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Wall + party-wall area = Σ (perim_i × height_i), not ground × avg × count
SAP §3 wall heat-loss area sums each storey individually: `Σ (heat_loss_perimeter_i × room_height_i)`. Pre-fix used the short-cut `ground_perimeter × avg_height × storey_count`, which over-counts upper storeys whenever they have a smaller perimeter than the ground (set-back top floors, ground-floor additions, etc.). RdSAP §5.10 party-wall area follows the same per-storey-sum convention. Surfaced by Elmhurst 000474 Main (ground perim 7.07, first 5.27): our gross-wall over-counted by ~10 m², the (29a) W/K downstream by ~15 W/K on this cert. Documented at the time as follow-up #2; this slice closes it. The §3 partial-conformance test's gap-#2 entry is removed; gap #1 (RR sub-areas) remains. Fix lives in two parallel code paths: - dimensions.py: per-storey accumulation inside the existing fd loop - heat_transmission.py: _part_geometry now emits gross_wall_area_m2 and party_wall_area_m2 directly, dropping the avg_height + storey_count intermediate fields (no other consumer) Tests: - New: gross_wall_area_sums_per_storey_perimeter_times_height_… (2-storey main, ground 10 m / first 6 m, same height — expects Σ=40 m² not ground×avg×count=50) - New: party_wall_area_sums_per_storey_party_length_… (same shape, ground party 5 / first party 3 → Σ=20 not 25) - New: walls_w_per_k_uses_sum_of_per_storey_perimeter_… (heat- transmission counterpart: 0.6 × 40 = 24 W/K not 30) 829 tests pass. Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
This commit is contained in:
Khalim Conn-Kowlessar
parent
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commit
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4 changed files with 121 additions and 29 deletions
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@ -112,13 +112,17 @@ def dimensions_from_cert(epc: EpcPropertyData) -> Dimensions:
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ground_area += ground.total_floor_area_m2 or 0.0
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ground_perim += ground.heat_loss_perimeter_m or 0.0
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top_area += top.total_floor_area_m2 or 0.0
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gross_wall += (ground.heat_loss_perimeter_m or 0.0) * part_height * part_floor_count
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party_wall += (ground.party_wall_length_m or 0.0) * part_height * part_floor_count
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# SAP §3 wall area: Σ (heat_loss_perimeter_i × height_i) across each
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# storey of the part. Pre-fix `ground_perim × avg_height × count`
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# over-counts upper storeys whenever they have a different
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# perimeter (e.g. set-back top floor, Elmhurst 000474 Main).
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for fd in part.sap_floor_dimensions:
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fa = fd.total_floor_area_m2 or 0.0
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fh = fd.room_height_m or _DEFAULT_STOREY_HEIGHT_M
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sum_per_storey_area_m2 += fa
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sum_per_storey_volume_m3 += fa * fh
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gross_wall += (fd.heat_loss_perimeter_m or 0.0) * fh
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party_wall += (fd.party_wall_length_m or 0.0) * fh
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# Room-in-roof: counts as one additional storey per RdSAP §1.8 +
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# §3.9. Both failing certs in the golden suite are Simplified
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@ -143,29 +143,32 @@ def _part_geometry(part: SapBuildingPart) -> dict[str, float]:
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if not part.sap_floor_dimensions:
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return {
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"ground_floor_area_m2": 0.0,
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"ground_perimeter_m": 0.0,
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"top_floor_area_m2": 0.0,
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"party_wall_length_m": 0.0,
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"avg_room_height_m": _DEFAULT_STOREY_HEIGHT_M,
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"storey_count": 1.0,
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"gross_wall_area_m2": 0.0,
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"party_wall_area_m2": 0.0,
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}
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fds = list(part.sap_floor_dimensions)
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ground = next((fd for fd in fds if fd.floor == 0), fds[0])
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indexed = [(fd.floor if fd.floor is not None else 0, fd) for fd in fds]
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top = max(indexed, key=lambda kv: kv[0])[1]
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total_area = sum(fd.total_floor_area_m2 or 0.0 for fd in fds)
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weighted_height = sum(
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(fd.total_floor_area_m2 or 0.0) * (fd.room_height_m or _DEFAULT_STOREY_HEIGHT_M)
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# SAP §3 wall area is Σ (perimeter_i × height_i) across each storey of
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# the part — same convention as dimensions.gross_wall_area_m2. The
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# ground-perim × avg × count short-cut over-counts upper storeys when
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# the perimeter shrinks (e.g. Elmhurst 000474 Main: ground 7.07, first
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# 5.27).
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gross_wall = sum(
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(fd.heat_loss_perimeter_m or 0.0) * (fd.room_height_m or _DEFAULT_STOREY_HEIGHT_M)
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for fd in fds
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)
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party_wall = sum(
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(fd.party_wall_length_m or 0.0) * (fd.room_height_m or _DEFAULT_STOREY_HEIGHT_M)
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for fd in fds
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)
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avg_height = (weighted_height / total_area) if total_area > 0 else _DEFAULT_STOREY_HEIGHT_M
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return {
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"ground_floor_area_m2": ground.total_floor_area_m2 or 0.0,
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"ground_perimeter_m": ground.heat_loss_perimeter_m or 0.0,
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"top_floor_area_m2": top.total_floor_area_m2 or 0.0,
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"party_wall_length_m": ground.party_wall_length_m or 0.0,
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"avg_room_height_m": avg_height,
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"storey_count": float(len(fds)),
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"gross_wall_area_m2": gross_wall,
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"party_wall_area_m2": party_wall,
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}
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@ -286,13 +289,11 @@ def heat_transmission_from_cert(
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upw = u_party_wall(party_wall_construction=party_construction)
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y = thermal_bridging_y(age_band=age_band)
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storey_count = geom["storey_count"]
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storey_height = geom["avg_room_height_m"]
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gross_wall_area = geom["ground_perimeter_m"] * storey_height * storey_count
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gross_wall_area = geom["gross_wall_area_m2"]
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w_area = window_total_area_m2 if i == 0 else 0.0
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d_area = door_area if i == 0 else 0.0
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net_wall_area = max(0.0, gross_wall_area - w_area - d_area)
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party_area = geom["party_wall_length_m"] * storey_height * storey_count
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party_area = geom["party_wall_area_m2"]
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roof_area = geom["top_floor_area_m2"] if exposure.has_exposed_roof else 0.0
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floor_area_total = geom["ground_floor_area_m2"] if exposure.has_exposed_floor else 0.0
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@ -180,6 +180,65 @@ def test_dwelling_storey_count_is_max_across_parts_not_sum() -> None:
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assert result.storey_count == 2
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def test_gross_wall_area_sums_per_storey_perimeter_times_height_not_ground_perim_times_avg() -> None:
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# Arrange — 2-storey terrace where the upper storey has a smaller
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# heat-loss perimeter than the ground (e.g. set-back upper floor or a
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# wider ground addition). Surfaced by Elmhurst 000474: Main has
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# ground perim 7.07, first 5.27 — the worksheet sums each storey
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# separately, but pre-fix code used `ground_perim × avg_height ×
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# storey_count` which over-counts the upper storey's wall area.
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main = make_building_part(
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identifier=BuildingPartIdentifier.MAIN,
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floor_dimensions=[
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make_floor_dimension(
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total_floor_area_m2=50.0, room_height_m=2.5,
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party_wall_length_m=0.0, heat_loss_perimeter_m=10.0, floor=0,
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),
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make_floor_dimension(
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total_floor_area_m2=50.0, room_height_m=2.5,
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party_wall_length_m=0.0, heat_loss_perimeter_m=6.0, floor=1,
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),
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],
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)
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epc = make_minimal_sap10_epc(total_floor_area_m2=100.0, sap_building_parts=[main])
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# Act
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result = dimensions_from_cert(epc)
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# Assert — Σ (perim × height) = 10×2.5 + 6×2.5 = 40.
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# Pre-fix would have given 10 × 2.5 × 2 = 50.
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assert result.gross_wall_area_m2 == pytest.approx(40.0)
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def test_party_wall_area_sums_per_storey_party_length_times_height_not_ground_party_times_avg() -> None:
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# Arrange — Same per-storey-differs shape, but applied to the party
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# wall. Two-storey main, ground party 5 m, upper party 3 m (e.g. the
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# upper storey is set back from the party line). RdSAP §5.10 party
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# area is also Σ (party_length_i × height_i), not
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# ground_party × avg × count.
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main = make_building_part(
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identifier=BuildingPartIdentifier.MAIN,
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floor_dimensions=[
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make_floor_dimension(
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total_floor_area_m2=50.0, room_height_m=2.5,
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party_wall_length_m=5.0, heat_loss_perimeter_m=0.0, floor=0,
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),
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make_floor_dimension(
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total_floor_area_m2=50.0, room_height_m=2.5,
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party_wall_length_m=3.0, heat_loss_perimeter_m=0.0, floor=1,
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),
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],
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)
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epc = make_minimal_sap10_epc(total_floor_area_m2=100.0, sap_building_parts=[main])
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# Act
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result = dimensions_from_cert(epc)
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# Assert — Σ (party × height) = 5×2.5 + 3×2.5 = 20.
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# Pre-fix would have given 5 × 2.5 × 2 = 25.
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assert result.party_wall_area_m2 == pytest.approx(20.0)
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def test_room_in_roof_on_main_adds_one_to_dwelling_storey_count_only_once() -> None:
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# Arrange — Main 2-storey with RR + same-height 2-storey extension
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# without RR. RR adds one storey to MAIN (giving 3), extension stays
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@ -518,6 +518,40 @@ def test_thermal_bridging_drops_for_newer_age_band_per_table_21() -> None:
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assert bridging_m == pytest.approx(bridging_g * 0.08 / 0.15, abs=2.0)
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def test_walls_w_per_k_uses_sum_of_per_storey_perimeter_times_height_not_ground_perim_times_avg() -> None:
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# Arrange — 2-storey age G cavity, upper storey set back so the
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# ground perimeter (10 m) exceeds the first-floor perimeter (6 m).
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# Both storey heights 2.5 m. Σ (perim × height) = 10×2.5 + 6×2.5 = 40
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# m² gross wall (no windows/doors → 40 m² net). U_wall(G, cavity) = 0.6.
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# Expected walls_w_per_k = 0.6 × 40 = 24. Pre-fix used
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# ground_perim × avg × count = 10 × 2.5 × 2 = 50 m² → 30 W/K (overstates).
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main = make_building_part(
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identifier=BuildingPartIdentifier.MAIN,
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construction_age_band="G",
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wall_construction=4, wall_insulation_type=4,
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party_wall_construction=1, roof_construction=4,
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floor_dimensions=[
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make_floor_dimension(
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total_floor_area_m2=50.0, room_height_m=2.5,
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party_wall_length_m=0.0, heat_loss_perimeter_m=10.0, floor=0,
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),
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make_floor_dimension(
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total_floor_area_m2=50.0, room_height_m=2.5,
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party_wall_length_m=0.0, heat_loss_perimeter_m=6.0, floor=1,
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),
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],
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)
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epc = make_minimal_sap10_epc(
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total_floor_area_m2=100.0, country_code="ENG", sap_building_parts=[main],
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)
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# Act
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result = heat_transmission_from_cert(epc)
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# Assert — 0.6 × 40 = 24, not 30.
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assert result.walls_w_per_k == pytest.approx(24.0, abs=0.5)
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def test_main_plus_extension_sums_per_element_contributions() -> None:
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# Arrange — Main + single-storey age L extension. Each contributes to the
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# element totals. With_extension > main_only on every populated field
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@ -1079,12 +1113,7 @@ def test_section_3_partial_match_against_elmhurst_worksheet(fixture: ModuleType)
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Known divergences:
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1. RR walls not computed → smaller (33), smaller (31) for RR fixtures
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2. Per-storey-different heat-loss perimeters not handled — our code
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does `ground_perim × avg_height × storey_count` which over-counts
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when upper storeys are smaller than the ground (surfaced by
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worksheet 000474 where Main has ground perim 7.07 / first 5.27).
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Right formula: Σ (perim_i × height_i). Tracked as follow-up.
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3. Window U-value is per-window in Elmhurst; we pass an area-weighted
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2. Window U-value is per-window in Elmhurst; we pass an area-weighted
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raw U so our effective transform approximates (27)
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"""
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# Arrange — every Elmhurst fixture has known window U=1.4 raw on
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@ -1110,9 +1139,8 @@ def test_section_3_partial_match_against_elmhurst_worksheet(fixture: ModuleType)
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assert result.total_w_per_k == pytest.approx(
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result.fabric_heat_loss_w_per_k + result.thermal_bridging_w_per_k, rel=1e-9
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)
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# External-area divergence direction depends on the fixture:
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# - RR fixtures: ours < worksheet (RR walls missing — gap #1)
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# - Non-RR with non-constant per-storey perim: ours > worksheet
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# (gap #2 — wall-area over-count). Just check non-zero until both
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# fixes land.
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# External-area: RR fixtures still under-count vs worksheet because RR
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# sub-areas are not modelled (gap #1). Non-RR fixtures should match
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# the worksheet's (31) once the per-storey-perimeter fix lands; for
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# now keep the looser non-zero check until RR closes.
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assert result.total_external_element_area_m2 > 0
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