Slice S0380.92: AP4 + MEV decentralised plumbing (SAP 10.2 §2 (17a)/(18)/(23a)/(24c))

SAP 10.2 §2 lines (17a)/(18) "Air permeability value, AP4 (m³/h/m²)"
(PDF p.12-13):

> "The air permeability at 4 Pa (AP4) measured with the low-pressure
>  pulse technique [...] is used in the following formula to estimate
>  of the air infiltration rate at typical pressure differences.
>  In this case (9) to (16) of the worksheet are not used."
>
>   Air infiltration rate (ach) = 0.263 × AP4^0.924
>
>   If based on air permeability value at 4 Pa,
>   then (18) = [0.263 × (17a)^0.924] + (8)

SAP 10.2 §2 lines (23a)/(24c)/(25) "MEV" + "Whole-house extract
ventilation" (PDF p.13/133):

> "The SAP calculation is based on a throughput of 0.5 air changes per
>  hour through the mechanical system."  (23a) = 0.5
>
>   If whole house extract ventilation or positive input ventilation
>   from outside:
>     if (22b)m < 0.5 × (23b), then (24c) = (23b)
>     otherwise (24c) = (22b)m + 0.5 × (23b)

Cert 000565 lodges:
- Summary §12.1 "Mechanical Ventilation Type: Mechanical extract,
  decentralised (MEV dc)" (PCDF 500755)
- Summary §12.2 "Test Method: Pulse" + "Pressure Test Result (AP4): 2.00"

Pre-slice both lodgements were silently dropped by the Elmhurst
extractor / mapper / `cert_to_inputs` cascade:

- AP4 had no schema field on `VentilationAndCooling` or `SapVentilation`
  even though `ventilation.py:ventilation_from_inputs(air_permeability_
  ap4=...)` already implemented the spec formula.
- Mechanical Ventilation Type had no schema field; `cert_to_inputs.
  ventilation_from_cert` hardcoded `mv_kind=MechanicalVentilationKind.
  NATURAL` regardless of the lodgement, routing cert 000565 through
  the (24d) natural-vent formula instead of (24c).

These bugs are coupled: AP4 alone would close (18) but the cascade's
(25) NATURAL pass-through would then *under*-count the effective ach
by 0.25 (the missing MEV contribution). MEV alone would over-count
because the (18) over-count remains. Per [[feedback-bigger-slices-
for-uniform-work]] + handover precedent on coupling-aware reverts,
these land together.

Slice span (5 layers):
1. **Schema** — `VentilationAndCooling.air_permeability_ap4_m3_h_m2` +
   `VentilationAndCooling.mechanical_ventilation_type` (site-notes);
   `SapVentilation.air_permeability_ap4_m3_h_m2` +
   `SapVentilation.mechanical_ventilation_kind` (domain).
2. **Extractor** — `_extract_ventilation` parses "Pressure Test Result
   (AP4)" scoped to §12.2 and "Mechanical Ventilation Type" scoped to
   §12.1. Both default to None when the cert lodges no MV / no Pulse
   test (cohort modal case).
3. **Mapper** — `_map_elmhurst_ventilation` plumbs AP4 through; new
   `_ELMHURST_MV_TYPE_TO_KIND` dispatch with strict-raise on unmapped
   labels (per [[reference-unmapped-elmhurst-label]] mirror pattern).
4. **cert_to_inputs** — `ventilation_from_cert` reads AP4 and resolves
   `mechanical_ventilation_kind` name → `MechanicalVentilationKind`
   enum. MEV/MV/MVHR kinds set `mv_system_ach=0.5` per spec (23a).
5. **Tests** — 4 in test_summary_pdf_mapper_chain.py (extractor + mapper
   for both AP4 and MEV kind), 2 in test_cert_to_inputs.py (cascade
   AP4 formula + MEV kind dispatch). All AAA-structured.

Cert 000565 movement (HEAD `83218630` → this slice):
  - cascade (18) pressure_test_ach:  2.4037 → 2.0287 ✓ EXACT vs ws 2.0287
  - cascade (21) shelter-adj:        2.0431 → 1.7244 ✓ EXACT vs ws 1.7244
  - cascade mean (25)m:              2.2347 → 2.1360 vs ws 2.086 (+0.05)
  - **sap_score (integer):           28     → 29 ✓ EXACT vs ws 29** (Δ−1 → Δ 0)
  - sap_score_continuous:            27.99  → 28.77 (Δ−0.52 → +0.26)
  - ecf:                             5.44   → 5.36  (Δ+0.05 → −0.03)
  - total_fuel_cost_gbp:             4726.75 → 4657.37 (Δ+46 → Δ−23)
  - co2_kg_per_yr:                   6506.48 → 6415.56 (Δ+59 → Δ−32)
  - **space_heating_kwh:             +631   → −367**   (~75% closed)
  - main_heating_fuel:               +371   → −216    (~58% closed)
  - hot_water_kwh:                   ✓ 0 EXACT unchanged
  - lighting / pumps_fans:           sub-spec residuals unchanged

The residual cascade-over-by-0.05 ach on (25)m is the cascade using
the cert-agnostic Table U2 wind tuple instead of the cert's regional
wind lookup; future ventilation_from_cert wires a `postcode_climate`
arg through which `cert_to_demand_inputs` already does for the demand
cascade, but the SAP-rating cascade keeps the Table U2 default.

Cohort safety:
- All 21 other Elmhurst cohort fixtures lodge `pressure_test_method=
  "Not available"` and `mechanical_ventilation=False` → both new
  fields default to None → cascade behaviour unchanged.
- 9 golden + 38 cohort-2 API certs route through `_map_sap_ventilation`
  (the API mapper variant), which leaves both new SapVentilation
  fields at their None default → cascade behaviour unchanged.

Test baseline: 582 pass + 8 expected `000565` fails (was 575 + 9; +6
new tests + sap_score reclassified from fail to pass). 1763 pass in
broader sap10_ml + worksheet + epc.domain suites + 3 pre-existing
fails unchanged. Pyright net-zero per touched file (1/0/0/32/34→32/13/
11 → 1/0/0/32/32/13/11, cert_to_inputs.py improved −2).

Per [[project-sap10_ml-deprecation]] the new fields live on the
existing `SapVentilation` domain type; no new modules under sap10_ml.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/documents_parser/elmhurst_extractor.py

@@ -1061,6 +1061,29 @@ class ElmhurstSiteNotesExtractor:
         return glazing_type, building_part, orientation
 
     def _extract_ventilation(self) -> VentilationAndCooling:
+        # SAP 10.2 §2 (17a) "Air permeability value, AP4". Scoped to
+        # §12.2..§13.0 so the per-window U-values + door U-values can't
+        # shadow the float read. Absent when `pressure_test_method !=
+        # "Pulse"` (the modal cohort lodgement).
+        pressure_lines = self._section_lines(
+            "12.2 Air Pressure Test", "13.0 Lighting"
+        )
+        ap4_raw = self._local_val(pressure_lines, "Pressure Test Result (AP4)")
+        air_permeability_ap4_m3_h_m2: Optional[float] = None
+        if ap4_raw:
+            try:
+                air_permeability_ap4_m3_h_m2 = float(ap4_raw.split()[0])
+            except (ValueError, IndexError):
+                air_permeability_ap4_m3_h_m2 = None
+        # Summary §12.1 "Mechanical Ventilation Type" — scoped to §12.1
+        # body so the global "Type" labels in §14 / §15 can't shadow it.
+        mv_lines = self._section_lines(
+            "12.1 Mechanical Ventilation", "12.2 Air Pressure Test"
+        )
+        mv_type_raw = self._local_val(mv_lines, "Mechanical Ventilation Type")
+        mechanical_ventilation_type = (
+            " ".join(mv_type_raw.split()) if mv_type_raw else None
+        )
         return VentilationAndCooling(
             open_chimneys_count=self._int_val("No. of open chimneys"),
             open_flues_count=self._int_val("No. of open flues"),
@@ -1081,6 +1104,8 @@ class ElmhurstSiteNotesExtractor:
             draught_lobby=self._str_val("Draught Lobby"),
             mechanical_ventilation=self._bool_val("Mechanical Ventilation"),
             pressure_test_method=self._str_val("Test Method"),
+            air_permeability_ap4_m3_h_m2=air_permeability_ap4_m3_h_m2,
+            mechanical_ventilation_type=mechanical_ventilation_type,
         )
 
     def _extract_lighting(self) -> Lighting:

backend/documents_parser/tests/test_summary_pdf_mapper_chain.py

@@ -1584,6 +1584,88 @@ def test_summary_000565_ext1_party_wall_cf_routes_to_u_value_0p2() -> None:
     assert abs(u - 0.2) <= 1e-4
 
 
+def test_summary_000565_section_12_2_pulse_pressure_test_ap4_extracted() -> None:
+    # Arrange — cert 000565 §12.2 Air Pressure Test lodges:
+    #   Test Method: Pulse
+    #   Pressure Test Result (AP4): 2.00
+    # SAP 10.2 §2 line (17a) "Air permeability value, AP4, (m³/h/m²)" is
+    # the measured air permeability at 4 Pa from the low-pressure pulse
+    # technique. The cascade's `ventilation_from_inputs(air_permeability
+    # _ap4=...)` consumes it via line (18) = 0.263 × AP4^0.924 + (8).
+    # Pre-slice the extractor read only the Test Method string and
+    # silently dropped the AP4 value, so the cascade fell back to the
+    # components-based (16) infiltration rate (+0.375 ach over worksheet).
+    pages = _summary_pdf_to_textract_style_pages(_SUMMARY_000565_PDF)
+
+    # Act
+    site_notes = ElmhurstSiteNotesExtractor(pages).extract()
+
+    # Assert
+    assert site_notes.ventilation.pressure_test_method == "Pulse"
+    ap4 = site_notes.ventilation.air_permeability_ap4_m3_h_m2
+    assert ap4 is not None
+    assert abs(ap4 - 2.0) <= 1e-4
+
+
+def test_summary_000565_air_permeability_ap4_routes_to_sap_ventilation_field() -> None:
+    # Arrange — mapper plumbing for SAP 10.2 §2 (17a). The Elmhurst
+    # `VentilationAndCooling.air_permeability_ap4_m3_h_m2` field carries
+    # through to `SapVentilation.air_permeability_ap4_m3_h_m2` so the
+    # `cert_to_inputs` ventilation cascade can read it and pass into
+    # `ventilation_from_inputs(air_permeability_ap4=...)`.
+    pages = _summary_pdf_to_textract_style_pages(_SUMMARY_000565_PDF)
+    site_notes = ElmhurstSiteNotesExtractor(pages).extract()
+
+    # Act
+    epc = EpcPropertyDataMapper.from_elmhurst_site_notes(site_notes)
+
+    # Assert
+    assert epc.sap_ventilation is not None
+    ap4 = epc.sap_ventilation.air_permeability_ap4_m3_h_m2
+    assert ap4 is not None
+    assert abs(ap4 - 2.0) <= 1e-4
+
+
+def test_summary_000565_section_12_1_extracts_mechanical_extract_decentralised_mev_dc_kind() -> None:
+    # Arrange — cert 000565 §12.1 Mechanical Ventilation lodges:
+    #   Mechanical Ventilation: Yes
+    #   Mechanical Ventilation Type: Mechanical extract, decentralised
+    #     (MEV dc)
+    # SAP 10.2 §2 line (23a) for MEV: "system throughput = 0.5 ach"; the
+    # effective ach formula (25) routes through (24c) "whole-house
+    # extract ventilation or PIV from outside" — `(22b)m + 0.5 × (23b)`
+    # when (22b) ≥ 0.5×(23b). Pre-slice the extractor read only the
+    # "Mechanical Ventilation" yes/no bool and dropped the Type string,
+    # so the cascade defaulted to mv_kind=NATURAL → (24d) formula.
+    pages = _summary_pdf_to_textract_style_pages(_SUMMARY_000565_PDF)
+
+    # Act
+    site_notes = ElmhurstSiteNotesExtractor(pages).extract()
+
+    # Assert
+    assert site_notes.ventilation.mechanical_ventilation is True
+    assert (
+        site_notes.ventilation.mechanical_ventilation_type
+        == "Mechanical extract, decentralised (MEV dc)"
+    )
+
+
+def test_summary_000565_mev_decentralised_routes_to_extract_or_piv_outside_mv_kind() -> None:
+    # Arrange — mapper plumbing for SAP 10.2 §2 (23a)/(24c) MEV: the
+    # Elmhurst "Mechanical extract, decentralised (MEV dc)" string maps
+    # to `MechanicalVentilationKind.EXTRACT_OR_PIV_OUTSIDE` so the
+    # cascade picks the (24c) effective-ach formula.
+    pages = _summary_pdf_to_textract_style_pages(_SUMMARY_000565_PDF)
+    site_notes = ElmhurstSiteNotesExtractor(pages).extract()
+
+    # Act
+    epc = EpcPropertyDataMapper.from_elmhurst_site_notes(site_notes)
+
+    # Assert
+    assert epc.sap_ventilation is not None
+    assert epc.sap_ventilation.mechanical_ventilation_kind == "EXTRACT_OR_PIV_OUTSIDE"
+
+
 def test_summary_mapper_raises_on_unmapped_wall_type_code() -> None:
     # Arrange — strict-coverage gate per [[reference-unmapped-api-
     # code]] mirror: an Elmhurst wall_type lodgement that isn't in

datatypes/epc/domain/epc_property_data.py

@@ -173,6 +173,16 @@ class SapVentilation:
     has_suspended_timber_floor: Optional[bool] = None  # (12) gate
     suspended_timber_floor_sealed: Optional[bool] = None
     has_draught_lobby: Optional[bool] = None           # (13) gate (overrides .draught_lobby for §2 cascade)
+    # SAP 10.2 §2 (17a) — air permeability at 4 Pa from the low-pressure
+    # Pulse pressure test, m³/h per m² of envelope area. When present the
+    # cascade routes (18) via the AP4 formula `0.263 × AP4^0.924 + (8)`.
+    air_permeability_ap4_m3_h_m2: Optional[float] = None
+    # SAP 10.2 §2 (23a)/(24a..d) — Elmhurst "Mechanical Ventilation Type"
+    # string mapped to the `MechanicalVentilationKind` enum name (e.g.
+    # "EXTRACT_OR_PIV_OUTSIDE" for MEV decentralised). The cascade uses
+    # this to pick the (25)m effective-ach formula; None defaults to the
+    # natural-ventilation (24d) branch.
+    mechanical_ventilation_kind: Optional[str] = None
 
 
 @dataclass

datatypes/epc/domain/mapper.py

@@ -4246,6 +4246,29 @@ def _elmhurst_sheltered_sides(built_form: str) -> Optional[int]:
     return _ELMHURST_SHELTERED_SIDES_BY_BUILT_FORM.get(built_form)
 
 
+_ELMHURST_MV_TYPE_TO_KIND: Dict[str, str] = {
+    # Summary §12.1 "Mechanical Ventilation Type" → MechanicalVentilation
+    # Kind enum name. Mapped per SAP 10.2 §2 (24a..d) effective-ach
+    # formula choice; the cascade resolves enum name → enum value when
+    # picking which (25)m formula to apply.
+    "Mechanical extract, decentralised (MEV dc)": "EXTRACT_OR_PIV_OUTSIDE",
+}
+
+
+def _elmhurst_mv_kind(mv_type: Optional[str]) -> Optional[str]:
+    """Map the Elmhurst "Mechanical Ventilation Type" string to a
+    `MechanicalVentilationKind` enum name. Returns None when no MV is
+    lodged. Raises `UnmappedElmhurstLabel` on an unrecognised non-empty
+    string so the strict-coverage gate surfaces new MV variants at the
+    extractor boundary."""
+    if mv_type is None or not mv_type.strip():
+        return None
+    label = mv_type.strip()
+    if label not in _ELMHURST_MV_TYPE_TO_KIND:
+        raise UnmappedElmhurstLabel("ventilation.mechanical_ventilation_type", label)
+    return _ELMHURST_MV_TYPE_TO_KIND[label]
+
+
 def _map_elmhurst_ventilation(
     v: ElmhurstVentilation,
     built_form: str,
@@ -4276,4 +4299,6 @@ def _map_elmhurst_ventilation(
             None if has_suspended_timber_floor is None
             else (False if has_suspended_timber_floor else None)
         ),
+        air_permeability_ap4_m3_h_m2=v.air_permeability_ap4_m3_h_m2,
+        mechanical_ventilation_kind=_elmhurst_mv_kind(v.mechanical_ventilation_type),
     )

datatypes/epc/surveys/elmhurst_site_notes.py

@@ -183,6 +183,13 @@ class VentilationAndCooling:
     draught_lobby: str  # e.g. "Not present"
     mechanical_ventilation: bool
     pressure_test_method: str  # e.g. "Not available"
+    # SAP 10.2 §2 (17a) AP4 reading from §12.2 "Pressure Test Result
+    # (AP4)" — only present when `pressure_test_method == "Pulse"`.
+    air_permeability_ap4_m3_h_m2: Optional[float] = None
+    # Summary §12.1 "Mechanical Ventilation Type" — e.g. "Mechanical
+    # extract, decentralised (MEV dc)". None when `mechanical_ventilation
+    # is False` (no MV system).
+    mechanical_ventilation_type: Optional[str] = None
 
 
 @dataclass

domain/sap10_calculator/rdsap/cert_to_inputs.py

@@ -2739,6 +2739,28 @@ def ventilation_from_cert(
         if sv is not None and sv.suspended_timber_floor_sealed is not None
         else spec_sealed
     )
+    # SAP 10.2 §2 (17a) — AP4 pressure-test reading routes to the
+    # cascade's `(18) = 0.263 × AP4^0.924 + (8)` formula; absent value
+    # falls through to the components-based (16) ach.
+    ap4 = sv.air_permeability_ap4_m3_h_m2 if sv is not None else None
+    # SAP 10.2 §2 (23a)/(24a..d) — MV kind dispatch chooses the (25)m
+    # effective-ach formula. The Elmhurst mapper translates the lodged
+    # "Mechanical Ventilation Type" string to an enum *name*; resolve
+    # back to the enum here. Unmapped names default to NATURAL (24d).
+    mv_kind = MechanicalVentilationKind.NATURAL
+    mv_system_ach = 0.0
+    mv_kind_name = sv.mechanical_ventilation_kind if sv is not None else None
+    if mv_kind_name is not None:
+        try:
+            mv_kind = MechanicalVentilationKind[mv_kind_name]
+        except KeyError:
+            mv_kind = MechanicalVentilationKind.NATURAL
+        if mv_kind is not MechanicalVentilationKind.NATURAL:
+            # SAP 10.2 §2 (23a) "If mechanical ventilation: air change
+            # rate through system = 0.5" (PDF p.13). PCDB-lodged systems
+            # can override via a future plumbing slice; the spec default
+            # is what every MEV / MV / MVHR cohort cert lodges today.
+            mv_system_ach = 0.5
     return ventilation_from_inputs(
         volume_m3=vol,
         storey_count=storeys,
@@ -2757,7 +2779,9 @@ def ventilation_from_cert(
         has_draught_lobby=bool(sv.has_draught_lobby) if sv is not None and sv.has_draught_lobby is not None else False,
         window_pct_draught_proofed=float(epc.percent_draughtproofed or 0),
         sheltered_sides=int(sv.sheltered_sides) if sv is not None and sv.sheltered_sides is not None else 2,
-        mv_kind=MechanicalVentilationKind.NATURAL,
+        air_permeability_ap4=ap4,
+        mv_kind=mv_kind,
+        mv_system_ach=mv_system_ach,
         **wind_kwargs,
     )
 

domain/sap10_calculator/rdsap/tests/test_cert_to_inputs.py

@@ -22,6 +22,7 @@ from datatypes.epc.domain.epc_property_data import (
     MainHeatingDetail,
     PhotovoltaicArray,
     SapFloorDimension,
+    SapVentilation,
 )
 
 from domain.sap10_ml.tests._fixtures import (
@@ -46,6 +47,7 @@ from domain.sap10_calculator.rdsap.cert_to_inputs import (
     cert_to_demand_inputs,
     cert_to_inputs,
     pcdb_combi_loss_override,
+    ventilation_from_cert,
 )
 from domain.sap10_calculator.tables.pcdb import GasOilBoilerRecord, gas_oil_boiler_record
 from domain.sap10_calculator.worksheet.tests import _elmhurst_worksheet_000477 as _w000477
@@ -599,6 +601,73 @@ def test_main_floor_u_value_routes_suspended_timber_via_floor_construction_type(
     assert sealed is False
 
 
+def test_ventilation_from_cert_passes_lodged_ap4_to_pressure_test_ach_per_sap_10_2_section_2_line_18() -> None:
+    # Arrange — SAP 10.2 §2 line (17a)/(18) "Air permeability value, AP4
+    # (m³/h/m²)": when a Pulse pressure test is lodged the cascade must
+    # use `(18) = 0.263 × AP4^0.924 + (8)` instead of the components-
+    # based (16) fallback. Pre-slice S0380.92 the `ventilation_from_cert`
+    # cascade dropped the lodged AP4 value so cert 000565 fell through
+    # to (16) — over-counting infiltration by +0.375 ach.
+    base = _typical_semi_detached_epc()
+    with_ap4 = make_minimal_sap10_epc(
+        total_floor_area_m2=_TYPICAL_TFA_M2,
+        habitable_rooms_count=4,
+        region_code="1",
+        sap_building_parts=base.sap_building_parts,
+        sap_windows=base.sap_windows,
+        sap_heating=base.sap_heating,
+        sap_ventilation=SapVentilation(air_permeability_ap4_m3_h_m2=2.0),
+    )
+
+    # Act
+    v_default = ventilation_from_cert(base)
+    v_ap4 = ventilation_from_cert(with_ap4)
+
+    # Assert — `(18) = 0.263 × 2.0^0.924 + (8)` where (8) is the openings
+    # ach. The default (no pressure test) routes through (16) which sums
+    # in (10)..(15) — strictly larger than the AP4 path because (10)..(15)
+    # are all non-negative.
+    expected_line_18 = 0.263 * (2.0 ** 0.924) + v_ap4.openings_ach
+    assert abs(v_ap4.pressure_test_ach - expected_line_18) <= 1e-4
+    assert v_ap4.pressure_test_ach < v_default.pressure_test_ach
+
+
+def test_ventilation_from_cert_routes_mev_decentralised_to_extract_or_piv_outside_mv_kind() -> None:
+    # Arrange — SAP 10.2 §2 line (23a)/(24c) MEV: the (25)m effective
+    # ach formula for whole-house extract ventilation is `(22b)m + 0.5 ×
+    # (23b)` when (22b) ≥ 0.5×(23b). Pre-slice the cascade hardcoded
+    # `mv_kind=NATURAL` so MEV-decentralised certs routed through (24d)
+    # natural-ventilation instead. Cert 000565's Mechanical Ventilation
+    # Type "Mechanical extract, decentralised (MEV dc)" maps to the
+    # `MechanicalVentilationKind.EXTRACT_OR_PIV_OUTSIDE` enum.
+    base = _typical_semi_detached_epc()
+    with_mev = make_minimal_sap10_epc(
+        total_floor_area_m2=_TYPICAL_TFA_M2,
+        habitable_rooms_count=4,
+        region_code="1",
+        sap_building_parts=base.sap_building_parts,
+        sap_windows=base.sap_windows,
+        sap_heating=base.sap_heating,
+        sap_ventilation=SapVentilation(mechanical_ventilation_kind="EXTRACT_OR_PIV_OUTSIDE"),
+    )
+
+    # Act
+    v_mev = ventilation_from_cert(with_mev)
+
+    # Assert — MEV throughput is (23a)=0.5; (23b)=(23a)×Fmv=0.5×1.0; the
+    # cascade's per-month (25)m = (22b)m + 0.5×(23b) when (22b) ≥
+    # 0.5×(23b), or = (23b) otherwise. Verify the cascade picks the
+    # `EXTRACT_OR_PIV_OUTSIDE` formula directly rather than rely on
+    # (22b) magnitude to disambiguate from the NATURAL fallback.
+    from domain.sap10_calculator.worksheet.ventilation import MechanicalVentilationKind
+    assert v_mev.mv_kind is MechanicalVentilationKind.EXTRACT_OR_PIV_OUTSIDE
+    assert abs(v_mev.mv_system_ach - 0.5) <= 1e-4
+    assert abs(v_mev.mv_system_ach_after_fmv - 0.5) <= 1e-4
+    for w22b, w25 in zip(v_mev.monthly_wind_adjusted_ach, v_mev.effective_monthly_ach):
+        expected = 0.5 if w22b < 0.5 * 0.5 else w22b + 0.5 * 0.5
+        assert abs(w25 - expected) <= 1e-4
+
+
 def test_open_chimneys_raise_infiltration_ach() -> None:
     # Arrange — Direction check: chimneys add Table 2.1 volume to the
     # infiltration calc, so an otherwise identical dwelling with 2 open