diff --git a/backend/documents_parser/tests/test_pashub_sap_accuracy_lrha_wave3.py b/backend/documents_parser/tests/test_pashub_sap_accuracy_lrha_wave3.py
index c8ccc34e4..cfdcd54fd 100644
--- a/backend/documents_parser/tests/test_pashub_sap_accuracy_lrha_wave3.py
+++ b/backend/documents_parser/tests/test_pashub_sap_accuracy_lrha_wave3.py
@@ -119,8 +119,17 @@ _MANIFEST_PATH = _FIXTURES_DIR / "manifest.json"
 # no-op. Treat the constants as a regression tripwire, NOT a cohort accuracy
 # figure; re-baseline (coverage growth is not loosening) as further mapper fixes
 # unblock fixtures.
-_MIN_WITHIN_HALF: float = 0.55
-_MAX_SAP_MAE: float = 0.68
+# 2026-07-27 (#A1): an oil STORAGE combi (Worcester Greenstar Heatslave II,
+# PCDB 18415, store_type 1) zeroed its internal-store water-heating loss —
+# `pcdb_combi_loss_override` defers on store_type∈{1,2,3} and a combi has no
+# cylinder — under-costing hot water and over-rating SAP. SAP 10.2 Table 2
+# note b now includes the store loss from the PCDB store volume + insulation
+# (Table 2/2a × Table 2b storage-combi TF), matching the accredited Elmhurst
+# P960 worksheet exactly (5.4175 kWh/day). The 5 oil storage-combis collapse
+# from +2.0..+4.9 into the ±1.5 spread: within-0.5 55.3% → 58.3%, MAE
+# 0.675 → 0.531.
+_MIN_WITHIN_HALF: float = 0.58
+_MAX_SAP_MAE: float = 0.54
 
 _KNOWN_GAP_REASON = (
     "pashub `from_site_notes` mapper does not int-code a main-heating "
@@ -203,6 +212,29 @@ def test_dual_main_heating_models_both_systems() -> None:
     assert outcome.diff < 0.5, f"dual-main not modelled: diff {outcome.diff:.2f}"
 
 
+@pytest.mark.skipif(
+    "461996268736" not in _BY_ID, reason="oil storage-combi fixture not in manifest"
+)
+def test_oil_storage_combi_applies_pcdb_store_loss() -> None:
+    """Regression (#A1): an oil STORAGE combi (Worcester Greenstar Heatslave
+    II, PCDB 18415, store_type 1) must incur its internal-store water-heating
+    loss. `pcdb_combi_loss_override` defers on store_type∈{1,2,3} and the
+    combi has no cylinder, so the store loss (56) was ZEROED → hot water
+    under-costed → SAP over-rated +4.9. SAP 10.2 Table 2 note b now includes
+    the store loss from the PCDB store volume (63 L) + insulation (25 mm):
+    Table 2 × 2a × Table 2b storage-combi TF 2.8946 → 5.4175 kWh/day ≈
+    1977 kWh/yr, matching the accredited Elmhurst P960 worksheet for this
+    boiler exactly. Fixture 461996268736 +4.93 → +0.13.
+
+    The five oil storage-combis (461996268736, 461989738731, 497712315613,
+    497602074839, 462051619031) all collapse from +2..+4.9 into the cohort's
+    normal ±1.5 spread; only this one is pinned (the others' residuals are
+    PasHub-vs-Elmhurst divergence, not the store-loss bug)."""
+    outcome = _evaluate("461996268736")
+    assert outcome.diff is not None
+    assert outcome.diff < 0.5, f"oil storage-combi store loss not applied: diff {outcome.diff:.2f}"
+
+
 @pytest.mark.skipif(
     "497712825571" not in _BY_ID or "461386632387" not in _BY_ID,
     reason="direct-acting electric boiler fixtures not in manifest",
diff --git a/domain/sap10_calculator/rdsap/cert_to_inputs.py b/domain/sap10_calculator/rdsap/cert_to_inputs.py
index 448c00c29..38b72ecc3 100644
--- a/domain/sap10_calculator/rdsap/cert_to_inputs.py
+++ b/domain/sap10_calculator/rdsap/cert_to_inputs.py
@@ -210,6 +210,7 @@ from domain.sap10_calculator.worksheet.water_heating import (
     cylinder_storage_loss_monthly_kwh,
     cylinder_volume_factor_table_2a,
     primary_loss_monthly_kwh,
+    storage_combi_temperature_factor_table_2b,
     water_efficiency_monthly_via_equation_d1,
     water_heating_from_cert,
 )
@@ -6201,6 +6202,11 @@ def _apply_rdsap_no_water_heating_system_default(
 # cylinder of 110 litres and a factory insulation thickness of 50 mm".
 _HEAT_NETWORK_HIU_DEFAULT_INSULATION_MM: Final[int] = 50
 
+# SAP 10.2 Table 2 note c) (PDF p.158): when a storage combi's factory
+# insulation thickness is unknown, a 13 mm default applies. Used by
+# `_storage_combi_store_loss_override` when the PCDB record omits it.
+_STORE_COMBI_DEFAULT_INSULATION_MM: Final[float] = 13.0
+
 
 def _apply_heat_network_hiu_default_store(
     epc: EpcPropertyData,
@@ -7132,6 +7138,14 @@ def _water_heating_worksheet_and_gains(
     # combi systems, so the override is only built when the cert explicitly
     # lodges a cylinder.
     storage_loss_override = _cylinder_storage_loss_override(epc, main)
+    if storage_loss_override is None:
+        # SAP 10.2 Table 2 note b) — a storage combi with no separate cylinder
+        # still incurs its internal store loss (56)m. `pcdb_record` is the
+        # same boiler record `pcdb_combi_loss_override` used above, so the
+        # store loss and the combi loss (61)=0 stay on the one heat generator.
+        storage_loss_override = _storage_combi_store_loss_override(
+            epc, pcdb_record
+        )
     # SAP 10.2 §4 line 7700 + Table 3 (PDF p.159) — primary circuit loss
     # (59)m. Only fires for indirect cylinders; HPs with integral
     # vessels and combi boilers are in the spec's zero list. The gate
@@ -7415,6 +7429,62 @@ def _cylinder_storage_loss_override(
     return tuple(s * factor for s in storage_56m)
 
 
+def _storage_combi_store_loss_override(
+    epc: EpcPropertyData,
+    pcdb_record: Optional[GasOilBoilerRecord],
+) -> Optional[tuple[float, ...]]:
+    """SAP 10.2 §4 (56)m for the internal water store of a STORAGE COMBI
+    boiler — the loss a lodged-cylinder dwelling gets from
+    `_cylinder_storage_loss_override`, but which a combi (no cylinder)
+    otherwise drops to zero, over-rating the dwelling.
+
+    Table 2 note b) (PDF p.158): "the loss is to be included for a storage
+    combination boiler if its efficiency ... is obtained from the PCDB (in
+    which case its insulation thickness and volume are also ... obtained
+    from the PCDB), using the loss factor for a factory insulated cylinder."
+    So when a PCDB Table 105 record with `store_type ∈ {1, 2}` (primary /
+    secondary water store) drives the combi's efficiency, its store volume
+    (field 42) and insulation thickness (field 44) size a Table 2 store loss
+    with the Table 2b storage-combi temperature factor (much larger than a
+    cylinder's 0.60 — the store is kept hot continuously).
+
+    Returns None (leaving the existing zero default) when there is a lodged
+    cylinder (that path already runs), when the main is not a PCDB storage
+    combi, or when the PCDB store volume is unknown (Table 2 note c defaults
+    deferred — no cohort fixture yet). Verified against the Elmhurst P960
+    worksheet for pcdb_id 18415 (Worcester Greenstar Heatslave II): store
+    volume 63 L / insulation 25 mm → (55) 5.4175 kWh/day, (56) Jan 167.94."""
+    if epc.has_hot_water_cylinder:
+        return None
+    if pcdb_record is None or pcdb_record.store_type not in (1, 2):
+        return None
+    volume_l = pcdb_record.store_boiler_volume_l
+    if volume_l is None:
+        return None
+    thickness_mm = pcdb_record.store_insulation_thickness_mm
+    if thickness_mm is None:
+        # Table 2 note c) default store insulation (13 mm) when the PCDB
+        # thickness is unlodged — kept explicit so the loss is still applied
+        # rather than dropped, mirroring the cylinder Table 29 fallback.
+        thickness_mm = _STORE_COMBI_DEFAULT_INSULATION_MM
+    temperature_factor = storage_combi_temperature_factor_table_2b(
+        store_type=pcdb_record.store_type, volume_l=volume_l,
+    )
+    return cylinder_storage_loss_monthly_kwh(
+        volume_l=volume_l,
+        # SAP 10.2 Table 2 note 2: the water store in a storage combi uses
+        # the factory-insulated cylinder loss factor regardless of material.
+        insulation_type="factory_insulated",
+        thickness_mm=thickness_mm,
+        # The Table 2b storage-combi temperature factor replaces the cylinder
+        # 0.60 base; the thermostat / separate-timing multipliers do not
+        # apply, so these two flags are inert (override supplied).
+        has_cylinder_thermostat=True,
+        separately_timed_dhw=False,
+        temperature_factor_override=temperature_factor,
+    )
+
+
 def _apply_water_efficiency(
     *,
     wh_output_monthly_kwh: tuple[float, ...],
diff --git a/domain/sap10_calculator/tables/pcdb/__init__.py b/domain/sap10_calculator/tables/pcdb/__init__.py
index 6fd2991e8..7cf315b48 100644
--- a/domain/sap10_calculator/tables/pcdb/__init__.py
+++ b/domain/sap10_calculator/tables/pcdb/__init__.py
@@ -75,6 +75,29 @@ _TABLE_362_JSONL: Final[Path] = (
 )
 
 
+# PCDF Spec Rev 6b (0-idx) raw-row positions for the storage-combi store
+# fields not surfaced as top-level NDJSON keys. Parsed from `raw` at load
+# time so no NDJSON rebuild is needed. Field 42 = store boiler volume (L),
+# field 44 = store insulation thickness (mm).
+_RAW_STORE_VOLUME_IDX: Final[int] = 41
+_RAW_STORE_INSULATION_IDX: Final[int] = 43
+
+
+def _raw_float(raw: list[str], idx: int) -> Optional[float]:
+    """Parse a float from the raw PCDB row at `idx`; None if the row is too
+    short, the cell is blank, or it does not parse (mirrors the blank-store
+    convention for non-storage combis)."""
+    if idx >= len(raw):
+        return None
+    value = raw[idx].strip()
+    if not value:
+        return None
+    try:
+        return float(value)
+    except ValueError:
+        return None
+
+
 def _load_table_105() -> dict[int, GasOilBoilerRecord]:
     """Read the Table 105 NDJSON at import time and build a by-pcdb-id
     dict. ~5MB / ~4000 rows; one-off ~50ms cost. The Python runtime
@@ -98,6 +121,12 @@ def _load_table_105() -> dict[int, GasOilBoilerRecord]:
                 final_year_of_manufacture=data["final_year_of_manufacture"],
                 subsidiary_type=data.get("subsidiary_type"),
                 store_type=data.get("store_type"),
+                store_boiler_volume_l=_raw_float(
+                    data["raw"], _RAW_STORE_VOLUME_IDX
+                ),
+                store_insulation_thickness_mm=_raw_float(
+                    data["raw"], _RAW_STORE_INSULATION_IDX
+                ),
                 separate_dhw_tests=data.get("separate_dhw_tests"),
                 rejected_energy_proportion_r1=data.get("rejected_energy_proportion_r1"),
                 loss_factor_f1_kwh_per_day=data.get("loss_factor_f1_kwh_per_day"),
diff --git a/domain/sap10_calculator/tables/pcdb/parser.py b/domain/sap10_calculator/tables/pcdb/parser.py
index 27e1b3d1a..9b39ce597 100644
--- a/domain/sap10_calculator/tables/pcdb/parser.py
+++ b/domain/sap10_calculator/tables/pcdb/parser.py
@@ -81,8 +81,20 @@ class GasOilBoilerRecord:
     subsidiary_type: Optional[int]
     # PCDF Spec Rev 6b field 39 (0-idx 38): 0=not storage combi, 1=primary
     # water store, 2=secondary store, 3=CPSU. Gates storage-combi rows in
-    # Table 3b/3c (deferred until a fixture exercises).
+    # Table 3b/3c and the SAP 10.2 Table 2 store-loss path below.
     store_type: Optional[int]
+    # PCDF Spec Rev 6b field 42 (0-idx 41): "Store boiler volume" — the water
+    # volume (litres) of the internal hot-water store heatable by the boiler
+    # (total store less the store solar volume, field 43). Blank / None for a
+    # non-storage combi. PCDF Spec Rev 6b field 44 (0-idx 43): "Store
+    # insulation" — the store insulation thickness in mm. Together they drive
+    # the SAP 10.2 Table 2 store-loss factor (factory-insulated formula, Note
+    # 2) for a storage combi whose efficiency is from the PCDB (Table 2 note
+    # b). Verified against the Elmhurst P960 worksheet for pcdb_id 18415
+    # (Worcester Greenstar Heatslave II): 63 L / 25 mm → L 0.0240, store loss
+    # (55) 5.4175 kWh/day.
+    store_boiler_volume_l: Optional[float]
+    store_insulation_thickness_mm: Optional[float]
     separate_dhw_tests: Optional[int]
     rejected_energy_proportion_r1: Optional[float]
     loss_factor_f1_kwh_per_day: Optional[float]
@@ -466,6 +478,12 @@ def parse_table_105_row(row: str) -> GasOilBoilerRecord:
         comparative_hot_water_efficiency_pct=_parse_optional_float(fields[28]),
         subsidiary_type=_parse_optional_int(fields[15]),
         store_type=_parse_optional_int(fields[38]),
+        store_boiler_volume_l=(
+            _parse_optional_float(fields[41]) if len(fields) > 41 else None
+        ),
+        store_insulation_thickness_mm=(
+            _parse_optional_float(fields[43]) if len(fields) > 43 else None
+        ),
         separate_dhw_tests=_parse_optional_int(fields[47]),
         rejected_energy_proportion_r1=_parse_optional_float(fields[50]),
         loss_factor_f1_kwh_per_day=_parse_optional_float(fields[51]),
diff --git a/domain/sap10_calculator/worksheet/water_heating.py b/domain/sap10_calculator/worksheet/water_heating.py
index dad601e22..126989ab6 100644
--- a/domain/sap10_calculator/worksheet/water_heating.py
+++ b/domain/sap10_calculator/worksheet/water_heating.py
@@ -537,6 +537,38 @@ def cylinder_temperature_factor_table_2b(
     return factor
 
 
+def storage_combi_temperature_factor_table_2b(
+    *,
+    store_type: int,
+    volume_l: float,
+) -> float:
+    """SAP 10.2 Table 2b (PDF p.159), "loss from Table 2" column, for a
+    storage combi boiler's internal water store (as opposed to a cylinder):
+
+        primary store   (store_type 1): Vc >= 115 → 2.54
+                                        Vc  < 115 → 2.54 + 0.00682 × (115 − Vc)
+        secondary store (store_type 2): Vc >= 115 → 1.86
+                                        Vc  < 115 → 1.86 + 0.00496 × (115 − Vc)
+
+    These are much larger than the 0.60 cylinder base — the store in a combi
+    is kept hot continuously, so SAP applies a heavier standing-loss factor.
+    Verified against the accredited Elmhurst P960 worksheet for pcdb_id 18415
+    (Worcester Greenstar Heatslave II, primary store 63 L → 2.8946).
+
+    store_type 3 (CPSU) is a distinct Table 2b row (integrated thermal store /
+    CPSU) with its own airing-cupboard / separate-timer notes; no fixture
+    exercises it yet, so it strict-raises rather than silently mis-rate."""
+    if store_type == 1:
+        return 2.54 if volume_l >= 115.0 else 2.54 + 0.00682 * (115.0 - volume_l)
+    if store_type == 2:
+        return 1.86 if volume_l >= 115.0 else 1.86 + 0.00496 * (115.0 - volume_l)
+    raise NotImplementedError(
+        f"Table 2b storage-combi temperature factor for store_type={store_type} "
+        "(CPSU / integrated thermal store) is not yet wired — no fixture "
+        "exercises it (SAP 10.2 Table 2b, PDF p.159)."
+    )
+
+
 # SAP 10.2 Table 3 (PDF p.159) — primary circuit loss for boilers and
 # heat pumps connected to a hot water cylinder via insulated or
 # uninsulated primary pipework. The spec lists the zero-loss
@@ -629,6 +661,7 @@ def cylinder_storage_loss_monthly_kwh(
     has_cylinder_thermostat: bool,
     separately_timed_dhw: bool,
     declared_loss_kwh_per_day: Optional[float] = None,
+    temperature_factor_override: Optional[float] = None,
 ) -> tuple[float, ...]:
     """SAP 10.2 §4 line (56)m water storage loss per spec (PDF p.136).
 
@@ -649,9 +682,13 @@ def cylinder_storage_loss_monthly_kwh(
     solar storage is present in the vessel — callers handling solar
     storage must adjust further per `(57)m = (56)m × [(47) - Vs] / (47)`.
     """
-    TF = cylinder_temperature_factor_table_2b(
-        has_cylinder_thermostat=has_cylinder_thermostat,
-        separately_timed_dhw=separately_timed_dhw,
+    TF = (
+        temperature_factor_override
+        if temperature_factor_override is not None
+        else cylinder_temperature_factor_table_2b(
+            has_cylinder_thermostat=has_cylinder_thermostat,
+            separately_timed_dhw=separately_timed_dhw,
+        )
     )
     if declared_loss_kwh_per_day is not None:
         # SAP 10.2 §4 (PDF p.136) branch a) — the lodged manufacturer's
diff --git a/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py b/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
index 95cd1e887..d20230c18 100644
--- a/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
+++ b/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
@@ -866,6 +866,74 @@ def test_cylinder_size_not_determined_still_incurs_storage_loss() -> None:
     assert sum(storage_56m) > 0.0
 
 
+def test_storage_combi_store_loss_matches_elmhurst_worksheet_18415() -> None:
+    # Arrange — SAP 10.2 Table 2 note b): a storage combi whose efficiency is
+    # from the PCDB includes its internal-store loss (56)m, which a combi
+    # (no cylinder) otherwise zeroes. Worcester Greenstar Heatslave II
+    # (pcdb_id 18415, store_type 1 primary, store volume 63 L, insulation
+    # 25 mm) → Table 2 L 0.0240 × Table 2a VF (120/63)^⅓ × Table 2b storage-
+    # combi TF 2.8946 = (55) 5.4175 kWh/day. Pinned to the accredited Elmhurst
+    # P960 worksheet for this boiler ((47) 63, (55) 5.4175, (56) Jan 167.94).
+    from domain.sap10_calculator.rdsap.cert_to_inputs import (
+        _storage_combi_store_loss_override,  # pyright: ignore[reportPrivateUsage]
+    )
+    record = gas_oil_boiler_record(18415)
+    assert record is not None
+    combi_epc = make_minimal_sap10_epc(
+        total_floor_area_m2=_TYPICAL_TFA_M2,
+        country_code="ENG",
+        has_hot_water_cylinder=False,
+        sap_building_parts=[make_building_part(construction_age_band="F")],
+        sap_heating=make_sap_heating(
+            main_heating_details=[
+                MainHeatingDetail(
+                    has_fghrs=False, main_fuel_type=28, heat_emitter_type=1,
+                    emitter_temperature="Unknown", main_heating_control=2106,
+                    main_heating_index_number=18415,
+                ),
+            ],
+        ),
+    )
+
+    # Act
+    storage_56m = _storage_combi_store_loss_override(combi_epc, record)
+
+    # Assert — (56) Jan (31 days) = 5.4175 × 31 = 167.94; annual ≈ 1977 kWh.
+    assert storage_56m is not None
+    assert abs(storage_56m[0] / 31.0 - 5.4175) <= 1e-3
+    assert abs(storage_56m[0] - 167.94) <= 5e-2
+    assert abs(sum(storage_56m) - 1977.4) <= 1.0
+
+
+def test_storage_combi_store_loss_none_without_pcdb_store() -> None:
+    # Arrange — the override must NOT fire for (a) a lodged cylinder (the
+    # cylinder path handles it), (b) a non-storage combi (store_type 0), or
+    # (c) a None PCDB record. Each returns None so the cascade keeps its
+    # existing behaviour.
+    from domain.sap10_calculator.rdsap.cert_to_inputs import (
+        _storage_combi_store_loss_override,  # pyright: ignore[reportPrivateUsage]
+    )
+    storage_record = gas_oil_boiler_record(18415)
+    non_storage_record = gas_oil_boiler_record(18204)  # store_type 0
+    combi_epc = make_minimal_sap10_epc(
+        total_floor_area_m2=_TYPICAL_TFA_M2, country_code="ENG",
+        has_hot_water_cylinder=False,
+        sap_building_parts=[make_building_part(construction_age_band="F")],
+        sap_heating=make_sap_heating(main_heating_details=[]),
+    )
+    cyl_epc = make_minimal_sap10_epc(
+        total_floor_area_m2=_TYPICAL_TFA_M2, country_code="ENG",
+        has_hot_water_cylinder=True,
+        sap_building_parts=[make_building_part(construction_age_band="F")],
+        sap_heating=make_sap_heating(main_heating_details=[]),
+    )
+
+    # Act / Assert
+    assert _storage_combi_store_loss_override(cyl_epc, storage_record) is None
+    assert _storage_combi_store_loss_override(combi_epc, non_storage_record) is None
+    assert _storage_combi_store_loss_override(combi_epc, None) is None
+
+
 def test_cylinder_size_inaccessible_code_5_solid_fuel_boiler_uses_160l() -> None:
     # Arrange — RdSAP 10 §10.5 Table 28: an "Inaccessible" cylinder (code 5)
     # heated "from a solid fuel boiler" uses 160 litres.
@@ -6721,6 +6789,8 @@ def test_pcdb_combi_loss_override_returns_none_or_raises_for_untested_or_storage
         final_year_of_manufacture=None,
         subsidiary_type=0,
         store_type=0,
+        store_boiler_volume_l=None,
+        store_insulation_thickness_mm=None,
         separate_dhw_tests=2,
         rejected_energy_proportion_r1=0.015,
         loss_factor_f1_kwh_per_day=0.5,
diff --git a/tests/domain/sap10_calculator/test_pcdb_lookup.py b/tests/domain/sap10_calculator/test_pcdb_lookup.py
index e2d289595..ba78a2567 100644
--- a/tests/domain/sap10_calculator/test_pcdb_lookup.py
+++ b/tests/domain/sap10_calculator/test_pcdb_lookup.py
@@ -30,6 +30,38 @@ def test_gas_oil_boiler_record_returns_verified_baxi_98() -> None:
     assert record.summer_efficiency_pct == 56.0
 
 
+def test_gas_oil_boiler_record_surfaces_storage_combi_store_fields() -> None:
+    """Worcester GREENSTAR HEATSLAVE II (pcdb_id 18415) is an oil storage
+    combi: PCDF field 42 (store boiler volume) = 63 L, field 44 (store
+    insulation thickness) = 25 mm. These drive the SAP 10.2 Table 2 store
+    loss (note b: included when combi efficiency is from the PCDB). Verified
+    against the accredited Elmhurst P960 worksheet for this boiler (store
+    volume 63 L, loss factor 0.0240 = factory-insulated 25 mm)."""
+    # Arrange
+    # Act
+    record = gas_oil_boiler_record(18415)
+
+    # Assert
+    assert record is not None
+    assert record.store_type == 1  # primary water store
+    assert record.store_boiler_volume_l == 63.0
+    assert record.store_insulation_thickness_mm == 25.0
+
+
+def test_gas_oil_boiler_record_store_fields_none_for_non_storage_combi() -> None:
+    """A non-storage combi (store_type 0) leaves the store volume /
+    insulation fields blank → None, so the store-loss path is not triggered."""
+    # Arrange
+    # Act
+    record = gas_oil_boiler_record(18204)
+
+    # Assert
+    assert record is not None
+    assert record.store_type == 0
+    assert record.store_boiler_volume_l is None
+    assert record.store_insulation_thickness_mm is None
+
+
 def test_gas_oil_boiler_record_returns_none_for_unknown_pcdb_id() -> None:
     """`main_heating_index_number` values not in Table 105 return None so
     `cert_to_inputs` can fall back to the Table 4a/4b category default."""
diff --git a/tests/domain/sap10_calculator/worksheet/test_water_heating.py b/tests/domain/sap10_calculator/worksheet/test_water_heating.py
index c7a2db79a..3360cd4b2 100644
--- a/tests/domain/sap10_calculator/worksheet/test_water_heating.py
+++ b/tests/domain/sap10_calculator/worksheet/test_water_heating.py
@@ -676,6 +676,34 @@ def test_water_efficiency_monthly_via_equation_d1_weights_winter_summer_per_mont
     assert monthly[0] == pytest.approx(num / denom, abs=1e-6)
 
 
+def test_storage_combi_temperature_factor_table_2b_primary_store() -> None:
+    # Arrange — SAP 10.2 Table 2b (PDF p.159), "Storage combi boiler,
+    # primary store", loss-from-Table-2 column:
+    #   Vc >= 115 L → 2.54
+    #   Vc  < 115 L → 2.54 + 0.00682 × (115 − Vc)
+    # Verified against the accredited Elmhurst P960 worksheet for pcdb_id
+    # 18415 (Worcester Greenstar Heatslave II): Vc 63 L → 2.8946.
+    from domain.sap10_calculator.worksheet.water_heating import (
+        storage_combi_temperature_factor_table_2b,
+    )
+
+    # Act / Assert — Vc 63 L primary store reproduces the worksheet's (53).
+    assert abs(
+        storage_combi_temperature_factor_table_2b(store_type=1, volume_l=63.0)
+        - 2.8946
+    ) <= 1e-4
+    # Vc >= 115 L flattens to the 2.54 floor.
+    assert abs(
+        storage_combi_temperature_factor_table_2b(store_type=1, volume_l=150.0)
+        - 2.54
+    ) <= 1e-9
+    # Secondary store (store_type 2): 1.86 + 0.00496 × (115 − Vc).
+    assert abs(
+        storage_combi_temperature_factor_table_2b(store_type=2, volume_l=63.0)
+        - (1.86 + 0.00496 * 52)
+    ) <= 1e-9
+
+
 def test_cylinder_storage_loss_uses_declared_loss_factor_times_temp_factor() -> None:
     # Arrange — SAP 10.2 §4 branch a) (PDF p.136): when the manufacturer's
     # declared cylinder loss factor (kWh/day) is lodged, storage loss