Merge pull request #1610 from Hestia-Homes/fix/1606-water-only-c6-half-standing-charge

[1592] Include the C6 half standing charge for water-only heat networks

domain/sap10_calculator/rdsap/cert_to_inputs.py

@@ -1995,16 +1995,15 @@ def _is_community_heating_hw_from_main(epc: EpcPropertyData) -> bool:
 def _heat_network_standing_charge_gbp(
     epc: EpcPropertyData, main: Optional[MainHeatingDetail]
 ) -> Optional[float]:
-    """SAP 10.2 Table 12 note (l) + §C3.2 heat-network standing charge, or
-    None when the dwelling is not on a heat network (caller then falls back
-    to the fuel-based `additional_standing_charges_gbp`).
+    """SAP 10.2 Table 12 note (l) + §C6 heat-network standing charge that
+    REPLACES the dwelling's fuel-based standing, or None when no replacement
+    applies (`_standing_charges_total_gbp` then composes the fuel-based
+    `additional_standing_charges_gbp` itself).
 
     A heat network carries the Table 12 £120/yr standing charge regardless
     of the network fuel — full when the SPACE heating is on the network
-    (§C3.2 "the total standing charge is the normal heat network standing
-    charge"), halved to £60 when ONLY DHW is provided by the heat network
-    (note (l)). This REPLACES the fuel-based gas/off-peak standing for a
-    heat-network main, so it must not be added on top of
+    (§C6 p.55 "the total standing charge is the normal heat network standing
+    charge"). Being a TOTAL, it must not be added on top of
     `additional_standing_charges_gbp` (which would double-count: a
     Summary-path community-gas main lodges Table-32 code 1 and already
     draws the £120 gas standing). Worksheet-validated: simulated case 14
@@ -2013,6 +2012,17 @@ def _heat_network_standing_charge_gbp(
     The API path under-counted this: an EPC community fuel (e.g. 20 = mains
     gas community) is not a Table-32 gas code, so `_is_gas_code` returned
     False and the standing came out £0 — cert 9390 lost the whole £120.
+
+    The £60 branch is reachable ONLY for WHC 914 (`_water_heating_main`
+    returns Main 2 for 914 alone, and 901/902's Main 1 would already have
+    hit the branch above). Main 2 is a MAIN heating system, so it carries a
+    space-heating fraction — meaning the space heating IS also a heat
+    network, which is precisely C6's "unless", pointing at the full £120.
+    #1608 tracks that; it is deliberately left as-is here.
+
+    NOTE: this is NOT the C6 half charge for water-only networks (WHC
+    950/951/952) — that one is additive and lives in
+    `_standing_charges_total_gbp`.
     """
     if is_heat_network_main(main):
         return _HEAT_NETWORK_STANDING_CHARGE_GBP
@@ -2021,6 +2031,49 @@ def _heat_network_standing_charge_gbp(
     return None
 
 
+def _standing_charges_total_gbp(
+    epc: EpcPropertyData,
+    main: Optional[MainHeatingDetail],
+    *,
+    main_fuel_code: Optional[int],
+    water_heating_fuel_code: Optional[int],
+    tariff: Tariff,
+) -> float:
+    """Total annual standing charge (£/yr) for worksheet (251) — the single
+    source of truth for how the heat-network and fuel-based standing charges
+    compose.
+
+    SAP 10.2 §C6 (PDF p.54-55) settles the composition, and phrases its two
+    cases deliberately differently:
+
+    - Space heating on a heat network → "the TOTAL standing charge IS the
+      normal heat network standing charge" — a REPLACEMENT. The dwelling's own
+      fuel standing does not apply on top (see
+      `_heat_network_standing_charge_gbp`).
+    - DHW-only heat network → "INCLUDE one-half of the normal heat network
+      standing charge IN the calculation of fuel costs unless the space heating
+      is also a heat network" — ADDITIVE. Only the full case is worded as a
+      total, so the half stacks on whatever Table 12 note (a) already charges
+      the dwelling for its own space-heating fuel (e.g. £24 off-peak electric
+      for a storage-heater main → £84).
+
+    The DHW-only half keys off the WATER heating code (950 / 951 / 952), not
+    the space main: the space main is by definition NOT a network on this
+    branch, so gating on it (as the £120 branch does) can never fire here.
+    """
+    network_standing = _heat_network_standing_charge_gbp(epc, main)
+    if network_standing is not None:
+        return network_standing
+    fuel_standing = additional_standing_charges_gbp(
+        main_fuel_code=main_fuel_code,
+        water_heating_fuel_code=water_heating_fuel_code,
+        tariff=tariff,
+    )
+    if epc.sap_heating.water_heating_code in _WATER_HEAT_NETWORK_ONLY_CODES:
+        return fuel_standing + _HEAT_NETWORK_STANDING_CHARGE_GBP / 2.0
+    return fuel_standing
+
+
 def _main_heating_efficiency(epc: EpcPropertyData) -> float:
     """SAP 10.2 (206) main system 1 efficiency as a 0..1 fraction.
 
@@ -7680,15 +7733,12 @@ def _fuel_cost(
         table_32_unit_price_p_per_kwh(60) * _PENCE_TO_GBP
     )
 
-    heat_network_standing = _heat_network_standing_charge_gbp(epc, main)
-    standing = (
-        heat_network_standing
-        if heat_network_standing is not None
-        else additional_standing_charges_gbp(
-            main_fuel_code=main_fuel_code,
-            water_heating_fuel_code=water_heating_fuel_code,
-            tariff=tariff,
-        )
+    standing = _standing_charges_total_gbp(
+        epc,
+        main,
+        main_fuel_code=main_fuel_code,
+        water_heating_fuel_code=water_heating_fuel_code,
+        tariff=tariff,
     )
 
     # Worksheet display convention: when a row's kWh is zero (no main 2, no
@@ -8555,16 +8605,16 @@ def cert_to_inputs(
             immersion_high_rate_fraction=_hw_immersion_high_frac,
         )
         _hw_extra_standing = 0.0
-    _heat_network_standing = _heat_network_standing_charge_gbp(epc, main)
     standing_charges_total = (
-        _heat_network_standing
-        if _heat_network_standing is not None
-        else additional_standing_charges_gbp(
+        _standing_charges_total_gbp(
+            epc,
+            main,
             main_fuel_code=_main_fuel_code(main),
             water_heating_fuel_code=_water_heating_fuel_code(epc),
             tariff=_rdsap_tariff(epc),
         )
-    ) + _hw_extra_standing
+        + _hw_extra_standing
+    )
 
     # SAP 10.2 Appendix C §C3.2 (PDF p.51) — heat-network distribution
     # pumping electricity (worksheet (313)/(372)/(472)). None for

tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py

@@ -62,11 +62,13 @@ from domain.sap10_calculator.rdsap.cert_to_inputs import (
     _heat_network_dlf,  # pyright: ignore[reportPrivateUsage]
     _heat_network_factor_fuel_code,  # pyright: ignore[reportPrivateUsage]
     _heat_network_standing_charge_gbp,  # pyright: ignore[reportPrivateUsage]
+    _standing_charges_total_gbp,  # pyright: ignore[reportPrivateUsage]
     _main_fuel_code,  # pyright: ignore[reportPrivateUsage]
     _mev_decentralised_kwh_per_yr_from_cert,  # pyright: ignore[reportPrivateUsage]
     _mev_default_data_iuf,  # pyright: ignore[reportPrivateUsage]
     _TABLE_4G_DEFAULT_MEV_SFP_W_PER_L_PER_S,  # pyright: ignore[reportPrivateUsage]
     dimensions_from_cert,
+    fuel_cost_section_from_cert,
     _table_12_factor_fuel_code,  # pyright: ignore[reportPrivateUsage]
     _table_12a_system_for_main,  # pyright: ignore[reportPrivateUsage]
     _is_electric_main,  # pyright: ignore[reportPrivateUsage]
@@ -9104,6 +9106,174 @@ def test_non_heat_network_main_returns_none_so_caller_uses_fuel_standing() -> No
     assert _heat_network_standing_charge_gbp(epc, main) is None
 
 
+def test_water_only_heat_network_half_charge_adds_to_the_dwellings_fuel_standing() -> None:
+    # Arrange — SAP 10.2 §C6 (PDF p.54): "Include one-half of the normal heat
+    # network standing charge in the calculation of fuel costs unless the space
+    # heating is also a heat network." WHC 950 is a DHW-only heat network, and
+    # this dwelling's space heating is electric storage (SAP 401) — not a
+    # network — so the "unless" does not fire and the half charge applies.
+    #
+    # C6 says "INCLUDE [...] IN the calculation of fuel costs", in deliberate
+    # contrast to the full case's "the TOTAL standing charge IS the normal heat
+    # network standing charge" (next paragraph, p.55). Only the full case is
+    # phrased as a replacement, so the half is ADDITIVE to whatever Table 12
+    # note (a) already charges the dwelling for its own fuel. Note (a): "The
+    # standing charge for off-peak electricity is added to space and water
+    # heating costs where either main heating or hot water uses off-peak
+    # electricity" — a Dual meter + SAP 401 is §12 Rule 2 → 7-hour → Table 32
+    # code 32 = £24/yr.
+    #
+    # Half of the RdSAP10 Table 32 code-51 heat-network row (£120/yr) is £60.
+    # Total £84. Cert 22032926 (22 Gwydyr Mansions) is the corpus instance.
+    from dataclasses import replace
+
+    from domain.sap10_calculator.tables.table_12a import Tariff
+
+    base = _typical_semi_detached_epc()
+    storage_main = MainHeatingDetail(
+        has_fghrs=False,
+        main_fuel_type=29,  # electricity
+        heat_emitter_type=0,
+        emitter_temperature=1,
+        main_heating_control=2401,
+        main_heating_category=7,  # electric storage heaters
+        sap_main_heating_code=401,
+    )
+    epc = replace(
+        base,
+        sap_energy_source=replace(base.sap_energy_source, meter_type="1"),  # Dual
+        sap_heating=make_sap_heating(
+            main_heating_details=[storage_main],
+            water_heating_code=950,  # DHW-only heat network (boilers)
+        ),
+    )
+
+    # Act
+    total = _standing_charges_total_gbp(
+        epc,
+        storage_main,
+        main_fuel_code=29,
+        water_heating_fuel_code=51,  # Table 32 heat-network row
+        tariff=Tariff.SEVEN_HOUR,
+    )
+
+    # Assert — £24 off-peak electric (note (a)) + £60 half network (C6).
+    assert abs(total - 84.0) <= 1e-9
+
+
+def test_water_only_code_on_a_heat_network_space_main_takes_the_full_charge_not_both() -> None:
+    # Arrange — SAP 10.2 §C6's "unless" clause. A water-only WHC (950) can sit
+    # on a cert whose SPACE main is ALSO a heat network (SAP 301). C6 gives the
+    # half "unless the space heating is also a heat network (see next
+    # paragraph)", and that next paragraph (p.55) says "the total standing
+    # charge is the normal heat network standing charge" — £120 flat.
+    #
+    # So the half must NOT stack onto the full charge here: £180 would be the
+    # double-count this branch exists to prevent, and £120 is not additive with
+    # the dwelling's fuel standing either ("the TOTAL standing charge IS").
+    from dataclasses import replace
+
+    from domain.sap10_calculator.tables.table_12a import Tariff
+
+    base = _typical_semi_detached_epc()
+    network_main = MainHeatingDetail(
+        has_fghrs=False,
+        main_fuel_type=20,  # EPC mains gas (community)
+        heat_emitter_type=1,
+        emitter_temperature=1,
+        main_heating_control=2306,
+        main_heating_category=6,
+        sap_main_heating_code=301,  # community boilers — a heat network
+    )
+    epc = replace(
+        base,
+        sap_heating=make_sap_heating(
+            main_heating_details=[network_main],
+            water_heating_code=950,
+            water_heating_fuel=20,
+        ),
+    )
+
+    # Act
+    total = _standing_charges_total_gbp(
+        epc,
+        network_main,
+        main_fuel_code=20,
+        water_heating_fuel_code=20,
+        tariff=Tariff.STANDARD,
+    )
+
+    # Assert — the full £120, not £120 + £60.
+    assert abs(total - 120.0) <= 1e-9
+
+
+def test_water_only_heat_network_half_charge_reaches_the_fuel_cost_worksheet() -> None:
+    # Arrange — the §C6 half charge must reach worksheet (251), not just the
+    # helper. A gas-boiler main (SAP 102, not a network) with WHC 950 keeps the
+    # dwelling on the STANDARD tariff, so `_fuel_cost` returns the real
+    # (240)..(255) result rather than the off-peak zero sentinel.
+    #
+    # Table 12 note (a): "The standing charge for gas fuels is added to space
+    # and water heating costs where the gas fuel is used for space heating" →
+    # Table 32 code 1 = £120. Plus the C6 half (£60) = £180.
+    from dataclasses import replace
+
+    base = _typical_semi_detached_epc()
+    epc = replace(
+        base,
+        sap_heating=make_sap_heating(
+            main_heating_details=[_gas_boiler_detail(sap_main_heating_code=102)],
+            water_heating_code=950,  # DHW-only heat network (boilers)
+        ),
+    )
+
+    # Act
+    fc = fuel_cost_section_from_cert(epc)
+
+    # Assert
+    assert fc is not None
+    assert abs(fc.additional_standing_charges_gbp - 180.0) <= 1e-9
+
+
+def test_water_only_heat_network_half_charge_reaches_the_off_peak_standing_scalar() -> None:
+    # Arrange — cert 22032926 (22 Gwydyr Mansions) in shape: electric storage
+    # main (SAP 401) on a Dual meter + WHC 950. Off-peak certs take the zero
+    # `FuelCostResult` sentinel from `_fuel_cost` and cost via the scalar
+    # `standing_charges_gbp` instead, so the §C6 half must reach BOTH paths —
+    # this is the one the corpus cert actually bills through.
+    #
+    # The lodged `water_heating_fuel` 20 ("mains gas (community)") is NOT a
+    # Table 32 gas code, so note (a) draws no gas standing — the dwelling has
+    # no gas meter, only the Economy-7 electric one. Hence £24 today.
+    from dataclasses import replace
+
+    base = _typical_semi_detached_epc()
+    storage_main = MainHeatingDetail(
+        has_fghrs=False,
+        main_fuel_type=29,  # electricity
+        heat_emitter_type=0,
+        emitter_temperature=1,
+        main_heating_control=2401,
+        main_heating_category=7,  # electric storage heaters
+        sap_main_heating_code=401,
+    )
+    epc = replace(
+        base,
+        sap_energy_source=replace(base.sap_energy_source, meter_type="1"),  # Dual
+        sap_heating=make_sap_heating(
+            main_heating_details=[storage_main],
+            water_heating_code=950,  # DHW-only heat network (boilers)
+            water_heating_fuel=20,  # EPC "mains gas (community)", as lodged
+        ),
+    )
+
+    # Act
+    standing: float = cert_to_inputs(epc).standing_charges_gbp
+
+    # Assert — £24 off-peak electric (Table 12 note (a)) + £60 half (C6).
+    assert abs(standing - 84.0) <= 1e-9
+
+
 def test_mev_default_data_iuf_is_table_329_system_type_10_value() -> None:
     # Arrange / Act — SAP 10.2 Table 4g note 3 (PDF p.176) directs the
     # default SFP to "the appropriate in-use factor for default data from

tests/infrastructure/epc_client/test_sap_accuracy_corpus.py

@@ -367,7 +367,26 @@ _MIN_WITHIN_HALF_SAP = 0.788
 # 100031768368 SAP 59.12 -> 61.21 (lodged 61, now within 0.5); 217091901
 # 60.82 -> 61.59 (lodged 62, now an exact match). 14 corpus certs lodge
 # exactly 280mm solid brick. Unit-pinned in test_rdsap_uvalues.
-_MAX_SAP_MAE = 0.628
+# MAE 0.627 -> 0.625 (within-0.5 held at 78.8%) via the SAP 10.2 §C6
+# water-only heat-network standing charge: "Include one-half of the normal
+# heat network standing charge in the calculation of fuel costs unless the
+# space heating is also a heat network" (p.54). The half was gated on the
+# SPACE main being a network, which WHC 950/951/952 never are by
+# definition, so it could not fire. C6 phrases the half as "INCLUDE [...]
+# IN the calculation of fuel costs" against the full case's "the TOTAL
+# standing charge IS", so it is ADDITIVE to the dwelling's own Table 12
+# note (a) fuel standing rather than replacing it.
+# Blast radius is 1 cert: of the three WHC-950 certs, only 22032926 (22
+# Gwydyr Mansions, electric storage main 401) has a non-network space
+# main. £24 Economy-7 -> £24 + £60 = £84; SAP 63.737 -> 60.328 (lodged 61),
+# |err| 2.737 -> 0.672. The other two (mains 304 / 301) are on network
+# space mains and correctly keep the full £120 via the first branch.
+# NOTE: this lands on a spec argument, not an oracle — there is no
+# accredited Elmhurst worksheet for a water-only network yet (#1592-C).
+# The -0.672 overshoot is expected to move again when the C4 plant-
+# efficiency and C3.2 pumping siblings land. Unit-pinned in
+# test_cert_to_inputs.
+_MAX_SAP_MAE = 0.626
 _MAX_CO2_MAE_TONNES = 0.072     # t CO2 / yr vs co2_emissions_current
 _MAX_PE_PER_M2_MAE = 3.0        # kWh / m2 / yr vs energy_consumption_current