diff --git a/tests/domain/modelling/test_heating_recommendation.py b/tests/domain/modelling/test_heating_recommendation.py
index deca9ffad..51d41f035 100644
--- a/tests/domain/modelling/test_heating_recommendation.py
+++ b/tests/domain/modelling/test_heating_recommendation.py
@@ -779,3 +779,15 @@ def test_ashp_sizing_selects_the_smallest_pump_meeting_the_design_heat_loss() ->
 
     # Assert — the 7 kW aroTHERM plus rung.
     assert pcdb_id == 110265
+
+
+def test_ashp_sizing_caps_at_the_largest_pump_for_a_high_heat_loss_dwelling() -> None:
+    # A very leaky dwelling (17.5 kW design heat loss, e.g. UPRN 10002468116)
+    # exceeds the domestic aroTHERM plus range, so sizing caps at the largest
+    # rung — the 12 kW aroTHERM plus (PCDB 110281, ~11.48 kW) — an honest, still
+    # far better than the fixed 5 kW pump whose PSR collapsed to ~0.25.
+    # Act
+    pcdb_id = select_ashp_pcdb_id(design_heat_loss_kw=17.5)
+
+    # Assert — capped at the 12 kW rung, not left undersized at 5 kW.
+    assert pcdb_id == 110281