feat(overlay): enable gas A/B bands — Elmhurst-validated (winter,summer)

Gas A = (90.6, 81.6) [PCDB 18790 Worcester Greenstar 8000 Life], B = (88.0, 79.4)
[PCDB 15029 Baxi Duo-tec Combi 24 HE], read off accredited Elmhurst RdSAP10
worksheets (206)/(217). Applied to gas regular (102) + combi (104); gas/LPG/biogas
share the codes. A-rated real cert 15017550: +1.78 SAP (the under-credit fix).
Oil A/B still pending (no slot). Corpus unmoved; 797 tests green (ADR-0068).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

domain/epc/property_overlays/main_heating_system_overlay.py

@@ -287,13 +287,25 @@ _BAND_TO_TABLE_4B_CODE: dict[tuple[int, BoilerEfficiencyBand], int] = {
 }
 
 # A/B exceed Table 4b's 84 % gas / 82–84 % oil ceiling, so they cannot borrow a
-# Table 4b code — they take SEDBUK-derived `(winter, summer)` slot values that
-# MUST be validated against an accredited Elmhurst build before they are enabled
-# (ADR-0068 gated pre-req; ~14.6k A-rated homes move up + ripple into
-# Recommendations / eligibility). PENDING VALIDATION: intentionally empty — until
-# the pairs are pinned, an A/B boiler takes no slot and keeps its condensing code
-# default, so no invented numbers ship.
-_BAND_SLOT_EFFICIENCY_PCT: dict[tuple[int, BoilerEfficiencyBand], tuple[float, float]] = {}
+# Table 4b code — they take SEDBUK-derived `(winter, summer)` slot values
+# validated against an **accredited Elmhurst build** (ADR-0068 gated pre-req).
+#
+# GAS (validated 2026-07-29 on Elmhurst RdSAP10, dwelling ref 000567):
+#   A → PCDB 18790 (Worcester Greenstar 8000 Life): worksheet (206)=90.6 / (217)=81.6
+#   B → PCDB 15029 (Baxi Duo-tec Combi 24 HE class): worksheet (206)=88.0 / (217)=79.4
+# Gas/LPG/biogas share codes 101–119, so one pair per band covers all three; applied
+# to both gas regular (102) and combi (104). (Enabling A moves ~14.6k A-rated homes
+# up ~+1–3 SAP — flagged to stakeholders; some cross a band → Recs/eligibility.)
+#
+# OIL (124–132): still pending — no oil slot yet, so an oil A/B boiler keeps its
+# condensing code default (127/130). Oil A/B is a tiny population; a follow-up can
+# pin it from an oil PCDB product or an oil Elmhurst build. No invented numbers ship.
+_BAND_SLOT_EFFICIENCY_PCT: dict[tuple[int, BoilerEfficiencyBand], tuple[float, float]] = {
+    (102, BoilerEfficiencyBand.A): (90.6, 81.6),
+    (104, BoilerEfficiencyBand.A): (90.6, 81.6),
+    (102, BoilerEfficiencyBand.B): (88.0, 79.4),
+    (104, BoilerEfficiencyBand.B): (88.0, 79.4),
+}
 
 
 def band_seasonal_efficiency_pct(

tests/domain/epc/test_boiler_efficiency_band_overlay.py

@@ -42,15 +42,33 @@ def test_c_and_below_bands_resolve_to_the_table_4b_pair(
     assert band_seasonal_efficiency_pct(base_code, band) == expected
 
 
+@pytest.mark.parametrize(
+    ("base_code", "band", "expected"),
+    [
+        # Gas A/B validated against accredited Elmhurst (ADR-0068): A → PCDB 18790
+        # (Worcester Greenstar 8000 Life) 90.6/81.6; B → PCDB 15029 (Baxi Duo-tec
+        # Combi 24 HE) 88.0/79.4. Gas/LPG/biogas share codes — one pair per band on
+        # both regular (102) and combi (104).
+        (102, BoilerEfficiencyBand.A, (90.6, 81.6)),
+        (104, BoilerEfficiencyBand.A, (90.6, 81.6)),
+        (102, BoilerEfficiencyBand.B, (88.0, 79.4)),
+        (104, BoilerEfficiencyBand.B, (88.0, 79.4)),
+    ],
+)
+def test_gas_a_and_b_bands_use_the_elmhurst_validated_slot(
+    base_code: int, band: BoilerEfficiencyBand, expected: tuple[float, float]
+) -> None:
+    # These exceed Table 4b's 84% ceiling (the A under-credit the fix removes).
+    assert band_seasonal_efficiency_pct(base_code, band) == expected
+
+
 @pytest.mark.parametrize("band", [BoilerEfficiencyBand.A, BoilerEfficiencyBand.B])
-@pytest.mark.parametrize("base_code", [102, 104, 127, 130])
-def test_a_and_b_bands_are_pending_elmhurst_validation(
+@pytest.mark.parametrize("base_code", [127, 130])
+def test_oil_a_and_b_bands_are_pending_validation(
     base_code: int, band: BoilerEfficiencyBand
 ) -> None:
-    # A/B exceed Table 4b's 84% ceiling — they are judgement values that MUST be
-    # pinned against an accredited Elmhurst build before they are enabled
-    # (ADR-0068 gated pre-req). Until then: no slot, so an A/B boiler keeps its
-    # condensing code default (no invented numbers ship).
+    # Oil A/B not yet validated — no slot, so an oil A/B boiler keeps its
+    # condensing code default (127/130). No invented numbers ship (ADR-0068).
     assert band_seasonal_efficiency_pct(base_code, band) is None
 
 
@@ -77,6 +95,10 @@ def test_non_banded_boilers_never_take_a_slot(
         ("Gas boiler, combi", BoilerEfficiencyBand.G, (66.0, 57.0)),
         ("Oil boiler, regular", BoilerEfficiencyBand.E, (71.0, 59.0)),
         ("Oil boiler, combi", BoilerEfficiencyBand.D, (77.0, 68.0)),
+        # Elmhurst-validated A/B upside on gas (the A under-credit fix).
+        ("Gas boiler, regular", BoilerEfficiencyBand.A, (90.6, 81.6)),
+        ("Gas boiler, combi", BoilerEfficiencyBand.A, (90.6, 81.6)),
+        ("Gas boiler, combi", BoilerEfficiencyBand.B, (88.0, 79.4)),
     ],
 )
 def test_overlay_sets_the_band_efficiency_slot_on_gas_and_oil_boilers(
@@ -96,9 +118,8 @@ def test_overlay_sets_the_band_efficiency_slot_on_gas_and_oil_boilers(
     [
         # No band → no slot (today's behaviour: keep the condensing code default).
         ("Gas boiler, combi", None),
-        # A/B pending Elmhurst → no slot yet.
-        ("Gas boiler, combi", BoilerEfficiencyBand.A),
-        # C on gas regular equals the code default — still set, but a no-op value.
+        # Oil A/B still pending Elmhurst → no slot yet.
+        ("Oil boiler, combi", BoilerEfficiencyBand.A),
         # Non-banded boilers ignore the band letter entirely.
         ("Electric boiler", BoilerEfficiencyBand.D),
         ("Solid fuel boiler", BoilerEfficiencyBand.G),