diff --git a/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py b/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
index b643bf0d..6b9daf5a 100644
--- a/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
+++ b/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
@@ -54,6 +54,7 @@ from domain.sap10_calculator.rdsap.cert_to_inputs import (
     _apply_heat_network_hiu_default_store,  # pyright: ignore[reportPrivateUsage]
     _cylinder_thermostat_present,  # pyright: ignore[reportPrivateUsage]
     _has_suspended_timber_floor_per_spec,  # pyright: ignore[reportPrivateUsage]
+    _heat_pump_apm_efficiencies,  # pyright: ignore[reportPrivateUsage]
     _heat_network_code_302_effective_factor,  # pyright: ignore[reportPrivateUsage]
     _heat_network_community_fuel_code,  # pyright: ignore[reportPrivateUsage]
     _heat_network_distribution_electricity,  # pyright: ignore[reportPrivateUsage]
@@ -4841,6 +4842,57 @@ def test_hot_water_from_pcdb_heat_pump_bills_at_app_n_wh_high_rate() -> None:
     assert abs(rate_immersion - 0.0750) <= 1e-6
 
 
+def test_heat_pump_water_efficiency_is_floored_at_100pct_per_app_n3_7() -> None:
+    # Arrange — SAP 10.2 Appendix N3.7 ("Thermal efficiency for water
+    # heating – heat pumps", PDF p.109): "multiply the thermal efficiency
+    # (ηwater) for water heating by the in-use factor in Table N8; subject
+    # to a MINIMUM EFFICIENCY OF 100%." Our `_heat_pump_apm_efficiencies`
+    # applied the in-use factor but omitted the floor, so an oversized heat
+    # pump whose PSR-extended ηwater × 0.60 in-use fell below 100% billed
+    # water heating at that sub-100% efficiency (over-counting HW fuel).
+    #
+    # Accredited anchor: golden fixture case 56 (PCDB 100061, the config of
+    # cert 100110101713). At HLC 107.82 W/K the PSR is 3.107, above the
+    # record's largest PSR 2.0, so the Appendix N2 extension takes ηwater,3
+    # from 198.9% toward 100% at 2 x 2.0 = 4.0 → 128.55%; × the 0.60 in-use
+    # factor (Open-EPC certs never lodge cylinder HX area → criteria fail)
+    # = 77.13% < 100% → the worksheet (216) reads 100.0000. In-range PSR
+    # (case 54, HLC large) keeps 0.60 × 198.9 = 119.34% (worksheet case 54
+    # (216) = 112.5% for its 187.5% record — both above the floor, unchanged).
+    from domain.sap10_calculator.tables.pcdb import heat_pump_record
+
+    record = heat_pump_record(100061)
+    assert record is not None
+    hp_main = MainHeatingDetail(
+        has_fghrs=False,
+        main_fuel_type=29,  # electricity (heat pump)
+        heat_emitter_type=1,  # radiators
+        emitter_temperature=0,
+        main_heating_control=2210,
+        main_heating_category=4,
+        sap_main_heating_code=None,
+        main_heating_index_number=100061,
+    )
+    epc = _typical_semi_detached_epc()  # no specified cylinder → in-use 0.60
+
+    # Act — oversized PSR (extension region) vs an in-range PSR.
+    _space_ext, water_ext = _heat_pump_apm_efficiencies(
+        main=hp_main, hp_record=record,
+        hlc_annual_avg_w_per_k=107.82,  # PSR 3.107 > largest 2.0
+        epc=epc,
+    ) or (None, None)
+    _space_in, water_in = _heat_pump_apm_efficiencies(
+        main=hp_main, hp_record=record,
+        hlc_annual_avg_w_per_k=400.0,  # PSR 0.837, in range
+        epc=epc,
+    ) or (None, None)
+
+    # Assert — extended HP water efficiency is floored at 100% (1.0); the
+    # in-range PSR keeps the un-floored 0.60 × 198.9% = 119.34%.
+    assert water_ext is not None and abs(water_ext - 1.0) < 1e-9
+    assert water_in is not None and abs(water_in - 0.60 * 198.9 / 100.0) < 1e-9
+
+
 def test_hot_water_immersion_off_peak_bills_at_table_13_blend() -> None:
     # Arrange — SAP 10.2 Table 12a (PDF p.191) "Immersion water heater"
     # row routes the WH column to Table 13 (PDF p.197). For an electric