S0380.239: system-build walls take masonry structural infiltration (0.35)

RdSAP 10 §2 (Ventilation, "Walls" row): "Structural infiltration: 0.25
for steel or timber frame or 0.35 for masonry construction ... System
build: treated as masonry." `_is_timber_or_steel_frame` wrongly included
wall_construction code 6 (system build) alongside code 5 (timber frame),
handing system-build dwellings the 0.25 structural ACH instead of 0.35.

On the cat-10 room-heater fixture (ref 001431, walls SY System Build →
code 6) this under-stated the infiltration rate (18) by exactly 0.10
(0.45 vs worksheet 0.55), dropping the effective air change (25), the
ventilation heat loss (38)m = 0.33 × (25)m × (5), and the heat-transfer
coefficient (39) — so space-heating demand (98) came out 404 kWh low
((211) 11158.6 vs worksheet 11563.2). Restrict the 0.25 branch to code 5
only; code 6 (and everything else) is masonry at 0.35.

Pins the rating-block (38)m ventilation heat loss mean = 83.3613 W/K at
abs 1e-4 and asserts the classifier treats the system-build wall as
masonry. §4 suite green (2415 passed, 1 skipped); no existing fixture
relied on system-build → 0.25.

Residual after this slice: SAP +0.03 / cost -£0.95 — a small fabric (33)
gap (-0.15 W/K) plus lighting (232) +1.0 kWh remain as separate causes.

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

backend/documents_parser/tests/test_room_heater_worksheet_001431.py

@@ -31,6 +31,7 @@ from datatypes.epc.domain.mapper import EpcPropertyDataMapper
 from domain.sap10_calculator.calculator import calculate_sap_from_inputs
 from domain.sap10_calculator.rdsap.cert_to_inputs import (
     SAP_10_2_SPEC_PRICES,
+    _is_timber_or_steel_frame,  # pyright: ignore[reportPrivateUsage]
     cert_to_inputs,
 )
 
@@ -44,6 +45,12 @@ _FIXTURE_DIR = (
 # heater is electric (efficiency (216) = 100 %), so (219) == (64) output.
 _WORKSHEET_LINE_219_WATER_FUEL_KWH = 1770.2313
 
+# P960 line ref (38)m "Ventilation heat loss calculated monthly" — rating
+# block, mean of the 12 printed monthly values
+# (90.1949 .. 86.1692) / 12. The dwelling is SY System Build (masonry per
+# RdSAP 10 §2), so the structural infiltration (11) = 0.35 not 0.25.
+_WORKSHEET_LINE_38_VENT_HEAT_LOSS_MEAN_W_PER_K = 83.3613
+
 _ABS_TOLERANCE = 0.0001
 
 
@@ -99,3 +106,46 @@ def test_electric_room_heater_water_fuel_matches_worksheet_line_219() -> None:
 
     # Assert
     assert abs(actual_water_fuel_kwh - expected_water_fuel_kwh) <= _ABS_TOLERANCE
+
+
+def test_system_build_wall_is_classified_masonry_for_structural_infiltration() -> None:
+    # Arrange — the dwelling's walls are SY System Build (wall_construction
+    # code 6). Per RdSAP 10 §2 (Ventilation, "Walls" row): "System build:
+    # treated as masonry", so it must NOT take the 0.25 steel/timber-frame
+    # structural infiltration — only code 5 (timber frame) does.
+    summary_pdf = next(_FIXTURE_DIR.glob("Summary_*.pdf"))
+    pages = _summary_pdf_to_pages(summary_pdf)
+    site_notes = ElmhurstSiteNotesExtractor(pages).extract()
+    epc = EpcPropertyDataMapper.from_elmhurst_site_notes(site_notes)
+    main_wall_construction_code = epc.sap_building_parts[0].wall_construction
+
+    # Act
+    is_frame = _is_timber_or_steel_frame(epc.sap_building_parts)
+
+    # Assert
+    assert main_wall_construction_code == 6
+    assert is_frame is False
+
+
+def test_electric_room_heater_ventilation_heat_loss_matches_worksheet_line_38() -> None:
+    # Arrange — with SY System Build treated as masonry the structural
+    # infiltration (11) = 0.35, lifting the effective air change (25) and
+    # the monthly ventilation heat loss (38)m = 0.33 × (25)m × (5) to the
+    # worksheet's rating-block values.
+    summary_pdf = next(_FIXTURE_DIR.glob("Summary_*.pdf"))
+    pages = _summary_pdf_to_pages(summary_pdf)
+    site_notes = ElmhurstSiteNotesExtractor(pages).extract()
+    epc = EpcPropertyDataMapper.from_elmhurst_site_notes(site_notes)
+    expected_vent_heat_loss_w_per_k = _WORKSHEET_LINE_38_VENT_HEAT_LOSS_MEAN_W_PER_K
+
+    # Act
+    rating = calculate_sap_from_inputs(
+        cert_to_inputs(epc, prices=SAP_10_2_SPEC_PRICES),
+    )
+    actual_vent_heat_loss_w_per_k = rating.intermediate["infiltration_w_per_k"]
+
+    # Assert
+    assert (
+        abs(actual_vent_heat_loss_w_per_k - expected_vent_heat_loss_w_per_k)
+        <= _ABS_TOLERANCE
+    )

domain/sap10_calculator/rdsap/cert_to_inputs.py

@@ -1393,13 +1393,19 @@ def _climate_source(
 
 
 def _is_timber_or_steel_frame(parts: list[SapBuildingPart]) -> bool:
-    """RdSAP 10 §5: wall_construction codes 5 (timber frame) and 6 (system
-    build steel frame) get the lower 0.25 structural ACH; everything else
-    is treated as 0.35 masonry."""
+    """RdSAP 10 §2 (Ventilation, "Walls" row): "Structural infiltration:
+    0.25 for steel or timber frame or 0.35 for masonry construction ...
+    System build: treated as masonry." So only wall_construction code 5
+    (timber frame) takes the lower 0.25 structural ACH; code 6 (system
+    build) is explicitly masonry (0.35), as is everything else.
+
+    (Park homes also take the timber-frame value per the same spec row,
+    but that is a dwelling-type flag, not a wall_construction code, and is
+    out of scope here.)"""
     if not parts:
         return False
     wc = parts[0].wall_construction
-    return isinstance(wc, int) and wc in (5, 6)
+    return isinstance(wc, int) and wc == 5
 
 
 def _living_area_fraction_default(habitable_rooms_count: Optional[int]) -> float: