S0380.238: single-point instantaneous water heaters incur no distribution loss

Water heating SAP code 909 (electric instantaneous) and 907 (single-point
gas) heat water at the point of use, serving one outlet with no
distribution pipework. Per SAP 10.2 §4 (p.23, l.1416): "'Single-point'
heaters, which are located at the point of use and serve only one outlet,
do not have distribution losses either." So worksheet (46)m = 0 and the
heat-required line collapses to SAP 10.2 worksheet l.7704
  (62)m = 0.85 × (45)m + (46)m + (57)m + (59)m + (61)m
        = 0.85 × (45)m   (all loss terms zero for a no-cylinder system).

`distribution_loss_monthly_kwh` already supported the
`is_instantaneous_at_point_of_use` flag (and its docstring already named
codes 907/909), but `water_heating_from_cert` hard-coded it to False, so
the cascade applied (46)m = 0.15 × (45)m to single-point heaters. That
0.15 distribution loss exactly cancelled the 0.85 reduction, leaving
(62)m = (45)m. On the cat-10 room-heater fixture (ref 001431, code 909)
that over-stated the water fuel (219) as 2082.6250 instead of the
worksheet's 1770.2313, and inflated the (65)m heat gains (692.47 vs
worksheet 442.55) which in turn suppressed space-heating demand.

Thread the cert's existing instantaneous flag (`_INSTANTANEOUS_WATER_CODES`
= {907, 909}) through `_water_heating_worksheet_and_gains` into both the
demand-pass and final `water_heating_from_cert` calls.

Pins (219) water fuel = 1770.2313 at abs 1e-4 via the extractor → mapper →
rating cascade. §4 suite green (2414 passed, 1 skipped); no existing
fixture exercised the 907/909 path. The residual space-heating fuel gap
((211) 11158.59 vs worksheet 11563.17) this exposes is a separate cause —
next slice.

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

backend/documents_parser/tests/test_room_heater_worksheet_001431.py

@@ -0,0 +1,101 @@
+"""Worksheet pins for the cat-10 electric-room-heater dwelling (ref 001431).
+
+Fixture: `sap worksheets/Recommendations Elmhurst Files/main heating/high
+heat retention storage heaters/electric room heaters/before/` — Summary
+(site-notes input) + P960 (the `(1)..(286)` worksheet ground truth). The
+dwelling lodges main `sap_main_heating_code=691` (electric room heaters),
+control `2601`, an `18 Hour` meter, and water heating `sap_code=909`
+(electric instantaneous, single-point at the point of use — NO cylinder,
+NO solar, NO WWHRS).
+
+Per [[feedback-worksheet-not-api-reference]] + [[feedback-zero-error-strict]]
+the worksheet PDF is the 1e-4 target. Each pin below is a P960 line ref
+transcribed to 4 d.p. and asserted via `abs(x - y) <= 1e-4` against the
+extractor → mapper → cascade output.
+
+Because the SAP 10.2 worksheet computes the rating block (UK-average
+climate, Table 12 regulated prices) separately from the EPC block
+(postcode climate, Table 32 prices), the rating-mode cascade
+(`cert_to_inputs`) is pinned against the rating block and the demand-mode
+cascade (`cert_to_demand_inputs`) against the EPC block.
+"""
+
+from __future__ import annotations
+
+import re
+import subprocess
+from pathlib import Path
+
+from backend.documents_parser.elmhurst_extractor import ElmhurstSiteNotesExtractor
+from datatypes.epc.domain.mapper import EpcPropertyDataMapper
+from domain.sap10_calculator.calculator import calculate_sap_from_inputs
+from domain.sap10_calculator.rdsap.cert_to_inputs import (
+    SAP_10_2_SPEC_PRICES,
+    cert_to_inputs,
+)
+
+_FIXTURE_DIR = (
+    Path(__file__).parents[3]
+    / "sap worksheets/Recommendations Elmhurst Files/main heating"
+    / "high heat retention storage heaters/electric room heaters/before"
+)
+
+# P960 line ref (219) "Water heating fuel used" — rating block. The water
+# heater is electric (efficiency (216) = 100 %), so (219) == (64) output.
+_WORKSHEET_LINE_219_WATER_FUEL_KWH = 1770.2313
+
+_ABS_TOLERANCE = 0.0001
+
+
+def _summary_pdf_to_pages(pdf: Path) -> list[str]:
+    """Summary PDF → one Textract-style token string per page (the same
+    `pdftotext -layout` → whitespace-split preprocessing the rest of the
+    documents_parser chain tests use)."""
+    page_count_text = subprocess.run(
+        ["pdfinfo", str(pdf)], capture_output=True, text=True
+    ).stdout
+    page_count_match = re.search(r"Pages:\s+(\d+)", page_count_text)
+    assert page_count_match is not None, f"no page count in {pdf}"
+    page_count = int(page_count_match.group(1))
+    pages: list[str] = []
+    for page_index in range(1, page_count + 1):
+        layout = subprocess.run(
+            [
+                "pdftotext", "-layout",
+                "-f", str(page_index), "-l", str(page_index),
+                str(pdf), "-",
+            ],
+            capture_output=True,
+            text=True,
+        ).stdout
+        pages.append(
+            "\n".join(
+                token
+                for line in layout.splitlines()
+                for token in re.split(r"\s{2,}", line.strip())
+                if token
+            )
+        )
+    return pages
+
+
+def test_electric_room_heater_water_fuel_matches_worksheet_line_219() -> None:
+    # Arrange — route the before/ Summary through the full extractor →
+    # mapper → rating cascade. Water heating SAP code 909 is a single-
+    # point electric instantaneous heater at the point of use, so per
+    # SAP 10.2 §4 (p.23, l.1416) it has NO distribution loss: worksheet
+    # (46)m = 0 and (62)m = 0.85 × (45)m collapses to the (219) fuel.
+    summary_pdf = next(_FIXTURE_DIR.glob("Summary_*.pdf"))
+    pages = _summary_pdf_to_pages(summary_pdf)
+    site_notes = ElmhurstSiteNotesExtractor(pages).extract()
+    epc = EpcPropertyDataMapper.from_elmhurst_site_notes(site_notes)
+    expected_water_fuel_kwh = _WORKSHEET_LINE_219_WATER_FUEL_KWH
+
+    # Act
+    rating = calculate_sap_from_inputs(
+        cert_to_inputs(epc, prices=SAP_10_2_SPEC_PRICES),
+    )
+    actual_water_fuel_kwh = rating.hot_water_kwh_per_yr
+
+    # Assert
+    assert abs(actual_water_fuel_kwh - expected_water_fuel_kwh) <= _ABS_TOLERANCE

domain/sap10_calculator/rdsap/cert_to_inputs.py

@@ -5653,6 +5653,7 @@ def _water_heating_worksheet_and_gains(
         primary_loss_monthly_kwh_override=primary_loss_override,
         has_electric_shower=has_electric_shower,
         electric_shower_count=electric_shower_count,
+        is_instantaneous_at_point_of_use=is_instantaneous,
     )
     solar_hw_override = _solar_hw_monthly_override(
         epc=epc,
@@ -5670,6 +5671,7 @@ def _water_heating_worksheet_and_gains(
         solar_water_heating_monthly_kwh_override=solar_hw_override,
         has_electric_shower=has_electric_shower,
         electric_shower_count=electric_shower_count,
+        is_instantaneous_at_point_of_use=is_instantaneous,
     )
     return wh_result, wh_result.heat_gains_monthly_kwh
 

domain/sap10_calculator/worksheet/water_heating.py

@@ -843,6 +843,7 @@ def water_heating_from_cert(
     electric_shower_monthly_kwh_override: Optional[tuple[float, ...]] = None,
     has_electric_shower: bool = False,
     electric_shower_count: int = 0,
+    is_instantaneous_at_point_of_use: bool = False,
 ) -> WaterHeatingResult:
     """SAP 10.2 §4 orchestrator — chain every line ref from (42) through
     (65) for a combi-gas dwelling with optional PCDB-backed combi loss.
@@ -912,7 +913,7 @@ def water_heating_from_cert(
     )
     distribution = distribution_loss_monthly_kwh(
         monthly_energy_content_kwh=energy_content,
-        is_instantaneous_at_point_of_use=False,
+        is_instantaneous_at_point_of_use=is_instantaneous_at_point_of_use,
     )
     combi = (
         combi_loss_monthly_kwh_override