diff --git a/domain/modelling/products.py b/domain/modelling/products.py
index 4fa6ee31..132e3896 100644
--- a/domain/modelling/products.py
+++ b/domain/modelling/products.py
@@ -63,6 +63,13 @@ _DISTRIBUTION_BY_RADIATORS: dict[int, float] = {
 }
 _MIN_RADIATORS = 4
 _MAX_RADIATORS = 12
+# Power-flush + inhibitor when reusing an existing wet system (ECOHT67).
+_DISTRIBUTION_FLUSH = 168.0
+# Fraction of a full new distribution charged when reusing an existing wet
+# system — a stand-in for partial radiator upsizing at low ASHP flow temps.
+# The headline uncertainty in the model; recalibrate against real reuse-job
+# costs / survey data (ADR-0025).
+_REUSE_DISTRIBUTION_FRACTION = 0.5
 
 
 class AshpExistingSystem(Enum):
@@ -133,4 +140,9 @@ class Products:
 
     def _distribution(self, inputs: AshpCostInputs) -> float:
         radiators: int = max(_MIN_RADIATORS, min(_MAX_RADIATORS, inputs.radiator_count))
-        return _DISTRIBUTION_BY_RADIATORS[radiators]
+        full: float = _DISTRIBUTION_BY_RADIATORS[radiators]
+        # An existing wet system is reused, not rebuilt: a flush plus a fraction
+        # of the full distribution to cover partial radiator upsizing.
+        if inputs.has_reusable_wet_system:
+            return _DISTRIBUTION_FLUSH + _REUSE_DISTRIBUTION_FRACTION * full
+        return full
diff --git a/tests/domain/modelling/test_products.py b/tests/domain/modelling/test_products.py
index 964ee7e5..6db7c458 100644
--- a/tests/domain/modelling/test_products.py
+++ b/tests/domain/modelling/test_products.py
@@ -63,3 +63,24 @@ def test_decommission_cost_varies_by_existing_system() -> None:
     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
+
+
+def test_reusable_wet_system_prices_a_flush_plus_half_the_distribution() -> None:
+    # Arrange — a gas dwelling whose wet system is reusable: instead of a full
+    # new distribution, the ASHP pays a power-flush plus half the radiator band
+    # (a documented estimate for partial radiator upsizing — ADR-0025).
+    products = Products()
+    inputs = AshpCostInputs(
+        existing_system=AshpExistingSystem.GAS,
+        is_small_property=False,
+        design_heat_loss_kw=8.0,
+        radiator_count=8,
+        has_reusable_wet_system=True,
+    )
+
+    # Act
+    cost: Cost = products.ashp_bundle_cost(inputs)
+
+    # Assert — decommission 720 + pump 9840 + cylinder 2382.60 + distribution
+    # (flush 168 + 0.5 x 4152 = 2244) = 15186.60.
+    assert abs(cost.total - 15186.60) <= 1e-9