diff --git a/domain/epc/property_overlays/main_heating_system_overlay.py b/domain/epc/property_overlays/main_heating_system_overlay.py
index f02839105..0ca83c206 100644
--- a/domain/epc/property_overlays/main_heating_system_overlay.py
+++ b/domain/epc/property_overlays/main_heating_system_overlay.py
@@ -92,6 +92,12 @@ _ELECTRICITY_FUEL = 29
 _HOUSE_COAL_FUEL = 33
 _ELECTRIC_ROOM_HEATER_CODES = frozenset(range(691, 702))
 
+# Heat pumps (SAP Table 4a 211-224 wet, 521-527 warm-air) are category 4 and
+# unambiguously electric (natural fuel 29). Modellable on the default code's SPF
+# without a PCDB index (ADR-0041).
+_HEAT_PUMP_CATEGORY = 4
+_HEAT_PUMP_CODES = frozenset(range(211, 225)) | frozenset(range(521, 528))
+
 # 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
@@ -169,6 +175,8 @@ def _category_for(code: int) -> Optional[int]:
     as a room-heater system, not whatever category the replaced system carried."""
     if code in _SOLID_FUEL_ROOM_HEATER_CODES:
         return _ROOM_HEATER_CATEGORY
+    if code in _HEAT_PUMP_CODES:
+        return _HEAT_PUMP_CATEGORY
     return None
 
 
@@ -176,7 +184,7 @@ def _natural_fuel_for(code: int) -> Optional[int]:
     """The fuel an archetype unambiguously implies (a coherent default), or None
     where the fuel is ambiguous and must come from the `main_fuel` override. A
     present `main_fuel` override wins by last-wins composition."""
-    if code in _ELECTRIC_ROOM_HEATER_CODES:
+    if code in _ELECTRIC_ROOM_HEATER_CODES or code in _HEAT_PUMP_CODES:
         return _ELECTRICITY_FUEL
     if code in _SOLID_FUEL_ROOM_HEATER_CODES:
         return _HOUSE_COAL_FUEL
diff --git a/tests/domain/epc/test_main_heating_system_overlay.py b/tests/domain/epc/test_main_heating_system_overlay.py
index 0658bbf65..458311723 100644
--- a/tests/domain/epc/test_main_heating_system_overlay.py
+++ b/tests/domain/epc/test_main_heating_system_overlay.py
@@ -472,10 +472,11 @@ def test_air_source_heat_pump_decodes_to_the_default_ashp_code() -> None:
 
 def test_air_source_heat_pump_drags_its_coherent_companions() -> None:
     # A heat pump is unambiguously electric (natural fuel 29) and SAP Table 4a
-    # category 4, billed on an off-peak (Dual) meter (§12 Rule 3 lists heat pumps
-    # 211-224). The overlay drags these code-derived companions so a system-only
-    # override is self-coherent on a cert that lodged a different system
-    # (ADR-0035 / ADR-0041).
+    # category 4. Unlike storage/CPSU it does NOT imply an off-peak tariff (SAP
+    # §12 Rule 3 is conditional, not forcing), so the coherent default meter is
+    # single-rate — forcing Dual would assume an off-peak split it may not have
+    # and mis-bill it. The overlay drags these code-derived companions so a
+    # system-only override is self-coherent (ADR-0035 / ADR-0041).
 
     # Act
     simulation = main_heating_overlay_for("Air source heat pump", 0)
@@ -486,4 +487,4 @@ def test_air_source_heat_pump_drags_its_coherent_companions() -> None:
     heating = simulation.heating
     assert heating.main_heating_category == 4
     assert heating.main_fuel_type == 29
-    assert heating.meter_type == "Dual"
+    assert heating.meter_type == "Single"