fix(water-heating): heat-network primary loss uses Table 3 h=3 all months

SAP 10.2 Table 3 (PDF p.160) verbatim: "For heat networks apply the
formula above with p = 1.0 and h = 3 for all months." The primary
circulation hours for a heat-network main are fixed at h=3 winter and
summer, independent of the cylinder-thermostat / separate-timing
lodgement that selects the h=5/h=11 rows for boiler systems.

`primary_loss_monthly_kwh` / `primary_circuit_hours_per_day_table_3` gain
a `heat_network` flag (→ (3, 3)); `_primary_loss_override` passes
`_is_heat_network_main(main)`. p=1.0 was already pinned via
`_HEAT_NETWORK_PIPEWORK_INSULATION_FRACTION`; only the hours were wrong.

Before, cert 8536 routed through the h=5/3 row because its community
biomass DHW fuel (31) collides with electricity code 31, so
`_separately_timed_dhw` returned False. The Table 3 heat-network rule
overrides that path: 8536 primary loss (59) 335.81 → 273.90, EXACT to
the faithful case-32 worksheet (storage (56) 376.58 also matches 376.94).

API eval within-0.5 57.0% → 56.9% (one offsetting-error cert crosses
out; signed err −0.205 → −0.202). Applied spec-uniformly per the
determinism principle — the heat-network primary hours are unambiguous.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

domain/sap10_calculator/rdsap/cert_to_inputs.py

@@ -6154,6 +6154,12 @@ def _primary_loss_override(
         pipework_insulation_fraction=pipework_p,
         has_cylinder_thermostat=epc.sap_heating.cylinder_thermostat == "Y",
         separately_timed_dhw=_separately_timed_dhw(epc, main),
+        # 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." The h=3
+        # row applies regardless of the thermostat / separate-timing
+        # lodgement (and so is robust to the community-fuel-as-electric
+        # collision that would otherwise route DHW to the h=5 row).
+        heat_network=_is_heat_network_main(main),
     )
     # SAP 10.2 §12.4.4 (PDF p.36-37): for back-boiler combos summer DHW
     # comes from an electric immersion, not from the boiler — the boiler

domain/sap10_calculator/worksheet/water_heating.py

@@ -550,13 +550,21 @@ def primary_circuit_hours_per_day_table_3(
     *,
     has_cylinder_thermostat: bool,
     separately_timed_dhw: bool,
+    heat_network: bool = False,
 ) -> tuple[float, float]:
-    """SAP 10.2 Table 3 (PDF p.159) — hours of primary circulation per
+    """SAP 10.2 Table 3 (PDF p.159-160) — hours of primary circulation
-    day, returned as `(winter_hours, summer_hours)`:
+    per day, returned as `(winter_hours, summer_hours)`:
         no thermostat                       → (11, 3)
         thermostat, not separately timed    → ( 5, 3)
         thermostat, separately timed        → ( 3, 3)
+
+    SAP 10.2 Table 3 (PDF p.160) verbatim: "For heat networks apply the
+    formula above with p = 1.0 and h = 3 for all months." So a heat-
+    network main uses h=3 winter and summer regardless of the cylinder-
+    thermostat / separate-timing lodgement.
     """
+    if heat_network:
+        return (3.0, 3.0)
     if not has_cylinder_thermostat:
         return (11.0, 3.0)
     if separately_timed_dhw:
@@ -569,6 +577,7 @@ def primary_loss_monthly_kwh(
     pipework_insulation_fraction: float,
     has_cylinder_thermostat: bool,
     separately_timed_dhw: bool,
+    heat_network: bool = False,
 ) -> tuple[float, ...]:
     """SAP 10.2 §4 line (59)m via Table 3 (PDF p.159):
         (59)m = n_m × 14 × [{0.0091 × p + 0.0245 × (1 − p)} × h + 0.0263]
@@ -576,6 +585,10 @@ def primary_loss_monthly_kwh(
     hours of primary circulation per day (winter / summer split per
     `primary_circuit_hours_per_day_table_3`).
 
+    `heat_network=True` selects the SAP 10.2 Table 3 (PDF p.160) heat-
+    network row — h=3 for all months — regardless of the cylinder-
+    thermostat / separate-timing lodgement.
+
     Returns 12 monthly values in calendar order Jan..Dec. Callers must
     gate this helper on the spec's zero-loss configurations
     (combi boilers, integral-vessel HPs, CPSUs, thermal stores ≤ 1.5 m
@@ -587,6 +600,7 @@ def primary_loss_monthly_kwh(
     winter_h, summer_h = primary_circuit_hours_per_day_table_3(
         has_cylinder_thermostat=has_cylinder_thermostat,
         separately_timed_dhw=separately_timed_dhw,
+        heat_network=heat_network,
     )
     return tuple(
         n * 14.0 * (

tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py

@@ -317,6 +317,54 @@ def test_heat_network_dhw_no_cylinder_applies_sap_10_2_hiu_default_store() -> No
     assert abs(sum(wh_result.solar_storage_monthly_kwh) - expected_storage_kwh) <= 1.0
 
 
+def test_heat_network_primary_loss_uses_p1_h3_all_months_per_table_3() -> None:
+    # Arrange — heat-network DHW (Table 4a code 301, cat 6, WHC 901) with
+    # the SAP p.24 HIU default store applied. SAP 10.2 Table 3 (PDF p.160)
+    # verbatim: "For heat networks apply the formula above with p = 1.0 and
+    # h = 3 for all months." So the primary loss is independent of the
+    # cylinder-thermostat / separately-timed hours (which would give h=5/3)
+    # and equals Σ n_m × 14 × (0.0091×1.0×3 + 0.0263) = 273.9 kWh/yr.
+    # Community biomass fuel (31) collides with electricity code 31, so
+    # `_separately_timed_dhw` would route to h=5/3 — but the Table 3 heat-
+    # network rule must override that to h=3 regardless of the DHW fuel.
+    # This reproduces cert 8536-0929-6500-0815-7206.
+    main = MainHeatingDetail(
+        has_fghrs=False,
+        main_fuel_type=31,
+        heat_emitter_type=1,
+        emitter_temperature=1,
+        main_heating_control=2106,
+        main_heating_category=6,
+        sap_main_heating_code=301,
+    )
+    part = make_building_part(construction_age_band="A")
+    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=[main],
+            water_heating_code=901,
+            water_heating_fuel=31,
+        ),
+    )
+
+    # Act
+    wh_result, _ = _water_heating_worksheet_and_gains(
+        epc=_apply_heat_network_hiu_default_store(epc),
+        water_efficiency_pct=100.0,
+        is_instantaneous=False,
+        primary_age="A",
+        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_heat_network_distribution_electricity_per_sap_10_2_appendix_c_3_2() -> None:
     # Arrange — heat-network main (Table 4a code 301 = community heating,
     # category 6). SAP 10.2 Appendix C §C3.2 (PDF p.51): distribution