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slice S-B31: Table 12c DLF on heat-network main and HW-from-main
Heat-network certs (cat=6) were under-predicted in cost — SAP bias
+6.31 across 13 sample certs, PE bias -15.6 (we under-predicted PE).
Root cause: missing distribution-loss-factor application.
SAP 10.2 spec references:
- Table 12 note (k): "Cost is per unit of heat generated (i.e.
before distribution losses); emission and primary factors are per
unit of fuel used by the heat generator."
- §C3.1: "Where a heat network is listed in the PCDB, the DLF is
already factored into the cost, CO2 and PE factors recorded
therein, so a DLF of 1 should be entered in worksheet (306) to
avoid double counting." (Implication: non-PCDB networks MUST
apply DLF.)
- Table 12c (p. 193): DLF by age band, 1.20 (A pre-1900) →
1.50 (K+ 2007+).
- RdSAP 10 §10.11 Table 29 cross-references Table 12c.
Mechanism: setting main_heating_efficiency = 1/DLF (and water_eff
when HW inherits from main via codes 901/902/914) makes the
calculator's main_fuel_kwh = q_useful × DLF = q_generated, which
multiplied by the per-kWh-generated unit price gives the cost the
spec mandates.
Affects:
- Heat-network main heating (sap_main_heating_code in 301-304 OR
main_heating_category == 6)
- HW from main on such certs (water_heating_code in 901/902/914)
Trade-off: CO2/PE for heat-network certs will under-predict ~20%
versus the spec's "fuel-burned × per-fuel-factor" formula, because
our architecture uses one main_fuel_kwh value for cost AND CO2/PE.
For SAP-rating purposes (the priority) this is acceptable; the PE
bias actually moves in the right direction here (cat=6 PE bias
-15.6 → -5.6) because the under-counting partially cancels a
pre-existing larger under-count.
Parity probe at 300 certs, seed=7:
SAP MAE 4.69 → 4.61 (-0.08)
SAP bias 0.98 → 0.87 (-0.11)
PE MAE 43.32 → 43.11 (-0.21)
cat=6 PE bias -15.6 → -5.6 (+10.0, correct direction)
cat=6 PE MAE 40.3 → 35.8 (-4.5)
cat=6 our_pe 158.5 → 225.0 (cert 230.6 — converged)
Cumulative across S-B23 → S-B31:
SAP MAE 5.34 → 4.61 (-0.73)
PE MAE 57.28 → 43.11 (-14.17)
PE bias 51.56 → 38.64 (-12.92)
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
This commit is contained in:
Khalim Conn-Kowlessar
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@ -185,6 +185,51 @@ _FORCE_SECONDARY_FOR_MAIN_CODES: Final[frozenset[int]] = frozenset(
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_PV_EXPORT_TARIFF_CODE: Final[int] = 60
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# SAP 10.2 Table 12c (page 193) — Distribution Loss Factor for heat
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# networks by dwelling age band, used when no PCDB record is available
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# (the modal RdSAP case). Per §C3.1: "Where a heat network is listed
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# in the PCDB, the DLF is already factored into the cost, CO2 and PE
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# factors recorded therein, so a DLF of 1 should be entered in
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# worksheet (306) to avoid double counting." For non-PCDB networks
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# (our case), DLF must be applied. K-or-newer (post-2007) = 1.50.
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_HEAT_NETWORK_DLF_BY_AGE: Final[dict[str, float]] = {
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"A": 1.20, "B": 1.26, "C": 1.33, "D": 1.37, "E": 1.41, "F": 1.43,
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"G": 1.45, "H": 1.46, "I": 1.48, "J": 1.49, "K": 1.50, "L": 1.50,
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"M": 1.50,
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}
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_HEAT_NETWORK_DLF_DEFAULT: Final[float] = 1.50
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# SAP 10.2 Table 4a codes for heat-network main heating systems:
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# 301 = boiler-driven community heating
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# 302 = boiler-driven community heating with CHP
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# 303 = community CHP only
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# 304 = electric heat-pump community heating
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_HEAT_NETWORK_MAIN_CODES: Final[frozenset[int]] = frozenset({301, 302, 303, 304})
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_HEAT_NETWORK_CATEGORY: Final[int] = 6
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def _is_heat_network_main(main: Optional[MainHeatingDetail]) -> bool:
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"""True when the cert's main heating is a heat network — either by
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SAP code (Table 4a 301-304) or by `main_heating_category` (6)."""
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if main is None:
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return False
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code = main.sap_main_heating_code
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if isinstance(code, int) and code in _HEAT_NETWORK_MAIN_CODES:
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return True
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return main.main_heating_category == _HEAT_NETWORK_CATEGORY
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def _heat_network_dlf(age_band: Optional[str]) -> float:
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"""RdSAP 10 §10.11 + SAP 10.2 Table 12c distribution loss factor by
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age band. Defaults to the K-or-newer value (1.50) when band missing."""
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if age_band is None:
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return _HEAT_NETWORK_DLF_DEFAULT
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return _HEAT_NETWORK_DLF_BY_AGE.get(
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age_band.upper(), _HEAT_NETWORK_DLF_DEFAULT
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)
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@dataclass(frozen=True)
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class PriceTable:
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"""Seam between the spec-correct SAP 10.2/10.3 Table 12 prices and
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@ -749,12 +794,29 @@ def cert_to_inputs(
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)
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eff = seasonal_efficiency(main_code, main_category, main_fuel)
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if _is_heat_network_main(main):
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# SAP 10.2 Table 12 note (k): heat-network unit prices are per
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# kWh of heat GENERATED (before distribution losses), not per
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# kWh of fuel consumed. Setting efficiency = 1/DLF makes the
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# calculator's `main_fuel_kwh = q_useful / (1/DLF) = q_useful
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# × DLF = q_generated`, so cost = q_generated × unit_price as
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# the spec requires.
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eff = 1.0 / _heat_network_dlf(primary_age)
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water_eff = _water_efficiency_with_category_inherit(
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water_heating_code=epc.sap_heating.water_heating_code,
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main_code=main_code,
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main_category=main_category,
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main_fuel=main_fuel,
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)
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if (
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_is_heat_network_main(main)
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and epc.sap_heating.water_heating_code in _WATER_INHERIT_FROM_MAIN_CODES
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):
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# HW from main on a heat-network cert: the DHW also incurs the
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# network's distribution losses. Same 1/DLF override as for
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# space heating so the delivered HW kWh reflects q_useful × DLF
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# = q_generated, matching the per-kWh-generated unit price.
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water_eff = 1.0 / _heat_network_dlf(primary_age)
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is_instantaneous = epc.sap_heating.water_heating_code in _INSTANTANEOUS_WATER_CODES
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hw_kwh = predicted_hot_water_kwh(
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total_floor_area_m2=epc.total_floor_area_m2,
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@ -80,6 +80,133 @@ def _typical_semi_detached_epc():
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)
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def test_heat_network_main_applies_table12c_dlf_to_main_heating_efficiency() -> None:
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# Arrange — heat-network main heating (Table 4a code 301 = community
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# heating with CHP/boilers; main_heating_category=6). Cert age band
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# E (1967-1975) lodges Table 12c DLF = 1.41.
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# Per SAP 10.2 §C3.1 + Table 12 note (k): unit price is per kWh of
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# heat GENERATED (i.e. before distribution losses), so the fuel-kwh
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# multiplied by the unit price must be q_generated = q_useful × DLF.
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# Setting main_heating_efficiency = 1/DLF makes our calculator's
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# main_fuel_kwh = q_useful × DLF, which gives the correct cost.
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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="E")
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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(main_heating_details=[main]),
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)
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# Act
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inputs = cert_to_inputs(epc)
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# Assert — DLF = 1.41 for age E → effective efficiency = 1/1.41 = 0.709.
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assert inputs.main_heating_efficiency == pytest.approx(1.0 / 1.41, abs=0.005)
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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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part = make_building_part(construction_age_band="E")
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hn_main = MainHeatingDetail(
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has_fghrs=False,
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main_fuel_type=20,
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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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hn_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=[hn_main],
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water_heating_code=901, # from main
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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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)
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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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def test_gas_boiler_main_efficiency_unchanged_by_dlf_override() -> None:
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# Arrange — regression check: the DLF override only fires for heat-
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# network main heating. A standard gas boiler (cat=2, code=102) must
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# still get its Table 4b winter efficiency (0.84).
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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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sap_main_heating_code=102, # gas combi pre-2005, 0.84 eff
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)
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part = make_building_part(construction_age_band="E")
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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(main_heating_details=[main]),
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)
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# Act
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inputs = cert_to_inputs(epc)
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# Assert — Table 4b code 102 winter efficiency = 0.84, no DLF override.
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assert inputs.main_heating_efficiency == pytest.approx(0.84, abs=0.001)
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def test_main_heating_fraction_does_not_override_table11_secondary_default() -> None:
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# Arrange — the S-B30 attempt assumed `main_heating_fraction=1` meant
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# "no secondary heating" and dropped the Table 11 default in that
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