diff --git a/tests/domain/epc/test_main_heating_system_overlay.py b/tests/domain/epc/test_main_heating_system_overlay.py
index e1b7a90d6..1a55de61d 100644
--- a/tests/domain/epc/test_main_heating_system_overlay.py
+++ b/tests/domain/epc/test_main_heating_system_overlay.py
@@ -6,6 +6,8 @@ calculator reads from the primary system; the overlay is whole-dwelling.
 
 from __future__ import annotations
 
+import pytest
+
 from domain.epc.property_overlays.main_heating_system_overlay import (
     main_heating_overlay_for,
 )
@@ -19,3 +21,41 @@ def test_gas_combi_overlays_the_primary_heating_code() -> None:
     assert simulation is not None
     assert simulation.heating is not None
     assert simulation.heating.sap_main_heating_code == 104
+
+
+@pytest.mark.parametrize(
+    ("main_heating_value", "code"),
+    [
+        ("Gas boiler, regular", 102),
+        ("Gas CPSU", 120),
+        ("Electric storage heaters, fan", 404),
+        ("Direct-acting electric", 191),
+    ],
+)
+def test_heating_archetypes_decode_to_their_sap_codes(
+    main_heating_value: str, code: int
+) -> None:
+    # Act
+    simulation = main_heating_overlay_for(main_heating_value, 0)
+
+    # Assert
+    assert simulation is not None
+    assert simulation.heating is not None
+    assert simulation.heating.sap_main_heating_code == code
+
+
+@pytest.mark.parametrize(
+    "main_heating_value",
+    ["Unknown", "", "Air source heat pump", "Community heating"],
+)
+def test_unresolvable_or_unmodelled_heating_produces_no_overlay(
+    main_heating_value: str,
+) -> None:
+    # Heat pumps (main_heating_index_number) and community heating (community
+    # codes) don't map to a Table 4b sap_main_heating_code yet — no overlay.
+
+    # Act
+    simulation = main_heating_overlay_for(main_heating_value, 0)
+
+    # Assert
+    assert simulation is None