Treat water-only heat-network primary pipework as insulated 🟥

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

tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py

@@ -1611,6 +1611,74 @@ def test_water_only_heat_network_cylinder_accrues_table_3_primary_loss() -> None
     assert all(m > 0.0 for m in wh_result.primary_loss_monthly_kwh)
 
 
+def test_water_only_heat_network_primary_loss_matches_heat_network_main() -> None:
+    # Arrange — SAP 10.2 Table 3 (PDF p.160): "For heat networks apply the
+    # formula above with p = 1.0 and h = 3 for all months", and §4.3 (p.24)
+    # pins the pipework literal: "Primary circuit loss for INSULATED
+    # pipework". Both scope on the DHW source, so the two ways a dwelling
+    # can take DHW from a heat network must accrue the SAME (59)m:
+    #
+    #   - WHC 901 "from main" where the space main IS the network, and
+    #   - WHC 950, a water-only network behind a warm-air space main.
+    #
+    # Without this the water-only cert falls back to the RdSAP §3 age-band
+    # default (band E → p = 0.0, uninsulated), billing a heat network for
+    # bare primary pipework the spec says is insulated.
+    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,
+    )
+    heat_network_main = MainHeatingDetail(
+        has_fghrs=False,
+        main_fuel_type=20,
+        heat_emitter_type=1,
+        emitter_temperature=1,
+        main_heating_control=2106,
+        main_heating_category=6,
+        sap_main_heating_code=301,
+    )
+
+    def primary_loss_for(
+        main: MainHeatingDetail, water_heating_code: int
+    ) -> tuple[float, ...]:
+        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=[main],
+                water_heating_code=water_heating_code,
+                cylinder_size=2,  # Normal / 110 L
+                cylinder_thermostat="Y",
+            ),
+        )
+        wh_result, _ = _water_heating_worksheet_and_gains(
+            epc=epc,
+            water_efficiency_pct=100.0,
+            is_instantaneous=False,
+            primary_age="E",
+            pcdb_record=None,
+        )
+        assert wh_result is not None
+        return wh_result.primary_loss_monthly_kwh
+
+    # Act — DHW from a water-only network vs DHW from a network space main.
+    water_only = primary_loss_for(warm_air_main, 950)
+    from_network_main = primary_loss_for(heat_network_main, 901)
+
+    # Assert — Table 3's heat-network row keys on the DHW source, not the
+    # space main, so both bill the same insulated p=1.0 / h=3 loss.
+    assert water_only == pytest.approx(from_network_main, abs=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