Accrue Table 3 primary-circuit loss for water-only heat-network cylinders 🟥

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py

@@ -1561,6 +1561,56 @@ def test_water_only_heat_network_dhw_fuel_independent_of_plant_type() -> None:
     assert heat_pump == pytest.approx(boilers, abs=1e-6)
 
 
+def test_water_only_heat_network_cylinder_accrues_table_3_primary_loss() -> None:
+    # Arrange — SAP 10.2 §4.3 (PDF p.24) scopes itself "Where hot water is
+    # provided by a heat network", then requires: "Primary circuit loss for
+    # insulated pipework and cylinderstat should be included (see Table 3)."
+    # A water-ONLY network (WHC 950) feeds the dwelling-side cylinder via
+    # primary pipework from the network connection, so the loss applies on
+    # the DHW source alone — whatever the SPACE main is.
+    #
+    # The space main here is warm air (Table 4a code 502, category 9),
+    # mirroring cert 9093 (whc 950 + warm-air main). That main is not a
+    # boiler, so no boiler-category branch can accrue the loss on its
+    # behalf: without a WHC-keyed gate the cylinder silently gets (59)m = 0.
+    part = make_building_part(construction_age_band="E")
+    warm_air_main = MainHeatingDetail(
+        has_fghrs=False,
+        main_fuel_type=26,
+        heat_emitter_type=1,
+        emitter_temperature=1,
+        main_heating_control=2106,
+        main_heating_category=9,
+        sap_main_heating_code=502,
+    )
+    epc = make_minimal_sap10_epc(
+        total_floor_area_m2=_TYPICAL_TFA_M2,
+        habitable_rooms_count=4,
+        country_code="ENG",
+        has_hot_water_cylinder=True,
+        sap_building_parts=[part],
+        sap_heating=make_sap_heating(
+            main_heating_details=[warm_air_main],
+            water_heating_code=950,  # hot-water-only heat network
+            cylinder_size=2,  # Normal / 110 L
+            cylinder_thermostat="Y",
+        ),
+    )
+
+    # Act
+    wh_result, _ = _water_heating_worksheet_and_gains(
+        epc=epc,
+        water_efficiency_pct=100.0,
+        is_instantaneous=False,
+        primary_age="E",
+        pcdb_record=None,
+    )
+
+    # Assert — the primary circuit runs, so (59)m is billed every month.
+    assert wh_result is not None
+    assert all(m > 0.0 for m in wh_result.primary_loss_monthly_kwh)
+
+
 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