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RdSAP 10 Specification §4.1 Table 5 "Ventilation parameters" (PDF p.28)
verbatim — "Extract fans" entry:
• Number of extract fans if known
• If number is unknown:
Not park home:
Age bands A to E all cases → 0
Age bands F to G all cases → 1
Age bands H to M up to 2 hab. rooms → 1
3 to 5 hab. rooms → 2
6 to 8 hab. rooms → 3
more than 8 hab. rooms → 4
Park home:
Age band F all cases → 0
Age bands G onwards all cases → 2
The Elmhurst Summary §12.0 renders "No. of intermittent extract fans: 0"
as the form for *unknown*; every other §2 chimney/flue line item follows
"number if known, or 0 if not present" and the cascade trusts the lodged
value verbatim. Only extract fans have a non-zero age-band default.
Pre-slice the cascade read the lodged 0 verbatim → cohort-wide -0.044
ACH ventilation deficit (= -2.6 W/K HLC, = -1.2% SH demand, = ~-0.3 SAP
per variant). All 25 cascade-OK corpus variants are age G + 4 habitable
rooms + not park home → Table 5 default = 1 fan.
New helper `_rdsap_extract_fans_default(age_band, habitable_rooms, *,
is_park_home)` + wiring in `ventilation_from_cert` applies
`max(lodged, table_5_default)` so the spec minimum fires when lodging
is below it.
Heating-systems corpus impact (25 cascade-OK variants):
oil 1, oil pcdb 1/2/3 +0.27..+0.29 → EXACT (<1e-4)
electric 1, solid fuel 5/6/7/8 +0.28..+0.43 → EXACT
pcdb 1, ashp +0.41 / +0.18 → ±0.02
electric 3/6/7/8/9, sf 4/9/10/11 +0.39..+0.60 → +0.08..+0.12
electric 5 -0.74 → -1.18 (Cluster B over-shoot)
electric 2 -0.24 → -0.46 (Cluster C HW gap)
gshp +1.09 → +0.94 (Cluster C HW gap)
solid fuel 2/3 +3.08 / +1.76 → +2.77 / +1.31
Cluster A (cohort-wide HLC deficit) is closed. The four remaining open
fronts (Clusters B + C) are now visible without offsetting bugs:
- Cluster B (Table 9c step 12 R sign): electric 5, solid fuel 2/3
- Cluster C (HW kWh cascade): gshp + electric 2 (Appendix N3)
solid fuel 2/3 (Table 4b HW efficiency)
Golden-fixture re-pins:
cert 0240 (age J, TFA 118): PE +2.18 → +5.80, CO2 +0.13 → +0.32
cert 0390-2954 (age F, TFA 360): PE -28.27 → -27.97, CO2 -2.74 → -2.71
Pyright net-zero (44 → 44). Extended handover suite: 893 → 895 pass.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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