feat(modelling): gate boiler upgrade on the existing boiler's efficiency

Don't offer a like-for-like gas boiler swap to a dwelling whose existing gas
boiler is already at least as efficient as the new condensing boiler (SAP 10.2
Table 4b codes 102/104 = 84% winter) — it gains nothing, and the dwelling gets
the tune-up (cylinder + controls) instead. `_already_condensing` compares the
existing code's Table 4b winter efficiency to 84%; a non-Table-4b code (solid
fuel) has no comparable efficiency and is never treated as already-condensing.

The gate is GAS-ONLY: a non-gas boiler → gas is a fuel switch whose value (cost
/ carbon) is not captured by winter efficiency, so oil/LPG/coal → gas is never
suppressed on efficiency grounds (only gated on the mains-gas connection).

This correctly demotes the gas-with-cylinder example (cert lodges code 114
"Regular, condensing", 84% winter) to a tune-up case — confirming that 114→102
is ~0 boiler-efficiency gain in both our calc and Elmhurst (both Table 4b 84%);
Elmhurst's uplift there came from the cylinder + flue, not the boiler. The
boiler-with-cylinder overlay stays validated by the lpg pin (code 115, non-
condensing + cylinder) and by recasting the 114 fixtures' code to a pre-1998
non-condensing boiler (110) in the boiler tests — the overlay overwrites the
code to 102 regardless, so only eligibility changes, not the delta-0 result.
New tests: an already-condensing gas boiler yields no boiler upgrade (but a
tune-up); an oil condensing boiler is not gated (the fuel switch survives).

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

domain/modelling/generators/heating_recommendation.py

@@ -27,6 +27,9 @@ from domain.modelling.products import (
 from domain.modelling.measure_type import MeasureType
 from domain.modelling.recommendation import Cost, MeasureOption, Recommendation
 from domain.modelling.simulation import EpcSimulation, HeatingOverlay
+from domain.sap10_calculator.tables.table_4b import (
+    table_4b_seasonal_efficiencies_pct,
+)
 from repositories.product.product_repository import ProductRepository
 
 _HEATING_SURFACE = "Heating & Hot Water"
@@ -191,6 +194,14 @@ _ELECTRIC_BOILER_SAP_CODE_RANGE = range(191, 197)
 _CYLINDER_JACKET_INSULATION_TYPE = 2
 _MIN_CYLINDER_INSULATION_MM = 80
 
+# The new condensing boiler's winter efficiency: SAP 10.2 Table 4b codes 102
+# (regular condensing) and 104 (condensing combi) both lodge 84% winter. A
+# like-for-like gas swap onto an existing gas boiler that already meets this
+# gains nothing, so it is not offered (the dwelling gets a tune-up instead). The
+# gate is gas-only: a non-gas boiler → gas is a fuel switch whose value is not
+# captured by winter efficiency alone, so it is never suppressed on efficiency.
+_NEW_BOILER_WINTER_EFFICIENCY_PCT = 84.0
+
 
 # --- ASHP cost interpretation (ADR-0025): read the dwelling into the typed
 # inputs the catalogue math needs. The modelling-layer half of the split; the
@@ -427,7 +438,11 @@ def _boiler_upgrade_eligible(epc: EpcPropertyData) -> bool:
     condensing boiler. The gas end-state is installable only with a mains-gas
     connection, so gas dwellings always qualify and a non-gas wet boiler
     (oil/LPG/solid) qualifies only where mains gas is present. Electric boilers
-    are left alone — electrification, not a gas swap, is their upgrade path."""
+    are left alone — electrification, not a gas swap, is their upgrade path. A
+    gas boiler that already meets the new condensing efficiency is not re-offered
+    a like-for-like swap (it gains nothing — the dwelling gets a tune-up
+    instead); a non-gas boiler is a fuel switch, so it is never gated on
+    efficiency."""
     main: MainHeatingDetail = epc.sap_heating.main_heating_details[0]
     code: Optional[int] = main.sap_main_heating_code
     if code is None:
@@ -436,7 +451,24 @@ def _boiler_upgrade_eligible(epc: EpcPropertyData) -> bool:
         return False
     if code in _ELECTRIC_BOILER_SAP_CODE_RANGE:
         return False
-    return epc.sap_energy_source.mains_gas
+    if not epc.sap_energy_source.mains_gas:
+        return False
+    if main.main_fuel_type in _GAS_FUEL_CODES and _already_condensing(code):
+        return False
+    return True
+
+
+def _already_condensing(sap_main_heating_code: int) -> bool:
+    """Whether an existing gas boiler already meets the new condensing boiler's
+    winter efficiency (SAP 10.2 Table 4b). Non-Table-4b codes (e.g. solid fuel)
+    have no comparable efficiency and so are never treated as already-condensing."""
+    efficiencies: Optional[tuple[float, float]] = table_4b_seasonal_efficiencies_pct(
+        sap_main_heating_code
+    )
+    if efficiencies is None:
+        return False
+    winter_efficiency_pct: float = efficiencies[0]
+    return winter_efficiency_pct >= _NEW_BOILER_WINTER_EFFICIENCY_PCT
 
 
 def _boiler_combi_overlay(epc: EpcPropertyData) -> HeatingOverlay:

tests/domain/modelling/test_elmhurst_cascade_pins.py

@@ -772,6 +772,11 @@ def test_boiler_with_cylinder_overlay_reproduces_the_relodged_after() -> None:
     before: EpcPropertyData = parse_recommendation_summary(
         "boiler_cyl_gas_001431_before.pdf"
     )
+    # The cert lodges code 114 (already condensing), which the efficiency gate
+    # excludes from a like-for-like swap; recast to a pre-1998 non-condensing
+    # boiler (110) so the upgrade is offered. The overlay overwrites the code to
+    # 102 regardless, so this changes only eligibility, not the validated result.
+    before.sap_heating.main_heating_details[0].sap_main_heating_code = 110
     after: EpcPropertyData = parse_recommendation_summary(
         "boiler_cyl_gas_001431_after.pdf"
     )

tests/domain/modelling/test_heating_recommendation.py

@@ -256,9 +256,17 @@ def test_existing_heat_pump_yields_no_ashp_bundle() -> None:
 
 
 def _gas_boiler_with_cylinder_baseline() -> EpcPropertyData:
-    """A mains-gas wet boiler (Table 4b code 114) heating an uninsulated, un-
-    thermostatted hot-water cylinder — the boiler-with-cylinder dwelling."""
-    return parse_recommendation_summary("boiler_cyl_gas_001431_before.pdf")
+    """A mains-gas wet boiler heating an uninsulated, un-thermostatted hot-water
+    cylinder — the boiler-with-cylinder dwelling. The cert lodges code 114
+    (already condensing), which the efficiency gate excludes from a like-for-like
+    swap; recast to a pre-1998 non-condensing boiler (code 110) so the boiler
+    upgrade is a genuine candidate (the overlay overwrites the code to 102
+    regardless of the before)."""
+    epc: EpcPropertyData = parse_recommendation_summary(
+        "boiler_cyl_gas_001431_before.pdf"
+    )
+    epc.sap_heating.main_heating_details[0].sap_main_heating_code = 110
+    return epc
 
 
 def test_gas_boiler_with_cylinder_dwelling_yields_a_boiler_upgrade_bundle() -> None:
@@ -329,6 +337,42 @@ def test_boiler_upgrade_skips_thermostat_when_already_present() -> None:
     assert overlay.cylinder_insulation_type == 2
 
 
+def test_already_condensing_gas_boiler_yields_no_boiler_upgrade() -> None:
+    # Arrange — the real cert: a mains-gas boiler already condensing (Table 4b
+    # code 114, 84% winter — the same as the new code 102). A like-for-like swap
+    # gains nothing, so the boiler upgrade is not offered; the dwelling still
+    # gets a tune-up for its cylinder + controls.
+    baseline: EpcPropertyData = parse_recommendation_summary(
+        "boiler_cyl_gas_001431_before.pdf"
+    )
+
+    # Act
+    recommendation: Recommendation | None = recommend_heating(baseline, _StubProducts())
+
+    # Assert
+    assert recommendation is not None
+    measure_types = {o.measure_type for o in recommendation.options}
+    assert "gas_boiler_upgrade" not in measure_types
+    assert "system_tune_up_zoned" in measure_types
+
+
+def test_non_gas_boiler_is_not_gated_on_efficiency() -> None:
+    # Arrange — an oil condensing boiler (Table 4b code 127, 84% winter — meets
+    # the new gas boiler's efficiency) on a mains-gas street. Unlike a gas
+    # boiler, the oil→gas fuel switch has value beyond efficiency, so it is NOT
+    # suppressed by the efficiency gate.
+    baseline: EpcPropertyData = _gas_boiler_with_cylinder_baseline()
+    baseline.sap_heating.main_heating_details[0].sap_main_heating_code = 127
+    baseline.sap_heating.main_heating_details[0].main_fuel_type = 28  # oil
+
+    # Act
+    recommendation: Recommendation | None = recommend_heating(baseline, _StubProducts())
+
+    # Assert — the boiler upgrade is still offered (the fuel switch).
+    assert recommendation is not None
+    assert "gas_boiler_upgrade" in {o.measure_type for o in recommendation.options}
+
+
 def test_electric_boiler_dwelling_yields_no_gas_boiler_upgrade() -> None:
     # Arrange — an electric boiler (Table 4a code 191) is left alone:
     # electrification, not a gas swap, is its upgrade path.