From 64207ea225245da5ee70003a4ff730b6529e30c2 Mon Sep 17 00:00:00 2001
From: Khalim Conn-Kowlessar <kconnkowlessar@gmail.com>
Date: Tue, 14 Jul 2026 23:30:51 +0000
Subject: [PATCH] =?UTF-8?q?Accrue=20Table=203=20primary-circuit=20loss=20f?=
 =?UTF-8?q?or=20water-only=20heat-network=20cylinders=20=F0=9F=9F=A5?=
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Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
---
 .../rdsap/test_cert_to_inputs.py              | 51 +++++++++++++++++++
 1 file changed, 51 insertions(+)

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..96227afaf 100644
--- a/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
+++ b/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
@@ -478,6 +478,57 @@ def test_heat_network_primary_loss_uses_p1_h3_all_months_per_table_3() -> None:
     assert abs(sum(wh_result.primary_loss_monthly_kwh) - expected_primary_kwh) <= 1e-6
 
 
+def test_water_only_heat_network_cylinder_accrues_primary_loss() -> None:
+    # Arrange — DHW from a water-ONLY community scheme (WHC 950) with a
+    # lodged 210 L cylinder, on a heat-network space main (Table 4a 301,
+    # cat 6) — fixture 507644414148's shape. SAP 10.2 §4 "Heat networks"
+    # (PDF p.17 line 1482): "Primary circuit loss for insulated pipework
+    # and cylinderstat should be included (see Table 3)" — the network
+    # feeds the dwelling-side cylinder via primary pipework whichever WHC
+    # lodges it, so the loss applies to 950/951/952 exactly as to
+    # 901/902/914 (pre-fix these fell through to zero). Table 3 heat-
+    # network row: p = 1.0 and h = 3 for all months.
+    main = MainHeatingDetail(
+        has_fghrs=False,
+        main_fuel_type=51,  # Table 12 community heat (from boilers)
+        heat_emitter_type=1,
+        emitter_temperature=1,
+        main_heating_control=2306,
+        main_heating_category=6,
+        sap_main_heating_code=301,
+    )
+    part = make_building_part(construction_age_band="F")
+    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=950,
+            water_heating_fuel=51,
+            cylinder_size=4,  # Large (>170 litres) → 210 L
+            cylinder_insulation_type=1,
+            cylinder_insulation_thickness_mm=50,
+        ),
+    )
+
+    # Act
+    wh_result, _ = _water_heating_worksheet_and_gains(
+        epc=epc,
+        water_efficiency_pct=100.0,
+        is_instantaneous=False,
+        primary_age="F",
+        pcdb_record=None,
+    )
+
+    # Assert — p=1.0, h=3 all months → 365 × 14 × (0.0091×3 + 0.0263).
+    assert wh_result is not None
+    expected_primary_kwh = 365.0 * 14.0 * (0.0091 * 3.0 + 0.0263)
+    assert abs(sum(wh_result.primary_loss_monthly_kwh) - expected_primary_kwh) <= 1e-6
+
+
 def test_dual_main_system_2_costed_at_its_own_fuel_price() -> None:
     # Arrange — SAP 10.2 §10a: main heating system 2's fuel cost (worksheet
     # line (213)) is billed at ITS OWN Table 32 fuel price, separately from