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SAP 10.2 §9.4.11 (PDF p.30) "Boiler interlock":
For the purposes of the SAP, an interlocked system is one in which
both the space and stored water heating are interlocked. If either
is not, the 5% seasonal efficiency reduction is applied to both
space and water heating; if both are interlocked no reductions are
made.
Table 4c (PDF p.169-170) lodges -5 for both Space and DHW columns on
the "No boiler interlock — regular boiler" row. Pre-slice the cascade
applied the -5pp adjustment ONLY to the `water_eff` scalar fallback
(`cert_to_inputs.py:4354`) and missed:
(a) the SH efficiency path (cascade kept the raw PCDB winter eff for
space heating);
(b) the PCDB Equation D1 monthly cascade (Eq D1 received raw
winter/summer values without the -5pp adjustment).
RdSAP §3 (PDF p.57) defines boiler interlock as "Assumed present if
there is a room thermostat and (for stored hot water systems heated
by the boiler) a cylinder thermostat. Otherwise not interlocked."
Cert pcdb 1 (Potterton KOA PCDB 716 + 110 L cylinder + Cylinder Stat:
No) reproduces the pattern: worksheet (210) = 60% = PCDB winter
65 - 5; worksheet (217)m monthly Eq D1 pivots on (winter 60,
summer 48) not (65, 53).
The SH path is further gated on `pcdb_main is not None` because
§9.4.11 only applies to "gas and liquid fuel boilers" — cert 000565
(ASHP Main 1) keeps its raw SH eff. The combi-fed-cylinder DHW path
(cert 000565 WHC 914 to PCDB combi Main 2) continues to receive its
existing -5pp via the `water_pcdb_main` gate (unchanged).
Corpus impact: pcdb 1 SAP residual +6.95 → +3.40; cost -£157.61 →
-£75.68; CO2 -845.81 → -397.02; PE -3135.30 → -1601.74. No other
variant has PCDB main + cylinder + no thermostat, so the other 24
corpus pins are unchanged.
Extended handover suite: 884 pass, 0 fail (was 883 + 1 new AAA test
pinning the §9.4.11 SH eff path).
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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