +98,16 @@ _ELECTRIC_ROOM_HEATER_CODES = frozenset(range(691, 702))
 _HEAT_PUMP_CATEGORY = 4
 _HEAT_PUMP_CODES = frozenset(range(211, 225)) | frozenset(range(521, 528))
 
+# Community / heat-network heating (SAP Table 4a 301-304) is category 6; the
+# calculator's `_is_heat_network` keys off code OR category 6. The boiler-driven
+# schemes (301/302/303) are dominantly mains gas (community) — RdSAP main_fuel 20
+# — per real community certs; the heat-pump scheme (304) is electricity
+# (community), added with its archetype (ADR-0041).
+_HEAT_NETWORK_CATEGORY = 6
+_HEAT_NETWORK_CODES = frozenset({301, 302, 303, 304})
+_COMMUNITY_BOILER_CODES = frozenset({301, 302, 303})
+_COMMUNITY_GAS_FUEL = 20
+
 # SAP Table 4c full boiler-control code: programmer + room thermostat + TRVs. The
 # landlord names the boiler, not its controls — but a gas boiler installed under
 # modern Building Regs must carry compliant controls, and this overlay already
@@ -182,6 +192,8 @@ def _category_for(code: int) -> Optional[int]:
         return _ROOM_HEATER_CATEGORY
     if code in _HEAT_PUMP_CODES:
         return _HEAT_PUMP_CATEGORY
+    if code in _HEAT_NETWORK_CODES:
+        return _HEAT_NETWORK_CATEGORY
     return None
 
 
@@ -193,6 +205,8 @@ def _natural_fuel_for(code: int) -> Optional[int]:
         return _ELECTRICITY_FUEL
     if code in _SOLID_FUEL_ROOM_HEATER_CODES:
         return _HOUSE_COAL_FUEL
+    if code in _COMMUNITY_BOILER_CODES:
+        return _COMMUNITY_GAS_FUEL
     return None