Slice S0380.156: SAP 10.2 Table 3 WHC=903 electric-immersion zero-loss guard

SAP 10.2 Table 3 (PDF p.160) verbatim:

  Primary loss is set to zero for the following:
      Electric immersion heater
      Combi boiler ...
      CPSU ...
      Boiler and thermal store within a single casing
      Separate boiler and thermal store connected by no more than 1.5
      m of insulated pipework
      Direct-acting electric boiler
      Heat pump (...) with hot water vessel integral to package

The Elmhurst WHC=903 lodging signals exactly the first row: "HW from
a separate electric immersion heater" — the cylinder is heated by an
immersion element inside the tank, no primary pipework between any
heat generator and the cylinder. The rule is universal: regardless
of what main heating exists for space heating, electric immersion
means no primary circuit means no primary loss.

Pre-slice `_primary_loss_applies` only consulted `water_heating_code`
in the Table 4a wet-boiler branch (codes 151-161 / 191-196). The Cat
4 HP branch returned True unconditionally when no PCDB record was
lodged; the Cat 1/2 boiler branch returned True unconditionally; the
PCDB Table 322 + Table 4b non-PCDB branches likewise. For the
electric 2 corpus variant (sap_main_heating_code=524 Cat 5 warm-air
ASHP, main_heating_category=4 per Elmhurst mapper, no PCDB record,
WHC=903 + cylinder), the Cat-4 branch falsely returned True and the
cascade added ~510 kWh/yr primary loss to a system with no primary
circuit at all.

Per-line walk discipline applied: cascade `water_heating_from_cert`
output dump showed `primary_loss_monthly_kwh_annual = 509.98` while
worksheet (59)m = 0 every month → spec lookup found Table 3 verbatim
"Electric immersion heater" zero-loss line.

Adds `_WHC_ELECTRIC_IMMERSION: Final[int] = 903` constant + a
top-of-function `if water_heating_code == _WHC_ELECTRIC_IMMERSION:
return False` guard that fires before any of the system-type-keyed
branches.

Closures electric 2:
  HW kWh 2849.22 → 2339.24 (matches worksheet (62)/(64) = 2384.12
  within the residual ~45 kWh storage-loss gap)
  ΔSAP −0.4584 → +0.8118 (cascade swung past the worksheet by +1.27
  — the pre-slice 'near-correct' value was offsetting cascade bugs
  per [[feedback-software-no-special-handling]]; the +0.81 residual
  exposes a separate upstream gap to chase in a follow-up slice)
  Δcost +£10.56 → −£18.71
  ΔCO2 +47.89 → −7.21 kg
  ΔPE +443.13 → −161.68 kWh

No regressions on the other 24 cohort variants — only electric 2 has
the (Cat 4 HP, no PCDB, WHC=903) combination in the corpus.
Extended handover suite: 900 pass / 0 fail (was 899 — +1 from the
new AAA test). Pyright net-zero (43 → 43).

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

backend/documents_parser/tests/test_heating_systems_corpus.py

@@ -218,10 +218,28 @@ class _CorpusExpectation:
 # per affected variant, SAP residuals shift ±0.15 across 16 variants;
 # the SH+Sec demand mismatch for electric 3/6/7 (Table 11 fraction
 # for codes 401/402) remains the open driver of those SAP residuals.
+#
+# Slice S0380.156 added the universal SAP 10.2 Table 3 (PDF p.160)
+# zero-loss guard for WHC=903 (electric immersion HW) at the top of
+# `_primary_loss_applies`. Pre-slice the Cat 4 HP branch returned
+# True unconditionally when no PCDB record was lodged — so for
+# electric 2 (sap_main_heating_code=524 Cat 5 warm-air ASHP, mapped
+# to main_heating_category=4, WHC=903 + cylinder), the cascade
+# falsely added ~510 kWh/yr primary loss to a system whose cylinder
+# is heated directly by an immersion element with no primary
+# pipework. Per Table 3 verbatim: "Primary loss is set to zero for
+# the following: Electric immersion heater ...". Electric 2 SAP
+# residual −0.4584 → +0.8118 (cascade swung past the worksheet — the
+# pre-slice 'near-correct' value was masking an offsetting upstream
+# gap that the spec-correct fix has exposed); cost +£10.56 →
+# −£18.71; CO2 +47.89 → −7.21 kg; PE +443.13 → −161.68. No
+# regressions on the other 24 variants — the new guard is gated on
+# WHC=903 and only electric 2 has the (Cat 4 HP, no PCDB, WHC=903)
+# combination in the corpus.
 _EXPECTATIONS: tuple[_CorpusExpectation, ...] = (
     _CorpusExpectation(variant='ashp', block='11a', expected_sap_resid=-0.0240, expected_cost_resid_gbp=+0.5536, expected_co2_resid_kg=+7.3267, expected_pe_resid_kwh=+36.3435),
     _CorpusExpectation(variant='electric 1', block='11a', expected_sap_resid=-0.0000, expected_cost_resid_gbp=-0.0000, expected_co2_resid_kg=+11.9451, expected_pe_resid_kwh=+48.6605),
-    _CorpusExpectation(variant='electric 2', block='11a', expected_sap_resid=-0.4584, expected_cost_resid_gbp=+10.5613, expected_co2_resid_kg=+47.8864, expected_pe_resid_kwh=+443.1346),
+    _CorpusExpectation(variant='electric 2', block='11a', expected_sap_resid=+0.8118, expected_cost_resid_gbp=-18.7061, expected_co2_resid_kg=-7.2129, expected_pe_resid_kwh=-161.6840),
     _CorpusExpectation(variant='electric 3', block='11a', expected_sap_resid=+0.1215, expected_cost_resid_gbp=-2.8003, expected_co2_resid_kg=+6.7227, expected_pe_resid_kwh=-5.9859),
     _CorpusExpectation(variant='electric 5', block='11a', expected_sap_resid=-1.1759, expected_cost_resid_gbp=+27.0929, expected_co2_resid_kg=+62.7232, expected_pe_resid_kwh=+438.0333),
     _CorpusExpectation(variant='electric 6', block='11a', expected_sap_resid=+0.1081, expected_cost_resid_gbp=-2.4918, expected_co2_resid_kg=+7.3225, expected_pe_resid_kwh=+0.1603),

domain/sap10_calculator/rdsap/cert_to_inputs.py

@@ -533,6 +533,13 @@ _INTERNAL_GAINS_DEFAULT_OVERSHADING: Final[OvershadingCategory] = (
 # because there's no cylinder and no primary circuit.
 _INSTANTANEOUS_WATER_CODES: Final[frozenset[int]] = frozenset({907, 909})
 
+# Elmhurst WHC code for "HW from a separate electric immersion heater":
+# cylinder lodged but heated by an immersion element inside the tank, no
+# primary pipework between any heat generator and the cylinder. SAP 10.2
+# Table 3 (PDF p.160) puts "Electric immersion heater" first in its
+# zero-loss list, so primary loss is zero whenever this code is lodged.
+_WHC_ELECTRIC_IMMERSION: Final[int] = 903
+
 
 # SAP 10.2 Appendix M equation (M1): EPV = 0.8 × kWp × S × ZPV, summed
 # per array. The module efficiency constant (0.8), orientation-dependent
@@ -4114,6 +4121,19 @@ def _primary_loss_applies(
         return False
     if main is None:
         return False
+    # SAP 10.2 Table 3 (PDF p.160) zero-loss list — verbatim:
+    # "Primary loss is set to zero for the following: Electric immersion
+    # heater ...". Elmhurst WHC=903 lodges "HW from a separate electric
+    # immersion heater": the cylinder is heated by an immersion element
+    # inside the tank, no primary pipework between any heat generator
+    # and the cylinder. Applies universally — regardless of which main
+    # heating system exists for space heating (Cat 4 HP, Cat 1/2 boiler,
+    # Table 4b non-PCDB, PCDB Table 322). Pre-slice the WHC check only
+    # gated the Table 4a wet-boiler branch; the other branches falsely
+    # returned True for HP / boiler mains with WHC=903, adding ~510
+    # kWh/yr primary loss to a system with no primary circuit at all.
+    if water_heating_code == _WHC_ELECTRIC_IMMERSION:
+        return False
     if main.main_heating_category == 4:
         if hp_record is None:
             # No PCDB record → assume separate-vessel (conservative; the

domain/sap10_calculator/rdsap/tests/test_cert_to_inputs.py

@@ -4192,6 +4192,110 @@ def test_sap_table_3_primary_loss_applies_to_solid_fuel_back_boiler_with_cylinde
     )
 
 
+def test_sap_table_3_primary_loss_skipped_for_whc_903_electric_immersion_with_heat_pump_main() -> None:
+    """SAP 10.2 Table 3 (PDF p.160) zero-loss list — verbatim:
+
+        Primary loss is set to zero for the following:
+            Electric immersion heater
+            Combi boiler ...
+            CPSU ...
+            ...
+
+    The rule is universal: when HW is heated by an electric immersion
+    inside the cylinder (no primary pipework between any heat generator
+    and the cylinder), primary loss is zero — regardless of the main
+    heating system. The Elmhurst WHC=903 lodging signals exactly this
+    arrangement: "HW from a separate electric immersion heater".
+
+    Pre-slice `_primary_loss_applies` only checked the WHC for the
+    Table 4a wet-boiler branch (codes 151-161 / 191-196). The Cat 4
+    heat-pump branch returned True unconditionally when no PCDB record
+    was lodged, the Cat 1/2 boiler branch returned True unconditionally,
+    and the PCDB Table 322 + Table 4b non-PCDB branches likewise. For
+    the warm-air HP cert with code 524 + main_heating_category=4 +
+    WHC=903 + cylinder, the cat-4 branch falsely returned True and the
+    cascade added ~510 kWh/yr primary loss to a system with no primary
+    circuit at all.
+
+    Worksheet evidence — electric 2 (sap_main_heating_code=524 Cat 5
+    warm-air ASHP, main_heating_category=4 per mapper, WHC=903 electric
+    immersion, 110 L cylinder + cylinder thermostat lodged): the P960
+    block-11a (59)m row reads 0.0000 every month, annual sum = 0.
+    """
+    # Arrange — electric 2 corpus variant: Table 4a code 524 (warm-air
+    # ASHP) + main_heating_category=4 (Cat 4 HP per Elmhurst mapper) +
+    # WHC=903 (electric immersion HW) + 110 L cylinder + cylinder
+    # thermostat lodged. No PCDB heat-pump record lodged.
+    import re
+    import subprocess
+    from pathlib import Path
+
+    from backend.documents_parser.elmhurst_extractor import ElmhurstSiteNotesExtractor
+    from datatypes.epc.domain.mapper import EpcPropertyDataMapper
+
+    corpus_electric_2 = (
+        Path(__file__).parents[4]
+        / "sap worksheets/heating systems examples/electric 2"
+    )
+    summary_pdf = next(corpus_electric_2.glob("Summary_*.pdf"))
+    info = subprocess.run(
+        ["pdfinfo", str(summary_pdf)], capture_output=True, text=True, check=True,
+    ).stdout
+    pc_match = re.search(r"Pages:\s+(\d+)", info)
+    assert pc_match is not None
+    pc = int(pc_match.group(1))
+    pages: list[str] = []
+    for i in range(1, pc + 1):
+        layout = subprocess.run(
+            ["pdftotext", "-layout", "-f", str(i), "-l", str(i),
+             str(summary_pdf), "-"],
+            capture_output=True, text=True, check=True,
+        ).stdout
+        tokens: list[str] = []
+        for line in layout.splitlines():
+            if not line.strip():
+                tokens.append("")
+                continue
+            parts = [p for p in re.split(r"\s{2,}", line.strip()) if p]
+            tokens.extend(parts)
+        pages.append("\n".join(tokens))
+    notes = ElmhurstSiteNotesExtractor(pages).extract()
+    epc = EpcPropertyDataMapper.from_elmhurst_site_notes(notes)
+
+    main = epc.sap_heating.main_heating_details[0]
+    assert epc.has_hot_water_cylinder is True
+    assert main.sap_main_heating_code == 524
+    assert main.main_heating_category == 4
+    assert main.main_heating_index_number is None
+    assert epc.sap_heating.water_heating_code == 903
+
+    # Act — drive §4 (45..65) via the cascade helper. Pre-slice the Cat
+    # 4 branch falsely returned True for `_primary_loss_applies` because
+    # WHC=903 was only consulted in the Table 4a wet-boiler branch.
+    wh_result, _ = _water_heating_worksheet_and_gains(
+        epc=epc,
+        water_efficiency_pct=1.0,
+        is_instantaneous=False,
+        primary_age="G",
+        pcdb_record=None,
+    )
+    assert wh_result is not None
+
+    # Assert — (59)m annual must be 0 because the cylinder is heated by
+    # an electric immersion element (no primary pipework between any
+    # heat generator and the cylinder), matching the worksheet's all-zero
+    # (59)m row.
+    annual_primary = sum(wh_result.primary_loss_monthly_kwh)
+    assert abs(annual_primary - 0.0) <= 1e-4, (
+        f"electric 2 (Table 4a code 524 Cat 4 HP, WHC=903 immersion) "
+        f"primary loss annual = {annual_primary:.4f} kWh/yr; pre-slice "
+        f"the cascade Cat-4 branch returned True even though WHC=903 "
+        f"means electric immersion heats the cylinder directly. Per "
+        f"SAP 10.2 Table 3 zero-loss list ('Electric immersion heater') "
+        f"primary loss must be 0."
+    )
+
+
 def test_sap_table_4f_circulation_pump_dispatches_per_central_heating_pump_age() -> None:
     """SAP 10.2 Table 4f (PDF p.174) "Electricity for fans, pumps and
     other auxiliary uses" — Heating system circulation pump rows: