diff --git a/backend/documents_parser/tests/test_heating_systems_corpus.py b/backend/documents_parser/tests/test_heating_systems_corpus.py
index cdffe4b4..69c0b1ae 100644
--- a/backend/documents_parser/tests/test_heating_systems_corpus.py
+++ b/backend/documents_parser/tests/test_heating_systems_corpus.py
@@ -511,7 +511,21 @@ _EXPECTATIONS: tuple[_CorpusExpectation, ...] = (
     # residual (SAP +1.18, cost −£27 / CO2 −50 / PE −562) — likely a
     # cascade-side §A.2.2 efficiency or tariff-routing gap; pinned as
     # forcing function for follow-up.
-    _CorpusExpectation(variant='no system', block='11a', expected_sap_resid=+1.1783, expected_cost_resid_gbp=-27.1485, expected_co2_resid_kg=-49.8272, expected_pe_resid_kwh=-562.4367),
+    # Slice S0380.179 closed `no system` via RdSAP 10 §10.7 (PDF p.55)
+    # "No water heating system": the cert lodges §15.0 water code 999
+    # (NON) + §15.1 "Cylinder Present: No", but per spec the calculation
+    # is done for an electric immersion heater on a Table 28 row-1 110 L
+    # cylinder with Table 29 row-1 age-band insulation (25 mm foam at age
+    # G). The P960 worksheet header confirms the engine's substitution
+    # (WHS 903 Single immersion, 110 L). Pre-slice the cascade trusted
+    # the lodged "no cylinder" → no storage loss (56) + a spurious Table
+    # 3a combi loss, and the wrong HW heat-gains propagated through §5/§7
+    # to over-state the base MIT (+0.25 K), over-stating space fuel by
+    # +228 kWh. `_apply_rdsap_no_water_heating_system_default` injects
+    # the default cylinder before the section cascades, closing HW fuel
+    # (219) 1935.37 → 2529.69 EXACT AND the space residual in one move.
+    # ΔSAP +1.18 → <1e-4, all four metrics EXACT.
+    _CorpusExpectation(variant='no system', block='11a', expected_sap_resid=+0.0000, expected_cost_resid_gbp=+0.0000, expected_co2_resid_kg=+0.0000, expected_pe_resid_kwh=-0.0000),
     # Slice S0380.170 unblocked the 5 community-heating variants. Per
     # SAP 10.2 Table 12 (PDF p.189) the heat-network fuel code comes
     # from the §14.1 Community Heat Source × Community Fuel Type pair:
@@ -920,6 +934,35 @@ def test_oil_6_absent_room_thermostat_applies_table_4f_pump_1_3_multiplier() ->
     )
 
 
+def test_no_system_assumes_rdsap_10_7_electric_immersion_default_cylinder() -> None:
+    # Arrange — the "no system" cert lodges §15.0 "Water Heating Code:
+    # NON / SapCode 999" and §15.1 "Hot Water Cylinder Present: No". Per
+    # RdSAP 10 §10.7 (PDF p.55) "No water heating system" verbatim: "the
+    # calculation is done for an electric immersion heater... for a
+    # cylinder defined by the first row of Table 28 (110 litres) and the
+    # first row of Table 29." The BRE-approved Elmhurst engine confirms
+    # it — the P960 worksheet header lodges "WHS: 903 Electric immersion,
+    # Single", a 110 L cylinder, and Table 29 age-band insulation (the
+    # corpus property is age G -> 25 mm foam), giving storage loss (56) =
+    # 594.32 kWh/yr. Worksheet HW (64) = (45) 1935.37 + (56) 594.32 =
+    # 2529.6927. Pre-slice the cascade trusted the lodged "no cylinder"
+    # so it added no storage loss (and a spurious Table 3a combi loss).
+    summary_pdf, _ = _variant_paths('no system')
+    pages = _summary_pdf_to_textract_style_pages(summary_pdf)
+    site_notes = ElmhurstSiteNotesExtractor(pages).extract()
+    epc = EpcPropertyDataMapper.from_elmhurst_site_notes(site_notes)
+
+    # Act — run the rating cascade and read the resolved HW fuel kWh.
+    inputs = cert_to_inputs(epc, prices=SAP_10_2_SPEC_PRICES)
+
+    # Assert — HW fuel = (45) + Table 29 110 L / 25 mm-foam storage loss
+    # = 2529.6927 (matches worksheet (64)/(219)).
+    assert abs(inputs.hot_water_kwh_per_yr - 2529.6927) <= 1e-3, (
+        f"no system HW {inputs.hot_water_kwh_per_yr:.4f} kWh != 2529.6927 "
+        f"(RdSAP 10 §10.7 electric-immersion default 110 L cylinder)"
+    )
+
+
 @pytest.mark.skipif(
     not _BLOCKED_BY_MISSING_MAIN_FUEL_TYPE,
     reason="all blocked variants have been unblocked (latest: S0380.170)",
diff --git a/domain/sap10_calculator/rdsap/cert_to_inputs.py b/domain/sap10_calculator/rdsap/cert_to_inputs.py
index 5fc529c2..83b4a378 100644
--- a/domain/sap10_calculator/rdsap/cert_to_inputs.py
+++ b/domain/sap10_calculator/rdsap/cert_to_inputs.py
@@ -50,7 +50,7 @@ Reference: RdSAP 10 specification (10-06-2025); SAP 10.2 specification
 from __future__ import annotations
 
 import math
-from dataclasses import dataclass
+from dataclasses import dataclass, replace
 from decimal import ROUND_HALF_UP, Decimal
 from typing import Callable, Final, Literal, Optional
 
@@ -4142,6 +4142,72 @@ _CYLINDER_SIZE_CODE_TO_LITRES: Final[dict[int, float]] = {
 # "Foam" → factory-applied per SAP 10.2 Table 2 Note 2).
 _CYLINDER_INSULATION_TYPE_FACTORY: Final[int] = 1
 
+# RdSAP 10 §10.7 (PDF p.55) "No water heating system": SAP water-heating
+# code 999 (Elmhurst §15.0 "NON") signals that no DHW system was
+# identified. Per spec the calculation is then done for an electric
+# immersion heater + a cylinder defined by the first row of Table 28
+# (110 litres) and the first row of Table 29 (age-band insulation).
+_WHC_NO_WATER_HEATING_SYSTEM: Final[int] = 999
+# Table 28 row 1 "Inaccessible — otherwise: 110 litres" → SAP cylinder
+# size code 2 (Normal, 110 L). The immersion is single unless the meter
+# is dual; the corpus "no system" cert's worksheet header lodges
+# "Immersion Heater Type: Single" so the single-immersion path is used.
+_CYLINDER_SIZE_CODE_NORMAL_110L: Final[int] = 2
+# RdSAP 10 Table 29 (PDF p.56) "Hot water cylinder insulation if not
+# accessible" — the §10.7 default cylinder uses the age-band insulation,
+# same rule as the inaccessible-cylinder path: A-F → 12 mm loose jacket
+# (not yet plumbed — strict-raise), G/H → 25 mm foam, I-M → 38 mm foam.
+_TABLE_29_DEFAULT_CYLINDER_INSULATION_MM_BY_AGE: Final[dict[str, int]] = {
+    "G": 25, "H": 25, "I": 38, "J": 38, "K": 38, "L": 38, "M": 38,
+}
+
+
+def _apply_rdsap_no_water_heating_system_default(
+    epc: EpcPropertyData,
+) -> EpcPropertyData:
+    """RdSAP 10 §10.7 (PDF p.55) — when no water heating system is
+    identified (`water_heating_code == 999`), substitute the spec
+    default: an electric immersion heater (single — dual handling not
+    yet exercised) on a Table 28 row-1 110 L cylinder with Table 29
+    row-1 age-band insulation and an assumed cylinder thermostat
+    (Table 29: "A cylinder thermostat should be assumed to be present
+    when DHW is from ... an immersion heater ...").
+
+    Returns `epc` unchanged when a real water heating system is lodged.
+    Otherwise returns a copy with `has_hot_water_cylinder=True` and the
+    `sap_heating` fields the WHC-903 electric-immersion + cylinder
+    cascade reads, so every downstream gate (storage loss, combi-loss
+    suppression, primary loss) sees the spec default. This mirrors the
+    Elmhurst engine's worksheet header for the corpus "no system" cert
+    (WHS 903, Single immersion, 110 L cylinder, 25 mm foam at age G).
+
+    Raises `UnmappedSapCode` for age bands A-F (12 mm loose jacket) —
+    no corpus member exercises that combination and the SAP 10.2 Table 2
+    loss-factor dispatch only has the factory-foam path plumbed.
+    """
+    if epc.sap_heating.water_heating_code != _WHC_NO_WATER_HEATING_SYSTEM:
+        return epc
+    age_band = (
+        epc.sap_building_parts[0].construction_age_band
+        if epc.sap_building_parts else None
+    )
+    band = (age_band or "")[:1].upper()
+    thickness_mm = _TABLE_29_DEFAULT_CYLINDER_INSULATION_MM_BY_AGE.get(band)
+    if thickness_mm is None:
+        raise UnmappedSapCode(
+            "rdsap_10_7_default_cylinder_insulation_age_band", age_band
+        )
+    sap_heating = replace(
+        epc.sap_heating,
+        water_heating_code=_WHC_ELECTRIC_IMMERSION,
+        water_heating_fuel=_STANDARD_ELECTRICITY_FUEL_CODE,
+        cylinder_size=_CYLINDER_SIZE_CODE_NORMAL_110L,
+        cylinder_insulation_type=_CYLINDER_INSULATION_TYPE_FACTORY,
+        cylinder_insulation_thickness_mm=thickness_mm,
+        cylinder_thermostat="Y",
+    )
+    return replace(epc, has_hot_water_cylinder=True, sap_heating=sap_heating)
+
 
 # SAP 10.2 Table 4a solid-fuel boiler sub-rows (PDF p.163) — independent
 # boilers (151, 153, 155, 159), open-fire + back boiler (156), closed
@@ -5483,6 +5549,12 @@ def cert_to_inputs(
     parity validation now relies on the Validation Cohort filter
     (inspection_date ≥ 2025-07-01) rather than a per-cert price
     override."""
+    # RdSAP 10 §10.7 (PDF p.55) — substitute the electric-immersion +
+    # default-cylinder assumption before any section cascade runs when no
+    # water heating system is lodged (code 999). Rebinding `epc` here
+    # means every downstream helper sees the spec default; the demand
+    # cascade reuses this entry point so it is covered too.
+    epc = _apply_rdsap_no_water_heating_system_default(epc)
     dim = dimensions_from_cert(epc)
     # SAP §3 heat transmission + §2 ventilation cascades — see the
     # respective `_from_cert` helpers for cert→inputs mapping rules.