Bill metal-framed API windows on the Table 24 metal U-column 🟩

On the gov-EPC API path a window's metal frame was unrepresentable: both
transmission tables were frameless (U_pvc, g, 0.70), so a cert lodging a metal
frame (pvc_frame:"false", RdSAP input 6-5) was billed at the PVC/wooden U and
frame factor 0.70 — over-rating metal-framed dwellings.

Add the RdSAP 10 Table 24 metal U-column (PDF p.50-51; mirrors u_window's metal
values) and a pvc_frame dimension to _api_glazing_transmission: an explicit
"false" routes the window to the metal U + SAP 10.2 Table 6c frame factor 0.8;
"true"/None/other keep the PVC column + 0.70, so no PVC cert regresses.
Secondary glazing keeps its frame-independent U (only FF changes). Wired into
_api_sap_window and _api_sap_roof_window (both full-schema, lodge pvc_frame);
the reduced-field path has no pvc_frame and is unchanged.

Spec: RdSAP 10 Table 24 metal column, SAP 10.2 Table 6c, RdSAP input 6-5 (p.80).
Corpus: within-0.5 80.6% -> 81.0%, MAE 0.578 -> 0.571. Cert 21067296
68.60 -> 66.96 (toward lodged 67). Closes #1667.

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

datatypes/epc/domain/mapper.py

@@ -5342,24 +5342,67 @@ _API_GLAZING_TYPE_GAP_TO_TRANSMISSION: Dict[
 }
 
 
+# RdSAP 10 Table 24 METAL-frame U-column (PDF p.50-51) — #1667. Only the U
+# differs from the PVC/wooden column above (the glazing g is frame-independent);
+# the frame factor becomes 0.8 (SAP 10.2 Table 6c) instead of 0.70. Secondary
+# glazing (types 4/11/12) has no separate metal U — its combined U is
+# frame-independent — so it is absent here and keeps the PVC-column U. Values
+# mirror `u_window`'s metal column (`rdsap_uvalues.py`): single 5.7; DG pre-2002
+# 6/12/16 = 3.7/3.4/3.3; triple pre-2002 6/12/16 = 2.9/2.6/2.5; DG/triple 2002+
+# = 2.2; 2022+ = 1.6.
+_API_GLAZING_TYPE_TO_METAL_U: Dict[int, float] = {
+    1: 3.4, 3: 3.4, 7: 3.4,   # DG pre-2002 / unknown-date, 12mm default
+    2: 2.2, 9: 2.2,           # DG (2) / triple (9) 2002+ (pre-2022)
+    13: 1.6, 14: 1.6,         # DG / DG-or-triple 2022+
+    5: 5.7, 15: 5.7,          # single glazing
+    6: 2.6, 8: 2.6, 10: 2.6,  # triple pre-2002 / unknown / known, 12mm default
+}
+_API_GLAZING_TYPE_GAP_TO_METAL_U: Dict[tuple[int, object], float] = {
+    (1, 6): 3.7, (1, 12): 3.4, (1, "16+"): 3.3,
+    (3, 6): 3.7, (3, 12): 3.4, (3, "16+"): 3.3,
+    (7, 6): 3.7, (7, 12): 3.4, (7, "16+"): 3.3,
+    (6, 6): 2.9, (6, 12): 2.6, (6, "16+"): 2.5,
+    (8, 6): 2.9, (8, 12): 2.6, (8, "16+"): 2.5,
+    (10, 6): 2.9, (10, 12): 2.6, (10, "16+"): 2.5,
+}
+
+# SAP 10.2 Table 6c window frame factor: 0.8 for metal, 0.70 for PVC/wooden.
+_METAL_FRAME_FACTOR: Final[float] = 0.8
+
+
 def _api_glazing_transmission(
     glazing_type: Optional[int],
     glazing_gap: object,
+    pvc_frame: object = None,
 ) -> Optional[tuple[float, float, float]]:
     """Resolve (U, g, frame_factor) for an API window from RdSAP 10 Table 24.
     Per-gap override takes precedence over the type-only default. Returns None
     only when `glazing_type` is absent (None → cascade default). A glazing_type
     PRESENT but unmapped raises `UnmappedApiCode` rather than silently routing
     the window to the u_window all-None default U=2.5 — the forcing function
-    that surfaced single-glazing (code 5 → 4.8) instead of letting it hide."""
+    that surfaced single-glazing (code 5 → 4.8) instead of letting it hide.
+
+    `pvc_frame` is the lodged RdSAP input 6-5 boolean (spec p.80): "true" =
+    wooden/PVC (grouped in Table 24), "false" = metal. Only an explicit "false"
+    routes the window to the Table 24 metal U-column + FF 0.8 (#1667); "true",
+    None or any other value keeps the PVC/wooden column + FF 0.70, so no
+    PVC-lodged cert regresses."""
     if glazing_type is None:
         return None
     gap_key = (glazing_type, glazing_gap)
     if gap_key in _API_GLAZING_TYPE_GAP_TO_TRANSMISSION:
-        return _API_GLAZING_TYPE_GAP_TO_TRANSMISSION[gap_key]
-    if glazing_type in _API_GLAZING_TYPE_TO_TRANSMISSION:
-        return _API_GLAZING_TYPE_TO_TRANSMISSION[glazing_type]
-    raise UnmappedApiCode("glazing_type", glazing_type)
+        base = _API_GLAZING_TYPE_GAP_TO_TRANSMISSION[gap_key]
+    elif glazing_type in _API_GLAZING_TYPE_TO_TRANSMISSION:
+        base = _API_GLAZING_TYPE_TO_TRANSMISSION[glazing_type]
+    else:
+        raise UnmappedApiCode("glazing_type", glazing_type)
+    if pvc_frame != "false":
+        return base
+    u_pvc, g_perp, _pvc_ff = base
+    metal_u = _API_GLAZING_TYPE_GAP_TO_METAL_U.get(
+        gap_key, _API_GLAZING_TYPE_TO_METAL_U.get(glazing_type, u_pvc)
+    )
+    return (metal_u, g_perp, _METAL_FRAME_FACTOR)
 
 
 # GOV.UK RdSAP 21 `glazing_type` integer → SAP 10.2 Table 6b cascade
@@ -5478,7 +5521,7 @@ def _api_sap_roof_window(w: Any) -> SapRoofWindow:
     Table 6e Note 2 inclination adjustment (+0.30 W/m²K at 45° pitch) to
     the inclined-position value the worksheet bills on (27a). Mirror of
     the site-notes `_map_elmhurst_roof_window`."""
-    transmission = _api_glazing_transmission(w.glazing_type, w.glazing_gap)
+    transmission = _api_glazing_transmission(w.glazing_type, w.glazing_gap, w.pvc_frame)
     vertical_u = transmission[0] if transmission is not None else 2.0
     g_perp = transmission[1] if transmission is not None else 0.76
     frame_factor = w.frame_factor
@@ -5548,7 +5591,7 @@ def _api_sap_window(w: Any) -> SapWindow:
     to the SAP 10.2 Table 6b cascade enum so the cascade's daylight g_L
     lookup picks the spec-correct value (e.g. RdSAP 21 code 1 = DG
     pre-2002, cascade g_L=0.80, not single-glazed 0.90)."""
-    transmission = _api_glazing_transmission(w.glazing_type, w.glazing_gap)
+    transmission = _api_glazing_transmission(w.glazing_type, w.glazing_gap, w.pvc_frame)
     frame_factor: Optional[float] = w.frame_factor
     if frame_factor is None and transmission is not None:
         frame_factor = transmission[2]

tests/datatypes/epc/domain/test_mapper_metal_window_frame.py

@@ -0,0 +1,100 @@
+"""#1667 — on the gov-EPC API path a window's metal frame was unrepresentable:
+the transmission lookup had no frame dimension, so every window was billed at
+the PVC/wooden U-column and frame factor 0.70 even where the cert lodges a metal
+frame (`pvc_frame:"false"`). RdSAP 10 Table 24 gives metal a higher U (+0.2 to
++0.9 W/m^2K) and SAP 10.2 Table 6c a frame factor of 0.8, so we over-rated
+metal-framed dwellings. The mapper must route a metal-lodged window to the
+Table 24 metal column + FF 0.8, leaving PVC-lodged windows unchanged.
+"""
+
+from __future__ import annotations
+
+import json
+from copy import deepcopy
+from pathlib import Path
+from typing import Any, cast
+
+from datatypes.epc.domain.mapper import EpcPropertyDataMapper
+from domain.sap10_calculator.calculator import Sap10Calculator
+
+_CORPUS = Path("backend/epc_api/json_samples/RdSAP-Schema-21.0.1/corpus.jsonl")
+
+
+def _corpus_cert(uprn: str) -> dict[str, Any]:
+    for line in _CORPUS.read_text().splitlines():
+        if uprn in line:
+            cert: dict[str, Any] = json.loads(line)
+            return cert
+    raise AssertionError(f"uprn {uprn} not found in the RdSAP-21.0.1 corpus")
+
+
+def _forced_pvc_frame(payload: dict[str, Any], value: str) -> dict[str, Any]:
+    """Deep-copy `payload`, setting every lodged `pvc_frame` to `value` — used to
+    build a PVC-framed twin of a metal-framed cert."""
+    twin = deepcopy(payload)
+
+    def walk(node: object) -> None:
+        if isinstance(node, dict):
+            node_dict = cast("dict[str, Any]", node)
+            for key, child in node_dict.items():
+                if key == "pvc_frame" and child is not None:
+                    node_dict[key] = value
+                else:
+                    walk(child)
+        elif isinstance(node, list):
+            for child in cast("list[Any]", node):
+                walk(child)
+
+    walk(twin)
+    return twin
+
+
+def _window_u_and_ff(payload: dict[str, Any]) -> tuple[set[float], set[float]]:
+    epc = EpcPropertyDataMapper.from_api_response(payload)
+    assert epc is not None
+    u_values = {
+        w.window_transmission_details.u_value
+        for w in epc.sap_windows
+        if w.window_transmission_details is not None
+    }
+    frame_factors = {w.frame_factor for w in epc.sap_windows if w.frame_factor is not None}
+    return u_values, frame_factors
+
+
+def test_metal_frame_window_bills_table_24_metal_u_and_frame_factor_0p8() -> None:
+    # Arrange — cert 21067296 lodges six pvc_frame:"false" (metal) double-glazed
+    # (type 3, 12 mm) windows; HEAD billed them on the PVC column (U 2.8, FF 0.70).
+    payload = _corpus_cert("21067296")
+
+    # Act
+    u_values, frame_factors = _window_u_and_ff(payload)
+
+    # Assert — Table 24 metal DG-12mm column (3.4) and SAP 10.2 Table 6c FF 0.8.
+    assert u_values == {3.4}
+    assert frame_factors == {0.8}
+
+
+def test_pvc_frame_window_stays_on_the_pvc_column() -> None:
+    # Arrange — the same cert forced to pvc_frame:"true"; the fix must not
+    # regress PVC/wooden-framed windows (the discriminator is the boolean).
+    payload = _forced_pvc_frame(_corpus_cert("21067296"), "true")
+
+    # Act
+    u_values, frame_factors = _window_u_and_ff(payload)
+
+    # Assert — PVC DG-12mm column (2.8) and FF 0.70, exactly as before.
+    assert u_values == {2.8}
+    assert frame_factors == {0.7}
+
+
+def test_metal_frame_fix_moves_cert_toward_lodged() -> None:
+    # Arrange — 21067296 scored 68.60 on HEAD (PVC assumption), lodged 67.
+    payload = _corpus_cert("21067296")
+    epc = EpcPropertyDataMapper.from_api_response(payload)
+    assert epc is not None
+
+    # Act
+    sap = Sap10Calculator().calculate(epc).sap_score_continuous
+
+    # Assert — the metal U-penalty pulls it to ~66.96, i.e. toward lodged 67.
+    assert abs(sap - 66.96) <= 0.05