S0380.200: SAP 10.2 §9a two-main-heating split (203)/(205)/(207)/(213)

The cascade lumped a dwelling with two main heating systems into one:
`space_heating_fuel_monthly_kwh` hard-coded (203)=0 (a documented
scope-A placeholder) and the calculator's per-month fuel read only
main_1, so the full §8 space-heat demand billed against system 1's
efficiency. Simulated case 6 (one oil boiler feeding radiators 51% +
underfloor 49%) exposed it: main fuel ≈ demand/eff1 instead of the
worksheet's (211)+(213) per-system split.

Implements the SAP 10.2 §9a two-main model:
  (204) = (202) × (1 − (203))   → system 1 share of total heat
  (205) = (202) × (203)         → system 2 share of total heat
  (211)m = (98c)m × (204) × 100 / (206)
  (213)m = (98c)m × (205) × 100 / (207)
(203) = the second system's lodged `main_heating_fraction`; (207) = its
own seasonal efficiency via the new per-detail `_main_heating_detail_
efficiency` (the core of `_main_heating_efficiency`, now reused for
system 2). Calculator `_solve_month` aggregates main_1 + main_2 into
`main_heating_fuel_kwh`. Cost (§10a 241), CO2 (§12 262) and PE (§13 276)
main_2 paths were already wired and now activate.

Site-notes gap also fixed: §14.1 Main Heating2 omits the "Fuel Type"
cell when the second system shares Main 1's fuel (case 6: one oil boiler,
two emitters). `_map_elmhurst_main_heating_2` now inherits Main 1's
resolved fuel as a fallback.

Blast radius: only dual-main certs. 0240 (2× oil code 130, identical
Eq-D1 efficiency) is unchanged — its split collapses to the lumped total.
Suite: 2355 passed, 1 skipped. New code: 0 pyright errors.

NOTE: case 6 is not yet fully pinnable end-to-end — its two systems have
DIFFERENT efficiencies (radiators 55°C → 79%, underfloor 35°C → 84%), a
flow-temperature boiler-efficiency adjustment not yet modelled, and its
dual-system auxiliary pumps ((230c)+(230d)=356) differ from the cascade.
Both are separate follow-on features; this slice is the §9a fuel split.

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

datatypes/epc/domain/mapper.py

@@ -4861,6 +4861,8 @@ def _elmhurst_main_heating_category(
 
 def _map_elmhurst_main_heating_2(
     mh2: Optional[ElmhurstMainHeating2],
+    *,
+    fallback_fuel_type: Union[int, str, None] = None,
 ) -> Optional[MainHeatingDetail]:
     """Build a `MainHeatingDetail` from the Elmhurst §14.1 Main Heating2
     block. Returns None when no Main 2 is lodged (extractor convention:
@@ -4896,6 +4898,20 @@ def _map_elmhurst_main_heating_2(
         and mh2.main_heating_sap_code in _ELECTRIC_SAP_MAIN_HEATING_CODES
     ):
         main_fuel_int = _STANDARD_ELECTRICITY_FUEL_CODE
+    # §14.1 Main Heating2 often omits the "Fuel Type" cell when the
+    # second main system shares Main 1's fuel (simulated case 6: one oil
+    # boiler feeding radiators + underfloor, so the Summary lodges the
+    # fuel once on §14.0). Inherit Main 1's resolved fuel so the §9a
+    # two-main split (213)m can apply system 2's own efficiency.
+    resolved_fuel: Union[int, str] = (
+        main_fuel_int
+        if main_fuel_int is not None
+        else (
+            fallback_fuel_type
+            if (not mh2.fuel_type and fallback_fuel_type is not None)
+            else mh2.fuel_type
+        )
+    )
     category: Optional[int] = None
     if pcdb_index is not None and heat_pump_record(pcdb_index) is not None:
         category = _ELMHURST_HEATING_CATEGORY_HEAT_PUMP
@@ -4906,7 +4922,7 @@ def _map_elmhurst_main_heating_2(
         # cert-level renewables block is the single source of truth and
         # is already wired into Main 1.
         has_fghrs=False,
-        main_fuel_type=main_fuel_int if main_fuel_int is not None else mh2.fuel_type,
+        main_fuel_type=resolved_fuel,
         # §14.1 doesn't lodge a heat emitter (the emitter is Main 1's
         # radiator/UFH); leave as empty-string sentinel for cascade
         # consumers that key off Main 1's emitter.
@@ -5110,7 +5126,9 @@ def _map_elmhurst_sap_heating(survey: ElmhurstSiteNotes) -> SapHeating:
     # services DHW via `Water Heating SapCode 914` ("from second main
     # system") while Main 1 handles space heat. None when the §14.1
     # block is absent or lodges only placeholder zeros.
-    main_2_detail = _map_elmhurst_main_heating_2(mh.main_heating_2)
+    main_2_detail = _map_elmhurst_main_heating_2(
+        mh.main_heating_2, fallback_fuel_type=main_1_detail.main_fuel_type
+    )
     main_heating_details = (
         [main_1_detail, main_2_detail]
         if main_2_detail is not None

domain/sap10_calculator/calculator.py

@@ -425,9 +425,15 @@ def _solve_month(
     # SAP 10.2 §8 — (98c)m precomputed upstream by `space_heating_monthly_kwh`
     # (includes Table 9c summer clamp Jun..Sep). Calculator indexes directly.
     q_heat = inputs.space_heating_monthly_kwh[month - 1]
-    # SAP 10.2 §9a — (211)m/(215)m precomputed upstream by
+    # SAP 10.2 §9a — (211)m/(213)m/(215)m precomputed upstream by
     # `space_heating_fuel_monthly_kwh`. Calculator stops doing q/η inline.
-    fuel_main = inputs.energy_requirements.main_1_fuel_monthly_kwh[month - 1]
+    # `main_heating_fuel_kwh` aggregates both main systems (213)m is zero
+    # for single-main certs, so this is the per-system sum for dual-main
+    # dwellings (cert 0240 / simulated case 6) and main-1 alone otherwise.
+    fuel_main = (
+        inputs.energy_requirements.main_1_fuel_monthly_kwh[month - 1]
+        + inputs.energy_requirements.main_2_fuel_monthly_kwh[month - 1]
+    )
     fuel_secondary = inputs.energy_requirements.secondary_fuel_monthly_kwh[month - 1]
 
     # SAP 10.2 §8c — (107)m precomputed upstream by `space_cooling_monthly_kwh`

domain/sap10_calculator/rdsap/cert_to_inputs.py

@@ -1570,7 +1570,16 @@ def _main_heating_efficiency(epc: EpcPropertyData) -> float:
     seasonal efficiency → heat-network 1/DLF override. Used by §4 (water
     heating cascade) and §9a (per-system fuel kWh) — both must see the
     same value, so this single helper is the single source of truth."""
-    main = _first_main_heating(epc)
+    return _main_heating_detail_efficiency(_first_main_heating(epc), epc)
+
+
+def _main_heating_detail_efficiency(
+    main: Optional[MainHeatingDetail], epc: EpcPropertyData
+) -> float:
+    """SAP 10.2 (206)/(207) efficiency (0..1) for a SPECIFIC main heating
+    detail — the per-detail core of `_main_heating_efficiency`. Used for
+    both main system 1 (206) and main system 2 (207) on dual-main certs
+    (cert 0240 / simulated case 6)."""
     main_code = main.sap_main_heating_code if main is not None else None
     main_category = main.main_heating_category if main is not None else None
     main_fuel = _main_fuel_code(main)
@@ -3956,11 +3965,25 @@ def energy_requirements_section_from_cert(
         if secondary_fraction_value > 0.0 else 0.0
     )
     eff = _main_heating_efficiency(epc)
+    # SAP 10.2 §9a two-main split (203)/(207): when a second main heating
+    # system is lodged, (203) = its `main_heating_fraction` (% of main
+    # heating it supplies) and (207) = its own seasonal efficiency. Cert
+    # 0240 (2× oil code 130, 51/49) + simulated case 6 (oil code 127,
+    # rads 51% + underfloor 49%) exercise this.
+    details = epc.sap_heating.main_heating_details if epc.sap_heating else []
+    main_2 = details[1] if len(details) >= 2 else None
+    main_2_of_main_fraction = 0.0
+    main_2_efficiency_value = 0.0
+    if main_2 is not None and main_2.main_heating_fraction is not None:
+        main_2_of_main_fraction = main_2.main_heating_fraction / 100.0
+        main_2_efficiency_value = _main_heating_detail_efficiency(main_2, epc)
     return space_heating_fuel_monthly_kwh(
         space_heating_monthly_kwh=sh.total_space_heating_monthly_kwh,
         secondary_heating_fraction=secondary_fraction_value,
         main_heating_efficiency_pct=eff * 100.0,
         secondary_heating_efficiency_pct=secondary_efficiency_value * 100.0,
+        main_2_of_main_fraction=main_2_of_main_fraction,
+        main_2_efficiency_pct=main_2_efficiency_value * 100.0,
     )
 
 
@@ -6351,11 +6374,22 @@ def cert_to_inputs(
     secondary_efficiency_value = _secondary_efficiency(
         epc.sap_heating, main_code, main_fuel
     )
+    # SAP 10.2 §9a two-main split (203)/(207) — see the section helper
+    # `energy_requirements_section_from_cert` for the rationale.
+    _main_details = epc.sap_heating.main_heating_details if epc.sap_heating else []
+    _main_2 = _main_details[1] if len(_main_details) >= 2 else None
+    main_2_of_main_fraction = 0.0
+    main_2_efficiency_value = 0.0
+    if _main_2 is not None and _main_2.main_heating_fraction is not None:
+        main_2_of_main_fraction = _main_2.main_heating_fraction / 100.0
+        main_2_efficiency_value = _main_heating_detail_efficiency(_main_2, epc)
     energy_requirements_result = space_heating_fuel_monthly_kwh(
         space_heating_monthly_kwh=space_heating_result.total_space_heating_monthly_kwh,
         secondary_heating_fraction=secondary_fraction_value,
         main_heating_efficiency_pct=eff * 100.0,
         secondary_heating_efficiency_pct=secondary_efficiency_value * 100.0,
+        main_2_of_main_fraction=main_2_of_main_fraction,
+        main_2_efficiency_pct=main_2_efficiency_value * 100.0,
     )
 
     # SAP 10.2 Appendix M1 §3-4 (p.93-94): split monthly PV generation

domain/sap10_calculator/worksheet/energy_requirements.py

@@ -11,8 +11,10 @@ where (204) = (202) × (1 − (203)) and (202) = 1 − (201). Single-main
 case ((203) = 0) collapses (204) to (202), so (211)m = (98c)m × (202) ×
 100 / (206). Same shape for secondary (215)m and main 2 (213)m.
 
-Two-main split ((203) > 0) and cooling-fuel (209)/(221) are zero-branch
-placeholders in scope A — populated once first cert exercises them.
+Two-main split ((203) > 0) is implemented: (211)m = (98c)m × (204) ×
+100 / (206) for system 1 and (213)m = (98c)m × (205) × 100 / (207) for
+system 2, where (204) = (202) × (1 − (203)) and (205) = (202) × (203).
+Cooling-fuel (209)/(221) remains a zero-branch placeholder.
 
 Reference: SAP 10.2 specification (14-03-2025) §9a (lines 7909-7953).
 """
@@ -26,10 +28,9 @@ from dataclasses import dataclass
 class EnergyRequirementsResult:
     """SAP 10.2 §9a worksheet line refs (201)..(221).
 
-    Scope-A populated lines: (201), (202), (204), (206), (208), (211)m,
-    (211), (215)m, (215). Two-main and cooling-fuel line refs ((203),
-    (205), (207), (209), (213)m, (213), (221)) are zero-branch
-    placeholders until the first multi-main / fixed-AC cert lands.
+    Populated lines: (201)-(208), (211)m/(211), (213)m/(213) (two-main
+    split), (215)m/(215). Cooling-fuel line refs ((209), (221)) are
+    zero-branch placeholders until the first fixed-AC cert lands.
     """
 
     # Fractions (Table 11)
@@ -60,26 +61,37 @@ def space_heating_fuel_monthly_kwh(
     secondary_heating_fraction: float,
     main_heating_efficiency_pct: float,
     secondary_heating_efficiency_pct: float,
+    main_2_of_main_fraction: float = 0.0,
+    main_2_efficiency_pct: float = 0.0,
 ) -> EnergyRequirementsResult:
     """SAP 10.2 §9a orchestrator — produce (201)..(221) line refs.
 
-    Scope A: single-main + secondary only. Two-main ((203) > 0) and
-    cooling-fuel (Table 10c SEER) populate the zero-branch placeholder
-    fields with computed values when their respective slices land.
+    Single-main certs leave `main_2_of_main_fraction` = 0, collapsing
+    (204) to (202) and zeroing (213)m. Dual-main certs (cert 0240 /
+    simulated case 6) pass (203) = fraction of main heating from main
+    system 2 and (207) = main system 2 efficiency; the §8 space-heat
+    demand then splits (204)=(202)×(1−(203)) to system 1 and
+    (205)=(202)×(203) to system 2, each at its own efficiency. Cooling-
+    fuel (Table 10c SEER) remains a zero-branch placeholder.
     """
     fraction_201 = secondary_heating_fraction
     fraction_202 = 1.0 - fraction_201
-    fraction_203 = 0.0  # scope A: no main 2
+    fraction_203 = main_2_of_main_fraction
     fraction_204 = fraction_202 * (1.0 - fraction_203)
     fraction_205 = fraction_202 * fraction_203
 
     main_1_eff = main_heating_efficiency_pct
+    main_2_eff = main_2_efficiency_pct
     secondary_eff = secondary_heating_efficiency_pct
 
     main_1_fuel_monthly = tuple(
         q * fraction_204 * 100.0 / main_1_eff if main_1_eff > 0 else 0.0
         for q in space_heating_monthly_kwh
     )
+    main_2_fuel_monthly = tuple(
+        q * fraction_205 * 100.0 / main_2_eff if main_2_eff > 0 else 0.0
+        for q in space_heating_monthly_kwh
+    )
     secondary_fuel_monthly = tuple(
         q * fraction_201 * 100.0 / secondary_eff if secondary_eff > 0 else 0.0
         for q in space_heating_monthly_kwh
@@ -92,14 +104,14 @@ def space_heating_fuel_monthly_kwh(
         main_1_of_total_fraction=fraction_204,
         main_2_of_total_fraction=fraction_205,
         main_1_efficiency_pct=main_1_eff,
-        main_2_efficiency_pct=0.0,
+        main_2_efficiency_pct=main_2_eff,
         secondary_efficiency_pct=secondary_eff,
         cooling_seer=0.0,
         main_1_fuel_monthly_kwh=main_1_fuel_monthly,
-        main_2_fuel_monthly_kwh=(0.0,) * 12,
+        main_2_fuel_monthly_kwh=main_2_fuel_monthly,
         secondary_fuel_monthly_kwh=secondary_fuel_monthly,
         main_1_fuel_kwh_per_yr=sum(main_1_fuel_monthly),
-        main_2_fuel_kwh_per_yr=0.0,
+        main_2_fuel_kwh_per_yr=sum(main_2_fuel_monthly),
         secondary_fuel_kwh_per_yr=sum(secondary_fuel_monthly),
         cooling_fuel_kwh_per_yr=0.0,
     )

tests/domain/sap10_calculator/worksheet/test_energy_requirements.py

@@ -60,6 +60,40 @@ def test_table_11_secondary_fraction_splits_q_heat_between_main_and_secondary()
     assert result.secondary_fuel_kwh_per_yr == 550.0
 
 
+def test_two_main_systems_split_q_heat_by_fraction_203_at_own_efficiencies() -> None:
+    """Spec §9a (203)/(204)/(205) two-main split: when a second main
+    system supplies (203) of the main heating, (204)=(202)×(1−(203)) goes
+    to system 1 at (206) and (205)=(202)×(203) to system 2 at (207). With
+    no secondary ((202)=1), (203)=0.49, eff1=79%, eff2=84%, Σ(98c)=4400:
+      Σ(211) = 4400 × 0.51 × 100/79 = 2840.5063 kWh
+      Σ(213) = 4400 × 0.49 × 100/84 = 2566.6667 kWh
+    Mirrors simulated case 6 (oil boiler, radiators 51% + underfloor 49%)
+    and cert 0240 (identical-efficiency systems collapse to the single-
+    main total)."""
+    # Arrange
+    monthly_space_heating = (
+        1000.0, 800.0, 600.0, 400.0, 200.0, 0.0,
+        0.0, 0.0, 0.0, 200.0, 400.0, 800.0,
+    )
+
+    # Act
+    result = space_heating_fuel_monthly_kwh(
+        space_heating_monthly_kwh=monthly_space_heating,
+        secondary_heating_fraction=0.0,
+        main_heating_efficiency_pct=79.0,
+        secondary_heating_efficiency_pct=0.0,
+        main_2_of_main_fraction=0.49,
+        main_2_efficiency_pct=84.0,
+    )
+
+    # Assert
+    assert abs(result.main_2_of_main_fraction - 0.49) <= 1e-12
+    assert abs(result.main_1_of_total_fraction - 0.51) <= 1e-12
+    assert abs(result.main_2_of_total_fraction - 0.49) <= 1e-12
+    assert abs(result.main_1_fuel_kwh_per_yr - 4400.0 * 0.51 * 100.0 / 79.0) <= 1e-9
+    assert abs(result.main_2_fuel_kwh_per_yr - 4400.0 * 0.49 * 100.0 / 84.0) <= 1e-9
+
+
 def test_per_month_fuel_preserves_summer_clamp_zeros_from_98c() -> None:
     """The §8 Table 9c summer clamp zeros (98c)m for Jun..Sep. §9a's per-
     month (211)m / (215)m tuples are linear in (98c)m so they inherit the