diff --git a/tests/domain/epc/test_main_heating_system_overlay.py b/tests/domain/epc/test_main_heating_system_overlay.py
index 458311723..7e322181e 100644
--- a/tests/domain/epc/test_main_heating_system_overlay.py
+++ b/tests/domain/epc/test_main_heating_system_overlay.py
@@ -488,3 +488,19 @@ def test_air_source_heat_pump_drags_its_coherent_companions() -> None:
     assert heating.main_heating_category == 4
     assert heating.main_fuel_type == 29
     assert heating.meter_type == "Single"
+
+
+def test_community_boilers_decode_to_the_heat_network_boiler_code() -> None:
+    # A community/heat-network scheme driven by boilers is SAP Table 4a code 301
+    # (the calculator derives the heat-source efficiency + DLF from it). It must
+    # decode to 301, not the Gas CPSU (120) the under-populated taxonomy forced —
+    # a single-dwelling gas wet boiler is the wrong picture for a heat network
+    # (ADR-0041). A generic "Community heating" with no named source stays None.
+
+    # Act
+    simulation = main_heating_overlay_for("Community heating, boilers", 0)
+
+    # Assert — SAP Table 4a code 301 (boiler-driven community heating).
+    assert simulation is not None
+    assert simulation.heating is not None
+    assert simulation.heating.sap_main_heating_code == 301