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 f0a2e14ac..20333b032 100644
--- a/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
+++ b/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
@@ -1449,6 +1449,62 @@ def test_hot_water_only_heat_network_whc_950_applies_table12c_dlf() -> None:
     assert abs(hw_network / baseline - 1.41) <= 0.02
 
 
+def test_water_only_heat_network_dhw_prices_heat_without_plant_efficiency() -> None:
+    # Arrange — SAP 10.2 §4.3 (PDF p.24): "The efficiency for water heating
+    # is incorporated in the price of heat from a heat network in Table 12."
+    # The dwelling buys DELIVERED heat at the Table 12 fuel-51 rate, so the
+    # RdSAP Table 4a plant efficiency (950 → 0.80) must NOT also divide on
+    # the dwelling side — worksheet (310) is "(64) × (305a) × (306)", with
+    # no efficiency term. C4 (p.53) scopes plant efficiency explicitly: "For
+    # heat networks not listed in the PCDB, FOR THE CALCULATION OF CO2
+    # EMISSIONS AND PRIMARY ENERGY, the efficiency to be used is that of the
+    # heat generator" — i.e. never for delivered energy.
+    #
+    # (305a) is the Table 4c(3) "DHW-only system, flat rate charging" row =
+    # 1.05 (p.169); RdSAP 10 (spec p.59) defaults a non-PCDB water-only
+    # network to flat-rate charging. (306) is the age-E Table 12c DLF = 1.41.
+    #
+    # So HW fuel = q_useful × 1.41 × 1.05, i.e. water_eff = 1/(DLF × 1.05).
+    # Asserted as fuel ÷ q_useful (the §4 output at 100% efficiency) to pin
+    # the efficiency absolutely, mirroring the WHC-901 heat-network test
+    # above. The main is an ordinary gas boiler — NOT a heat network — since
+    # §4.3 keys on the DHW source ("Where hot water is provided by a heat
+    # network"), not on the space main.
+    part = make_building_part(construction_age_band="E")
+    gas_main = MainHeatingDetail(
+        has_fghrs=False,
+        main_fuel_type=26,
+        heat_emitter_type=1,
+        emitter_temperature=1,
+        main_heating_control=2106,
+        main_heating_category=2,
+    )
+    hw_network_epc = make_minimal_sap10_epc(
+        total_floor_area_m2=_TYPICAL_TFA_M2,
+        habitable_rooms_count=4,
+        country_code="ENG",
+        sap_building_parts=[part],
+        sap_heating=make_sap_heating(
+            main_heating_details=[gas_main],
+            water_heating_code=950,  # hot-water-only heat network (boilers)
+        ),
+    )
+
+    # Act
+    hw_fuel: float = cert_to_inputs(hw_network_epc).hot_water_kwh_per_yr
+    wh_result, _ = _water_heating_worksheet_and_gains(
+        epc=hw_network_epc,
+        water_efficiency_pct=100.0,
+        is_instantaneous=False,
+        primary_age="E",
+        pcdb_record=None,
+    )
+
+    # Assert — delivered HW fuel = q_useful × (305a) × (306); no 1/0.80.
+    assert wh_result is not None
+    assert abs(hw_fuel / wh_result.output_kwh_per_yr - 1.41 * 1.05) <= 1e-6
+
+
 def test_gas_boiler_main_efficiency_unchanged_by_dlf_override() -> None:
     # Arrange — regression check: the DLF override only fires for heat-
     # network main heating. A standard gas boiler (cat=2, code=102) must