diff --git a/backend/documents_parser/tests/test_heating_systems_corpus.py b/backend/documents_parser/tests/test_heating_systems_corpus.py
index b3b298ed..34f2b549 100644
--- a/backend/documents_parser/tests/test_heating_systems_corpus.py
+++ b/backend/documents_parser/tests/test_heating_systems_corpus.py
@@ -546,9 +546,25 @@ _EXPECTATIONS: tuple[_CorpusExpectation, ...] = (
     # (worksheet (464)/(466)/(364)/(366) per SAP 10.2 §13b spec) +
     # (2) community-HP COP cascade for CH3 + (3) heat-network overall
     # factor (486)/(386) calc — separate follow-up slices).
-    _CorpusExpectation(variant='community heating 1', block='11b', expected_sap_resid=+0.5915, expected_cost_resid_gbp=-13.6289, expected_co2_resid_kg=-787.2531, expected_pe_resid_kwh=-3827.1887),
+    #
+    # Slice S0380.172 closed the CH1 (boiler) + CH3 (HP) CO2 / PE
+    # residuals via SAP 10.2 Table 4a (PDF p.164) heat-network heat-
+    # source efficiency scaling: code 301 (boilers) eff = 80%, code
+    # 304 (HP) eff = 300%. Spec block 13a (467) = (307+310) × 100 /
+    # heat_source_eff × Table 12 PE factor; cascade meters network_
+    # input directly so PE/CO2 factors are scaled by 1/heat_source_eff
+    # at lookup time. CH1 ΔCO2 −787 → −126 (~84% closed) and ΔPE
+    # −3827 → −967 (~75% closed); CH3 ΔCO2 +1614 → +473 (~71%
+    # closed) and ΔPE +11879 → +1749 (~85% closed). Code 302 (CHP+
+    # boilers) is omitted from the scaling table — the 35%/65% split
+    # requires the displaced-electricity credit line per spec block
+    # 13b (464)/(466); follow-up slice scope. Residual CH1/CH3 gap is
+    # the WHC=901 HW path (cascade reads cert-lodged "Mains gas" as
+    # HW fuel; should fall through to main fuel for community heating)
+    # + the Elmhurst 0.8523 multiplier on heat-network energy column.
+    _CorpusExpectation(variant='community heating 1', block='11b', expected_sap_resid=+0.5915, expected_cost_resid_gbp=-13.6289, expected_co2_resid_kg=-126.4571, expected_pe_resid_kwh=-967.3648),
     _CorpusExpectation(variant='community heating 2', block='11b', expected_sap_resid=-0.0076, expected_cost_resid_gbp=+0.1744, expected_co2_resid_kg=-1430.3212, expected_pe_resid_kwh=+1506.0355),
-    _CorpusExpectation(variant='community heating 3', block='11b', expected_sap_resid=+0.5915, expected_cost_resid_gbp=-13.6289, expected_co2_resid_kg=+1613.7837, expected_pe_resid_kwh=+11878.7588),
+    _CorpusExpectation(variant='community heating 3', block='11b', expected_sap_resid=+0.5915, expected_cost_resid_gbp=-13.6289, expected_co2_resid_kg=+472.5996, expected_pe_resid_kwh=+1748.7395),
     _CorpusExpectation(variant='community heating 4', block='11b', expected_sap_resid=-0.0076, expected_cost_resid_gbp=+0.1744, expected_co2_resid_kg=-4397.0794, expected_pe_resid_kwh=+494.6090),
     _CorpusExpectation(variant='community heating 6', block='11b', expected_sap_resid=-8.0295, expected_cost_resid_gbp=+185.0120, expected_co2_resid_kg=-2934.9021, expected_pe_resid_kwh=+7864.5950),
 )
@@ -881,3 +897,15 @@ def test_community_heating_mapper_populates_chp_split_fields(
     main_1 = main_heating_details[0]
     assert main_1.community_heating_chp_fraction == expected_chp_fraction
     assert main_1.community_heating_boiler_fuel_type == expected_boiler_fuel_code
+
+
+# S0380.172 — Heat-network heat-source-eff scaling residual coverage.
+#
+# Per SAP 10.2 Table 4a (PDF p.164): "Boilers (RdSAP)" eff=80%, "Heat
+# pump (RdSAP)" eff=300%. The cascade's CO2/PE factor functions scale
+# Table 12 factors by 1/heat_source_eff so that network_input × scaled
+# factor lands on the spec block 13a (467) / 12b (367) "(307+310) ×
+# 100 / eff × Table 12 factor" formula. SAP code 302 (CHP+boilers) is
+# excluded — 35%/65% split + displaced-electricity credit is follow-up.
+# Coverage is asserted via the residual-pin test above (CH1 / CH3
+# closure; CH2 / CH4 / CH6 unchanged).
diff --git a/domain/sap10_calculator/rdsap/cert_to_inputs.py b/domain/sap10_calculator/rdsap/cert_to_inputs.py
index 79b370d0..9c959f7b 100644
--- a/domain/sap10_calculator/rdsap/cert_to_inputs.py
+++ b/domain/sap10_calculator/rdsap/cert_to_inputs.py
@@ -825,6 +825,31 @@ _HEAT_NETWORK_MAIN_CODES: Final[frozenset[int]] = frozenset({301, 302, 303, 304}
 _HEAT_NETWORK_CATEGORY: Final[int] = 6
 
 
+# SAP 10.2 Table 4a (PDF p.164) heat-network heat-source efficiency by
+# SAP code. Verbatim:
+#   301 "Boilers (RdSAP)" → 80%
+#   302 "CHP and boilers (RdSAP)" → 75% (overall — per RdSAP 10 §C)
+#   304 "Heat pump (RdSAP)" → 300% (= COP 3.0)
+# Used by the block 13a/12b PE/CO2 cascade to convert delivered network
+# input (post-DLF) into FUEL input by dividing by the heat-source
+# efficiency: spec (467) = (307+310) × 100 / (467a). The cascade meters
+# heat-network input directly (eff = 1/DLF for cost via Table 12
+# heat-network rate), so PE/CO2 factors are scaled by 1/heat_source_eff
+# at lookup time to land at the spec's fuel-input × Table-12-factor.
+#
+# Code 302 (CHP+boilers) is omitted here because the 35%/65% heat-
+# fraction split applies different efficiencies to the two heat sources
+# (CHP 75% overall + boilers 80%) and a single composite efficiency
+# can't model the displaced-electricity credit line per spec block
+# 13b (464)/(466). The cascade for code 302 keeps the current
+# 1/DLF override (giving large CO2/PE residuals on CH2/CH4/CH6 —
+# follow-up slice scope).
+_HEAT_NETWORK_HEAT_SOURCE_EFFICIENCY: Final[dict[int, float]] = {
+    301: 0.80,
+    304: 3.00,
+}
+
+
 def _is_heat_network_main(main: Optional[MainHeatingDetail]) -> bool:
     """True when the cert's main heating is a heat network — either by
     SAP code (Table 4a 301-304) or by `main_heating_category` (6)."""
@@ -836,6 +861,28 @@ def _is_heat_network_main(main: Optional[MainHeatingDetail]) -> bool:
     return main.main_heating_category == _HEAT_NETWORK_CATEGORY
 
 
+def _heat_network_heat_source_efficiency_scaling(
+    main: Optional[MainHeatingDetail],
+) -> float:
+    """Return the multiplicative scaling factor to apply to Table 12
+    CO2 / PE factors when the main is a heat-network boiler (SAP 301) or
+    heat pump (SAP 304). Cascade computes CO2/PE = network_input ×
+    Table_12_factor; spec block 13a/12b computes (network_input /
+    heat_source_eff) × Table_12_factor. Equivalent transform: scale the
+    factor by 1/heat_source_eff. Returns 1.0 for code 302 (CHP+boilers
+    — separate split-formula path) and non-heat-network mains.
+    """
+    if not _is_heat_network_main(main):
+        return 1.0
+    code = main.sap_main_heating_code if main is not None else None
+    if not isinstance(code, int):
+        return 1.0
+    eff = _HEAT_NETWORK_HEAT_SOURCE_EFFICIENCY.get(code)
+    if eff is None:
+        return 1.0
+    return 1.0 / eff
+
+
 def _heat_network_dlf(age_band: Optional[str]) -> float:
     """RdSAP 10 §10.11 + SAP 10.2 Table 12c distribution loss factor by
     age band. Defaults to the K-or-newer value (1.50) when band missing.
@@ -2413,7 +2460,16 @@ def _main_heating_co2_factor_kg_per_kwh(
       annual factor is the safe degenerate value)
     """
     if not _is_electric_main(main):
-        return _co2_factor_kg_per_kwh(main)
+        # Heat-network mains (SAP codes 301 / 304) are non-electric per
+        # `_is_electric_main` but require a heat-source-efficiency scaling
+        # per spec block 12b (363)/(367) = network_input × 100 /
+        # heat_source_eff × Table 12 CO2 factor. The cascade meters
+        # network_input directly so scale the factor by 1/eff to land at
+        # the spec's fuel-input × factor.
+        return (
+            _co2_factor_kg_per_kwh(main)
+            * _heat_network_heat_source_efficiency_scaling(main)
+        )
     if tariff is Tariff.STANDARD:
         monthly = _effective_monthly_co2_factor(
             main_fuel_monthly_kwh, _STANDARD_ELECTRICITY_FUEL_CODE,
@@ -2470,7 +2526,15 @@ def _main_heating_primary_factor(
     unknown dual-rate codes, zero-fuel)."""
     fuel = _main_fuel_code(main)
     if not _is_electric_main(main):
-        return primary_energy_factor(fuel)
+        # PE-side mirror of `_main_heating_co2_factor_kg_per_kwh`
+        # heat-network heat-source-eff scaling. Spec block 13a (463)/
+        # (467) = network_input × 100 / heat_source_eff × Table 12 PE
+        # factor; cascade meters network_input directly so scale by
+        # 1/eff at lookup time.
+        return (
+            primary_energy_factor(fuel)
+            * _heat_network_heat_source_efficiency_scaling(main)
+        )
     if tariff is Tariff.STANDARD:
         monthly = _effective_monthly_pe_factor(
             main_fuel_monthly_kwh, _STANDARD_ELECTRICITY_FUEL_CODE,