feat(modelling): ASHP decommission cost by existing system

Slice 2 of ADR-0025 costing. _decommission maps the existing system to its
Southern Housing line: gas/oil flat 720, LPG 960 (tank+fuel removal),
electric-storage 570/840 by property-size band. Unmapped systems raise for
now -- the no-system/electric-other/other fallbacks land in the next slice.

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

domain/modelling/products.py

@@ -29,6 +29,11 @@ _ASHP_MEASURE_TYPE = "air_source_heat_pump"
 # Decommission an existing electric-storage system, by property size band.
 _DECOMMISSION_ELECTRIC_STORAGE_SMALL = 570.0
 _DECOMMISSION_ELECTRIC_STORAGE_LARGE = 840.0
+# Decommission an existing wet (boiler) system — flat across property size for
+# gas and oil; LPG carries the extra tank/fuel removal (ECOHT06-08, 03-04).
+_DECOMMISSION_GAS = 720.0
+_DECOMMISSION_OIL = 720.0
+_DECOMMISSION_LPG = 960.0
 
 # Heat-pump install (MONOBLOC, brand-neutral), by kW size band — design heat
 # loss is rounded up to the next band (ECOHT09-13).
@@ -112,11 +117,19 @@ class Products:
         return _PUMP_TOP_PRICE
 
     def _decommission(self, inputs: AshpCostInputs) -> float:
-        return (
+        if inputs.existing_system is AshpExistingSystem.ELECTRIC_STORAGE:
-            _DECOMMISSION_ELECTRIC_STORAGE_SMALL
+            return (
-            if inputs.is_small_property
+                _DECOMMISSION_ELECTRIC_STORAGE_SMALL
-            else _DECOMMISSION_ELECTRIC_STORAGE_LARGE
+                if inputs.is_small_property
-        )
+                else _DECOMMISSION_ELECTRIC_STORAGE_LARGE
+            )
+        if inputs.existing_system is AshpExistingSystem.GAS:
+            return _DECOMMISSION_GAS
+        if inputs.existing_system is AshpExistingSystem.OIL:
+            return _DECOMMISSION_OIL
+        if inputs.existing_system is AshpExistingSystem.LPG:
+            return _DECOMMISSION_LPG
+        raise ValueError(f"no decommission rate for {inputs.existing_system}")
 
     def _distribution(self, inputs: AshpCostInputs) -> float:
         radiators: int = max(_MIN_RADIATORS, min(_MAX_RADIATORS, inputs.radiator_count))

tests/domain/modelling/test_products.py

@@ -37,3 +37,29 @@ def test_ashp_bundle_cost_composes_an_electric_storage_full_distribution_dwellin
     # (7 rads) 3618 = 16290.60, with the separate 25% ASHP contingency.
     assert abs(cost.total - 16290.60) <= 1e-9
     assert abs(cost.contingency_rate - 0.25) <= 1e-9
+
+
+def _large_no_reuse(system: AshpExistingSystem) -> AshpCostInputs:
+    """A large dwelling, 8 kW band, 8 radiators, no reusable wet system — so the
+    only thing varying with ``system`` is the decommission line."""
+    return AshpCostInputs(
+        existing_system=system,
+        is_small_property=False,
+        design_heat_loss_kw=8.0,
+        radiator_count=8,
+        has_reusable_wet_system=False,
+    )
+
+
+def test_decommission_cost_varies_by_existing_system() -> None:
+    # Arrange — common: pump (8 kW) 9840 + cylinder 2382.60 + distribution (8
+    # rads) 4152 = 16374.60; only decommission differs by system.
+    products = Products()
+    common = 16374.60
+
+    # Act / Assert — gas and oil are flat 720; LPG 960; electric-storage large
+    # 840 (small 570 is pinned by the tracer above).
+    assert abs(products.ashp_bundle_cost(_large_no_reuse(AshpExistingSystem.GAS)).total - (common + 720.0)) <= 1e-9
+    assert abs(products.ashp_bundle_cost(_large_no_reuse(AshpExistingSystem.OIL)).total - (common + 720.0)) <= 1e-9
+    assert abs(products.ashp_bundle_cost(_large_no_reuse(AshpExistingSystem.LPG)).total - (common + 960.0)) <= 1e-9
+    assert abs(products.ashp_bundle_cost(_large_no_reuse(AshpExistingSystem.ELECTRIC_STORAGE)).total - (common + 840.0)) <= 1e-9