ASHP sizing selects the smallest pump meeting the dwelling's design heat loss 🟥

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

domain/modelling/generators/heating_recommendation.py

@@ -137,6 +137,31 @@ _ASHP_OVERLAY = HeatingOverlay(
 )
 
 
+# ASHP sizing ladder (ADR-0049): the Vaillant aroTHERM plus "& AI VIH RW" family,
+# one representative PCDB Table 362 record per nominal size, ascending by rated
+# max output (kW). The pump is sized to the dwelling's design heat loss so SAP
+# 10.2 Appendix N reads the heat-pump efficiency near its PSR peak (~1.0), rather
+# than a single fixed unit that is grossly undersized on high-heat-loss dwellings
+# — where the PSR collapses and the ASHP scores at/below the resistance baseline.
+# 110257 (the 5 kW rung) is the previously-fixed anchor, validated against the
+# relodged after-cert.
+_ASHP_SIZING_LADDER: tuple[tuple[float, int], ...] = (
+    (3.76, 110249),   # aroTHERM plus 3.5 kW
+    (4.37, 110257),   # aroTHERM plus 5 kW
+    (6.40, 110265),   # aroTHERM plus 7 kW
+    (7.93, 110273),   # aroTHERM plus 10 kW
+    (11.48, 110281),  # aroTHERM plus 12 kW
+)
+
+
+def select_ashp_pcdb_id(design_heat_loss_kw: float) -> int:
+    """The PCDB heat-pump record to install for a dwelling with
+    `design_heat_loss_kw`: the smallest ladder rung whose rated output meets the
+    load (PSR >= 1.0), capped at the largest rung when the load exceeds the
+    domestic aroTHERM plus range (ADR-0049)."""
+    raise NotImplementedError
+
+
 # --- Gas boiler upgrade (Heating/HW expansion): replace an existing wet boiler
 # with a modern gas condensing boiler. Validated against Elmhurst before/after
 # re-lodgements (cert 001431): the upgrade always targets mains gas — gas->gas

tests/domain/modelling/test_heating_recommendation.py

@@ -7,7 +7,10 @@ later slices. Detection + pricing only; impact is produced by scoring (ADR-0016)
 
 from datatypes.epc.domain.epc_property_data import EpcPropertyData
 from domain.geospatial.planning_restrictions import PlanningRestrictions
-from domain.modelling.generators.heating_recommendation import recommend_heating
+from domain.modelling.generators.heating_recommendation import (
+    recommend_heating,
+    select_ashp_pcdb_id,
+)
 from domain.modelling.measure_type import MeasureType
 from domain.modelling.product import Product
 from domain.modelling.recommendation import Recommendation
@@ -764,3 +767,15 @@ def test_boiler_upgrade_leaves_adequate_controls_unchanged() -> None:
     ).overlay.heating
     assert overlay is not None
     assert overlay.main_heating_control is None
+
+
+def test_ashp_sizing_selects_the_smallest_pump_meeting_the_design_heat_loss() -> None:
+    # A 6 kW design heat loss needs a pump whose rated output covers it: the 7 kW
+    # aroTHERM plus (PCDB 110265, ~6.40 kW), the smallest ladder rung meeting the
+    # load — not the fixed 5 kW unit (4.37 kW) that would sit below the load and
+    # collapse SAP's Appendix-N PSR efficiency.
+    # Act
+    pcdb_id = select_ashp_pcdb_id(design_heat_loss_kw=6.0)
+
+    # Assert — the 7 kW aroTHERM plus rung.
+    assert pcdb_id == 110265