diff --git a/tests/domain/epc/test_water_heating_overlay.py b/tests/domain/epc/test_water_heating_overlay.py
index 58ba813f7..f71ddcee4 100644
--- a/tests/domain/epc/test_water_heating_overlay.py
+++ b/tests/domain/epc/test_water_heating_overlay.py
@@ -86,6 +86,33 @@ def test_from_main_wood_logs_decodes_to_the_wood_fuel_not_house_coal() -> None:
     assert simulation.heating.water_heating_fuel == 6
 
 
+@pytest.mark.parametrize(
+    ("water_heating_value", "fuel"),
+    [
+        # Biomass (community) — RdSAP fuel 31. Under a coherent community main
+        # the calculator scores DHW from the main, so this is a faithful,
+        # score-neutral relabel off house coal; the fuel is still emitted so an
+        # individual-main dwelling would score biomass, not coal (ADR-0043).
+        ("From main system, biomass (community)", 31),
+        # Dual fuel (mineral + wood) — RdSAP fuel 9.
+        ("From main system, dual fuel (mineral and wood)", 9),
+        # Biodiesel (community) — RdSAP fuel 34.
+        ("From main system, biodiesel (community)", 34),
+    ],
+)
+def test_biomass_family_water_heating_decodes_off_house_coal(
+    water_heating_value: str, fuel: int
+) -> None:
+    # Act
+    simulation = water_heating_overlay_for(water_heating_value, 0)
+
+    # Assert — from main system (901), each to its own RdSAP fuel, not coal 33.
+    assert simulation is not None
+    assert simulation.heating is not None
+    assert simulation.heating.water_heating_code == 901
+    assert simulation.heating.water_heating_fuel == fuel
+
+
 @pytest.mark.parametrize("water_heating_value", ["Unknown", ""])
 def test_unresolvable_water_heating_produces_no_overlay(
     water_heating_value: str,