feat(modelling): classify ASHP existing system by fuel code

Slice 8 of ADR-0025 costing. _existing_system keys on the heating fuel code,
not the mains_gas flag -- the 001431 electric fixtures all lodge mains_gas=True
(gas available at the property) while heating electrically (fuel 30), which the
flag-based check misread as gas (and would have wrongly reused a non-existent
wet system). Electric/gas/oil/LPG map to their categories; empty details ->
NONE; unrecognised -> OTHER (gas-line fallback).

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

domain/modelling/generators/heating_recommendation.py

@@ -125,9 +125,11 @@ _KW_PER_M2 = 0.05
 # latter three from habitable rooms); fallback ~1 radiator per 13 m2.
 _RADIATOR_ROOM_OFFSET = 3
 _RADIATOR_M2_PER_RADIATOR = 13.0
-# SAP main-heating code lodged when a dwelling has no heating system.
-_NO_SYSTEM_SAP_CODE = 999
-# main_fuel_type codes (gov API enum and/or Table 12) for off-gas wet fuels.
+# main_fuel_type codes (gov API enum and/or Table 12) by fuel. Classification
+# keys on the heating *fuel*, NOT the `mains_gas` flag — that flag means gas is
+# available at the property, which is True even for electrically-heated dwellings
+# on a gas street (every 001431 electric fixture lodges mains_gas=True).
+_GAS_FUEL_CODES = frozenset({26, 1})
 _OIL_FUEL_CODES = frozenset({28, 4, 71, 73, 75, 76})
 _LPG_FUEL_CODES = frozenset({27, 2, 3, 5, 9})
 
@@ -149,21 +151,23 @@ def ashp_cost_inputs(epc: EpcPropertyData) -> AshpCostInputs:
 
 
 def _existing_system(epc: EpcPropertyData) -> AshpExistingSystem:
-    """Classify the dwelling's pre-retrofit system for decommission + reuse.
-    Mains gas is the most reliable signal (`mains_gas`); electricity keys on the
-    fuel code; oil/LPG on their fuel codes; an absent system on SAP code 999.
-    The storage-vs-other-electric split is deliberately not made — both price
-    the same decommission line (ADR-0025)."""
-    main: MainHeatingDetail = epc.sap_heating.main_heating_details[0]
-    if main.sap_main_heating_code == _NO_SYSTEM_SAP_CODE:
+    """Classify the dwelling's pre-retrofit system for decommission + reuse,
+    keyed on the heating *fuel code* (not the misleading `mains_gas` flag).
+    Electricity, gas, oil and LPG map to their categories; a dwelling with no
+    lodged main system to NONE; anything unrecognised to OTHER (which prices the
+    gas-line decommission fallback). The storage-vs-other-electric split is
+    deliberately not made — both price the same decommission line (ADR-0025)."""
+    details: list[MainHeatingDetail] = epc.sap_heating.main_heating_details
+    if not details:
         return AshpExistingSystem.NONE
-    if epc.sap_energy_source.mains_gas:
-        return AshpExistingSystem.GAS
-    if main.main_fuel_type == _ELECTRICITY_FUEL:
+    fuel = details[0].main_fuel_type
+    if fuel == _ELECTRICITY_FUEL:
         return AshpExistingSystem.ELECTRIC_STORAGE
-    if main.main_fuel_type in _OIL_FUEL_CODES:
+    if fuel in _GAS_FUEL_CODES:
+        return AshpExistingSystem.GAS
+    if fuel in _OIL_FUEL_CODES:
         return AshpExistingSystem.OIL
-    if main.main_fuel_type in _LPG_FUEL_CODES:
+    if fuel in _LPG_FUEL_CODES:
         return AshpExistingSystem.LPG
     return AshpExistingSystem.OTHER
 

tests/domain/modelling/test_ashp_cost_inputs.py

@@ -5,6 +5,9 @@ size band, design heat loss (floor-area proxy), radiator count, and whether a
 wet system can be reused — the catalogue math (Products) stays EPC-free.
 """
 
+import copy
+
+from datatypes.epc.domain.epc_property_data import EpcPropertyData
 from domain.modelling.generators.heating_recommendation import ashp_cost_inputs
 from domain.modelling.products import AshpCostInputs, AshpExistingSystem
 from tests.domain.modelling._elmhurst_recommendation import (
@@ -29,3 +32,53 @@ def test_mains_gas_dwelling_maps_to_a_reusable_wet_gas_system() -> None:
     assert abs(inputs.design_heat_loss_kw - 4.5) <= 1e-9
     assert inputs.radiator_count == 10
     assert inputs.has_reusable_wet_system is True
+
+
+def test_electric_dwelling_has_no_reusable_wet_system() -> None:
+    # Arrange — an electric storage-heater dwelling (no wet system).
+    epc = parse_recommendation_summary(
+        "hhr_storage_from_electric_storage_001431_before.pdf"
+    )
+
+    # Act
+    inputs: AshpCostInputs = ashp_cost_inputs(epc)
+
+    # Assert — electric, so a full new wet distribution is needed.
+    assert inputs.existing_system is AshpExistingSystem.ELECTRIC_STORAGE
+    assert inputs.has_reusable_wet_system is False
+
+
+def test_classification_keys_on_fuel_not_the_mains_gas_flag() -> None:
+    # Arrange — the 001431 electric fixtures all lodge mains_gas=True (gas is
+    # available at the property) while heating electrically (fuel 30). The
+    # classifier must key on the fuel, not the flag, or it would misread these
+    # as gas and wrongly reuse a non-existent wet system.
+    epc = parse_recommendation_summary(
+        "hhr_storage_from_electric_storage_001431_before.pdf"
+    )
+
+    # Act / Assert
+    assert epc.sap_energy_source.mains_gas is True
+    inputs: AshpCostInputs = ashp_cost_inputs(epc)
+    assert inputs.existing_system is AshpExistingSystem.ELECTRIC_STORAGE
+    assert inputs.has_reusable_wet_system is False
+
+
+def test_oil_and_lpg_dwellings_are_reusable_wet_systems() -> None:
+    # Arrange — no oil/LPG fixture exists, so mutate an off-gas dwelling's main
+    # fuel to oil (28) and LPG (27); both are wet boiler systems.
+    base: EpcPropertyData = parse_recommendation_summary(
+        "hhr_storage_from_electric_storage_001431_before.pdf"
+    )
+
+    def _with_fuel(code: int) -> EpcPropertyData:
+        clone: EpcPropertyData = copy.deepcopy(base)
+        clone.sap_energy_source.mains_gas = False
+        clone.sap_heating.main_heating_details[0].main_fuel_type = code
+        clone.sap_heating.main_heating_details[0].sap_main_heating_code = 199
+        return clone
+
+    # Act / Assert
+    assert ashp_cost_inputs(_with_fuel(28)).existing_system is AshpExistingSystem.OIL
+    assert ashp_cost_inputs(_with_fuel(27)).existing_system is AshpExistingSystem.LPG
+    assert ashp_cost_inputs(_with_fuel(28)).has_reusable_wet_system is True