feat(elmhurst-extractor): capture section 15.1 Immersion Heater (Dual/Single)

The Elmhurst Summary section 15.1 "Hot Water Cylinder" block lodges
"Immersion Heater: Dual" / "Single"; the extractor dropped it, so the
Summary path left immersion_heating_type = None while the API path already
captured it. Capturing it drives SAP Table 13's high-rate-fraction
DHW-cost split (RdSAP 10 section 10.5 p.54: 1 = dual, 2 = single) and
brings the two front-ends to parity.

Three-file change: WaterHeating.immersion_type field +
_extract_water_heating parse (scoped to the 15.1..15.2 slice) +
_elmhurst_immersion_type_code mapper (strict-raise on an unmapped label,
mirroring _elmhurst_cylinder_insulation_code).

Safe to land now that the preceding commit zeroes the high-rate fraction
for 18-/24-hour tariffs: the 20 solid-fuel corpus certs (solid fuel 4-11:
WHC 903 dual immersion, 18-hour meter, 110 L) carry a dual immersion, but
their 18-hour tariff bills 100% low-rate per Table 12a's 7-/10-hour scope
— so they stay EXACT instead of regressing to the 10-hour-column ~0.10.
7-/10-hour Summary immersion certs now correctly cost the Table 13
high-rate fraction instead of falling to the immersion=None 100%-low
default.

Regression gate green (3 pre-existing fails unrelated); API gauge
unchanged (Summary-path-only): 57.6% within 0.5, mean|err| 1.185.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/documents_parser/elmhurst_extractor.py

@@ -1531,6 +1531,9 @@ class ElmhurstSiteNotesExtractor:
         if cylinder_thermostat is None:
             if "Cylinder thermostat (Already installed)" in self._lines:
                 cylinder_thermostat = True
+        # §15.1 "Immersion Heater" lodging ("Dual" / "Single"). Scoped to
+        # the §15.1..§15.2 slice so the lookup can't collide elsewhere.
+        immersion_type = self._local_val(cylinder_lines, "Immersion Heater")
         return WaterHeating(
             water_heating_code=self._str_val("Water Heating Code"),
             water_heating_sap_code=self._int_val("Water Heating SapCode"),
@@ -1540,6 +1543,7 @@ class ElmhurstSiteNotesExtractor:
             cylinder_insulation_label=cylinder_insulation_label,
             cylinder_insulation_thickness_mm=cylinder_insulation_thickness_mm,
             cylinder_thermostat=cylinder_thermostat,
+            immersion_type=immersion_type,
         )
 
     def _extract_baths_and_showers(self) -> BathsAndShowers:

backend/documents_parser/tests/test_heating_systems_corpus.py

@@ -960,6 +960,24 @@ def test_heating_systems_corpus_residual_matches_pin(
     )
 
 
+def test_solid_fuel_5_captures_section_15_1_dual_immersion() -> None:
+    # Arrange — solid fuel 5 (cert 001431: House Coal main SAP 153, WHC 903
+    # electric immersion HW, 18-hour meter, 110 L Normal cylinder). The
+    # Elmhurst Summary §15.1 "Hot Water Cylinder" block lodges "Immersion
+    # Heater: Dual". The extractor must surface it and the mapper map it to
+    # the SAP10 `immersion_heating_type` code 1 (dual) per RdSAP 10 §10.5.
+    summary_pdf, _p960 = _variant_paths('solid fuel 5')
+    pages = _summary_pdf_to_textract_style_pages(summary_pdf)
+
+    # Act
+    site_notes = ElmhurstSiteNotesExtractor(pages).extract()
+    epc = EpcPropertyDataMapper.from_elmhurst_site_notes(site_notes)
+
+    # Assert
+    assert site_notes.water_heating.immersion_type == "Dual"
+    assert epc.sap_heating.immersion_heating_type == 1
+
+
 def test_oil_6_no_room_thermostat_applies_table_4c2_minus_5pp_space_efficiency() -> None:
     # Arrange — oil 6 (B30K standard liquid-fuel boiler, Table 4b code
     # 126 winter 80 / summer 68) lodges "Main Heating Controls Sap: SAP

backend/documents_parser/tests/test_summary_pdf_mapper_chain.py

@@ -43,6 +43,7 @@ from datatypes.epc.domain.mapper import (
     UnmappedApiCode,
     UnmappedElmhurstLabel,
     _elmhurst_glazing_type_code,  # pyright: ignore[reportPrivateUsage]
+    _elmhurst_immersion_type_code,  # pyright: ignore[reportPrivateUsage]
 )
 from domain.sap10_calculator.calculator import calculate_sap_from_inputs
 from domain.sap10_calculator.rdsap.cert_to_inputs import (
@@ -1539,6 +1540,37 @@ def test_summary_mapper_raises_on_unmapped_cylinder_insulation_label() -> None:
     assert excinfo.value.value == "Polyester wool"
 
 
+def test_elmhurst_immersion_type_code_maps_dual_and_single() -> None:
+    # Arrange — Elmhurst Summary §15.1 "Immersion Heater" lodges "Dual"
+    # or "Single". RdSAP 10 §10.5 (PDF p.54): an immersion is "assumed
+    # dual" on a dual/off-peak meter; the SAP10 cascade code is 1 = dual,
+    # 2 = single (cert_to_inputs `_IMMERSION_TYPE_DUAL`).
+
+    # Act
+    dual = _elmhurst_immersion_type_code("Dual", cylinder_present=True)
+    single = _elmhurst_immersion_type_code("Single", cylinder_present=True)
+    no_cylinder = _elmhurst_immersion_type_code("Dual", cylinder_present=False)
+    absent = _elmhurst_immersion_type_code(None, cylinder_present=True)
+
+    # Assert
+    assert dual == 1
+    assert single == 2
+    assert no_cylinder is None
+    assert absent is None
+
+
+def test_elmhurst_immersion_type_code_raises_on_unmapped_label() -> None:
+    # Arrange — a lodged §15.1 "Immersion Heater" label outside the
+    # {Dual, Single} set must strict-raise (mirror of the cylinder-size /
+    # cylinder-insulation helpers) rather than silently drop the field.
+
+    # Act / Assert
+    with pytest.raises(UnmappedElmhurstLabel) as excinfo:
+        _elmhurst_immersion_type_code("Triple", cylinder_present=True)
+    assert excinfo.value.field == "immersion_type"
+    assert excinfo.value.value == "Triple"
+
+
 def test_all_seven_ashp_cohort_certs_extract_without_unmapped_label_raise() -> None:
     # Arrange — coverage forcing function: every cohort cert must
     # extract through `from_elmhurst_site_notes` without triggering an

datatypes/epc/domain/mapper.py

@@ -5167,6 +5167,15 @@ _ELMHURST_CYLINDER_INSULATION_LABEL_TO_SAP10: Dict[str, int] = {
 }
 
 
+# Elmhurst §15.1 "Immersion Heater" label → SAP10 `immersion_heating_type`
+# cascade code. RdSAP 10 §10.5 (PDF p.54) + cert_to_inputs
+# `_IMMERSION_TYPE_DUAL`/`_IMMERSION_TYPE_SINGLE`: 1 = dual, 2 = single.
+_ELMHURST_IMMERSION_TYPE_LABEL_TO_SAP10: Dict[str, int] = {
+    "Dual": 1,
+    "Single": 2,
+}
+
+
 # Elmhurst §15.0 "Water Heating Fuel Type" labels that route to solid-
 # fuel Table 32 codes (Anthracite, House coal, Wood logs/pellets, etc.).
 # Used by `_resolve_elmhurst_inaccessible_cylinder_size` to detect the
@@ -5282,6 +5291,23 @@ def _elmhurst_cylinder_insulation_code(
     return code
 
 
+def _elmhurst_immersion_type_code(
+    immersion_type_label: Optional[str], cylinder_present: bool,
+) -> Optional[int]:
+    """Map an Elmhurst §15.1 "Immersion Heater" label ("Dual" / "Single")
+    to the SAP10 `immersion_heating_type` cascade code (1 = dual, 2 =
+    single per RdSAP 10 §10.5 p.54). Returns None when no cylinder is
+    present or the label is genuinely absent. Raises `UnmappedElmhurstLabel`
+    when the label IS lodged but isn't in the mapping dict — same
+    strict-fallback as `_elmhurst_cylinder_insulation_code`."""
+    if not cylinder_present or immersion_type_label is None:
+        return None
+    code = _ELMHURST_IMMERSION_TYPE_LABEL_TO_SAP10.get(immersion_type_label)
+    if code is None:
+        raise UnmappedElmhurstLabel("immersion_type", immersion_type_label)
+    return code
+
+
 def _resolve_elmhurst_inaccessible_cylinder_insulation(
     age_band: str,
 ) -> tuple[int, int]:
@@ -5850,6 +5876,14 @@ def _map_elmhurst_sap_heating(survey: ElmhurstSiteNotes) -> SapHeating:
             and survey.water_heating.cylinder_thermostat is not None
             else None
         ),
+        # §15.1 "Immersion Heater" (Dual / Single) → SAP10
+        # `immersion_heating_type` code (1 = dual, 2 = single). Drives the
+        # Table 13 high-rate-fraction split for WHC-903 electric immersion
+        # DHW on a 7-/10-hour off-peak tariff (18-/24-hour bill 100% low).
+        immersion_heating_type=_elmhurst_immersion_type_code(
+            survey.water_heating.immersion_type,
+            survey.water_heating.hot_water_cylinder_present,
+        ),
         water_heating_code=survey.water_heating.water_heating_sap_code,
         water_heating_fuel=water_heating_fuel,
         secondary_heating_type=mh.secondary_heating_sap_code,

datatypes/epc/surveys/elmhurst_site_notes.py

@@ -367,6 +367,11 @@ class WaterHeating:
     # §15.1 "Cylinder Thermostat" lodging (Yes / No). False or absent
     # keeps the cascade's no-thermostat Table 2b temperature factor.
     cylinder_thermostat: Optional[bool] = None
+    # §15.1 "Immersion Heater" lodging ("Dual" / "Single"). Drives the
+    # SAP10 `immersion_heating_type` code (1 = dual, 2 = single) used by
+    # the Table 13 high-rate-fraction DHW-cost split. None when no
+    # cylinder is present or the line is absent.
+    immersion_type: Optional[str] = None
 
 
 @dataclass