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fix(water-heating): heat-network DHW with no cylinder uses SAP 10.2 HIU default store, not combi keep-hot
A heat-network main with DHW from the network and no lodged cylinder was billed the Table 3a keep-hot 600 kWh/yr combi loss (cat 6 sat in `_TABLE_3A_COMBI_LOSS_MAIN_HEATING_CATEGORIES`). A heat network is not a combi boiler — SAP 10.2 §4 line 7702 says combi loss is 0 for non-combi systems. SAP 10.2 p.24 "Heat networks" (c): when neither a PCDB Heat Interface Unit nor a lodged cylinder applies, "a measured loss of 1.72 kWh/day should be used, corrected using Table 2b. This is equivalent to a cylinder of 110 litres and a factory insulation thickness of 50 mm". RdSAP 10 Table 29 (p.56): a cylinder thermostat is assumed present when DHW is from a heat network (Table 2b temperature factor 0.60). New `_apply_heat_network_hiu_default_store` rebinds the 110 L / 50 mm- factory store (thermostat present) onto a heat-network DHW cert with no cylinder and no PCDB index, mirroring `_apply_rdsap_no_water_heating_ system_default`. The injected store routes storage loss (56) ≈ 376.7 kWh/yr (= 1.72 × 0.60 × 365) + primary loss (59) through the existing machinery and zeroes the combi (61) loss via the has_hot_water_cylinder gate. Verified against the user's faithful case-32 worksheet: storage (56) 376.58 vs worksheet 376.94. Cert 8536 storage 0→376.6, combi 600→0. API eval within-0.5 56.8% → 57.0%; signed err −0.218 → −0.205. Reworked `test_heat_network_main_with_hw_from_main_dlf_scales_hot_water_kwh` to assert the DLF scaling directly (fuel ÷ §4 output = 1.41) since the old two-cert baseline premise (both combi-600) no longer holds. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
This commit is contained in:
Khalim Conn-Kowlessar
parent
ba56647401
commit
e6543c76ca
2 changed files with 123 additions and 35 deletions
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@ -5151,6 +5151,56 @@ def _apply_rdsap_no_water_heating_system_default(
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return replace(epc, has_hot_water_cylinder=True, sap_heating=sap_heating)
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# SAP 10.2 p.24 "Heat networks" (c): the default storage loss for heat-
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# network DHW with no PCDB HIU and no lodged cylinder is "equivalent to a
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# cylinder of 110 litres and a factory insulation thickness of 50 mm".
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_HEAT_NETWORK_HIU_DEFAULT_INSULATION_MM: Final[int] = 50
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def _apply_heat_network_hiu_default_store(
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epc: EpcPropertyData,
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) -> EpcPropertyData:
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"""SAP 10.2 p.24 "Heat networks" (c) — when domestic hot water is
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provided by a heat network and neither a PCDB Heat Interface Unit nor
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a lodged hot-water cylinder applies:
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"a measured loss of 1.72 kWh/day should be used, corrected using
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Table 2b. This is equivalent to a cylinder of 110 litres and a
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factory insulation thickness of 50 mm."
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RdSAP 10 Table 29 (PDF p.56) assumes a cylinder thermostat is present
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when DHW is from a heat network → Table 2b base temperature factor
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0.60 (no ×1.3 absent-thermostat penalty).
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Mirrors `_apply_rdsap_no_water_heating_system_default`: rebinds the
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110 L / 50 mm-factory store onto `epc` (keeping the community water-
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heating code + fuel) so every downstream §4 gate — storage loss (56),
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primary loss (59) and combi-loss suppression — sees the spec default.
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A heat network is NOT a combi boiler, so the injected cylinder zeroes
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the Table 3a keep-hot (61) loss via the `has_hot_water_cylinder` gate
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in `_water_heating_worksheet_and_gains`; without this the cascade
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wrongly billed a 600 kWh/yr keep-hot loss on heat-network DHW.
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No-op (returns `epc` unchanged) unless the DHW main is a heat network,
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no cylinder is lodged, and the network is not in the PCDB (an indexed
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network uses the PCDB HIU loss per branch (a))."""
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if epc.has_hot_water_cylinder:
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return epc
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dhw_main = _water_heating_main(epc)
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if not _is_heat_network_main(dhw_main):
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return epc
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if dhw_main is not None and dhw_main.main_heating_index_number is not None:
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return epc
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sap_heating = replace(
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epc.sap_heating,
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cylinder_size=_CYLINDER_SIZE_CODE_NORMAL_110L,
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cylinder_insulation_type=_CYLINDER_INSULATION_TYPE_FACTORY,
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cylinder_insulation_thickness_mm=_HEAT_NETWORK_HIU_DEFAULT_INSULATION_MM,
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cylinder_thermostat="Y",
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)
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return replace(epc, has_hot_water_cylinder=True, sap_heating=sap_heating)
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# SAP 10.2 Table 4a solid-fuel boiler sub-rows (PDF p.163) — independent
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# boilers (151, 153, 155, 159), open-fire + back boiler (156), closed
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# room heater + back boiler (158), range cooker boiler (160, 161).
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@ -6601,6 +6651,11 @@ def cert_to_inputs(
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# means every downstream helper sees the spec default; the demand
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# cascade reuses this entry point so it is covered too.
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epc = _apply_rdsap_no_water_heating_system_default(epc)
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# SAP 10.2 p.24 "Heat networks" (c) — heat-network DHW with no PCDB
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# HIU and no lodged cylinder uses a default 110 L / 50 mm-factory
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# store (storage + primary loss, no combi keep-hot). Rebinds before
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# the §4 cascade so every loss gate sees the spec default.
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epc = _apply_heat_network_hiu_default_store(epc)
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dim = dimensions_from_cert(epc)
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# SAP §3 heat transmission + §2 ventilation cascades — see the
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# respective `_from_cert` helpers for cert→inputs mapping rules.
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@ -48,6 +48,7 @@ from domain.sap10_calculator.exceptions import (
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)
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from domain.sap10_calculator.rdsap.cert_to_inputs import (
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SAP_10_2_SPEC_PRICES,
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_apply_heat_network_hiu_default_store, # pyright: ignore[reportPrivateUsage]
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_has_suspended_timber_floor_per_spec, # pyright: ignore[reportPrivateUsage]
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_heat_network_code_302_effective_factor, # pyright: ignore[reportPrivateUsage]
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_heat_network_community_fuel_code, # pyright: ignore[reportPrivateUsage]
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@ -268,6 +269,54 @@ def test_heat_network_dlf_uses_first_non_empty_building_part_age_band() -> None:
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assert abs(inputs.main_heating_efficiency - 1.0 / 1.20) <= 1e-9
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def test_heat_network_dhw_no_cylinder_applies_sap_10_2_hiu_default_store() -> None:
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# Arrange — community heat-network main (Table 4a code 301, cat 6),
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# DHW from the network (WHC 901), NO cylinder lodged. SAP 10.2 p.24
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# "Heat networks" (c): when neither a PCDB HIU nor a lodged cylinder
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# applies, "a measured loss of 1.72 kWh/day should be used, corrected
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# using Table 2b. This is equivalent to a cylinder of 110 litres and a
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# factory insulation thickness of 50 mm". RdSAP 10 Table 29: heat-
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# network DHW → cylinder thermostat assumed present → Table 2b temp
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# factor 0.60. A heat network is NOT a combi boiler, so the Table 3a
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# keep-hot combi loss must NOT apply.
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main = MainHeatingDetail(
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has_fghrs=False,
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main_fuel_type=20, # mains gas (community)
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heat_emitter_type=1,
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emitter_temperature=1,
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main_heating_control=2106,
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main_heating_category=6,
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sap_main_heating_code=301,
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)
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part = make_building_part(construction_age_band="A")
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epc = make_minimal_sap10_epc(
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total_floor_area_m2=_TYPICAL_TFA_M2,
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habitable_rooms_count=4,
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country_code="ENG",
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sap_building_parts=[part],
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sap_heating=make_sap_heating(
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main_heating_details=[main], water_heating_code=901,
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),
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)
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# Act — preprocess the HIU default store, then run the §4 cascade.
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epc_with_store = _apply_heat_network_hiu_default_store(epc)
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wh_result, _ = _water_heating_worksheet_and_gains(
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epc=epc_with_store,
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water_efficiency_pct=100.0,
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is_instantaneous=False,
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primary_age="A",
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pcdb_record=None,
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)
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# Assert — combi keep-hot loss suppressed; storage loss = the SAP p.24
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# default 1.72 kWh/day × Table 2b 0.60 × 365 ≈ 376.7 kWh/yr.
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assert wh_result is not None
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assert sum(wh_result.combi_loss_monthly_kwh) == 0.0
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expected_storage_kwh = 1.72 * 0.60 * 365.0
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assert abs(sum(wh_result.solar_storage_monthly_kwh) - expected_storage_kwh) <= 1.0
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def test_heat_network_distribution_electricity_per_sap_10_2_appendix_c_3_2() -> None:
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# Arrange — heat-network main (Table 4a code 301 = community heating,
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# category 6). SAP 10.2 Appendix C §C3.2 (PDF p.51): distribution
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@ -432,15 +481,13 @@ def test_heat_network_distribution_electricity_none_for_individual_main() -> Non
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def test_heat_network_main_with_hw_from_main_dlf_scales_hot_water_kwh() -> None:
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# Arrange — when main heating is a heat network AND water heating
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# inherits from main (water_heating_code=901), the HW also incurs
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# the network's distribution losses. The water efficiency must be
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# overridden to 1/DLF so that the delivered HW kWh (and therefore
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# cost/CO2/PE applied to it) reflects q_useful × DLF.
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# Compare against a gas-boiler baseline at the same age band: the
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# heat-network HW kWh should be greater by the ratio 0.80/(1/DLF) =
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# DLF × 0.80 = 0.80 × 1.41 = 1.128 (i.e. ~13% higher) since the
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# non-heat-network baseline inherits water efficiency 0.80 from
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# the heat-network main's pre-DLF efficiency.
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# inherits from main (water_heating_code=901), the HW also incurs the
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# network's distribution losses: the water efficiency is overridden to
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# 1/DLF, so the delivered HW FUEL kWh = q_useful (the §4 output) × DLF.
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# Asserted directly as fuel ÷ output to isolate the DLF scaling from
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# the §4 loss structure (the SAP 10.2 p.24 HIU default store now
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# supplies storage + primary loss in q_useful — see
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# `_apply_heat_network_hiu_default_store`).
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part = make_building_part(construction_age_band="E")
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hn_main = MainHeatingDetail(
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@ -463,34 +510,20 @@ def test_heat_network_main_with_hw_from_main_dlf_scales_hot_water_kwh() -> None:
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),
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)
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# Comparable gas-boiler baseline that ALSO inherits a 0.80 water
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# efficiency through `water_heating_code=901` for direct comparison.
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# Use sap_main_heating_code = None so cascade returns 0.80 default.
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gas_main = MainHeatingDetail(
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has_fghrs=False,
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main_fuel_type=26,
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heat_emitter_type=1,
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emitter_temperature=1,
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main_heating_control=2106,
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main_heating_category=2,
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)
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gas_epc = make_minimal_sap10_epc(
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total_floor_area_m2=_TYPICAL_TFA_M2,
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habitable_rooms_count=4,
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country_code="ENG",
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sap_building_parts=[part],
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sap_heating=make_sap_heating(
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main_heating_details=[gas_main],
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water_heating_code=901,
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),
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# Act — HW fuel kWh from the full cascade, and the §4 output (q_useful)
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# from the worksheet on the same (HIU-store-applied) epc.
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hn_hw_fuel = cert_to_inputs(hn_epc).hot_water_kwh_per_yr
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wh_result, _ = _water_heating_worksheet_and_gains(
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epc=_apply_heat_network_hiu_default_store(hn_epc),
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water_efficiency_pct=100.0,
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is_instantaneous=False,
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primary_age="E",
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pcdb_record=None,
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)
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# Act
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hn_hw = cert_to_inputs(hn_epc).hot_water_kwh_per_yr
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gas_hw = cert_to_inputs(gas_epc).hot_water_kwh_per_yr
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# Assert — DLF (1.41) for age E × 0.80 baseline / (1/1.41) HN = 1.128.
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assert hn_hw / gas_hw == pytest.approx(1.41 * 0.80 / 1.0, abs=0.02)
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# Assert — delivered HW fuel = q_useful × DLF; age E → Table 12c 1.41.
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assert wh_result is not None
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assert abs(hn_hw_fuel / wh_result.output_kwh_per_yr - 1.41) <= 1e-6
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def test_hot_water_only_heat_network_whc_950_applies_table12c_dlf() -> None:
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