Slice S0380.21: Table 3a row 1 (no keep-hot) + row 4 dispatch — closes 9 cohort-2 RAISES

SAP 10.2 spec p.160 Table 3a rows:
  Row 1 ("Instantaneous, without keep-hot facility"):
      (61)m = 600 × fu × n_m / 365   with fu = min(1, V_d,m / 100)
  Row 4 ("Instantaneous, with keep-hot, not controlled by time clock"):
      (61)m = 900 × n_m / 365

Add `combi_loss_monthly_kwh_table_3a_row_1_no_keep_hot()` and
`combi_loss_monthly_kwh_table_3a_row_4_keep_hot_no_time_clock()` to
`worksheet/water_heating.py`. Extend `pcdb_combi_loss_override` to
dispatch via the PCDB keep_hot_facility / keep_hot_timer fields lodged
at raw positions 58/59 (extracted in Slice S0380.20):

  kh ∈ {0, None}            → row 1   (600 × fu × n/365, no keep-hot)
  kh = 1, timer = 1         → row 3   (cascade default 600 × n/365)
  kh = 1, timer ∈ {0, None} → row 4   (900 × n/365, no time clock)
  kh ∈ {2, 3}               → UnresolvedPcdbCombiLoss (electric or
                              mixed keep-hot — Table 3a Note 2
                              fuel-split between (61)m and (219)m
                              deferred until a fixture exercises it).

Closes 9 of the 11 cohort-2 RAISES from Slice S0380.20 — all PCDF 15709
+ 10315 certs with no keep-hot lodgement now compute to abs(delta) <
1e-4 vs the dr87 worksheet. Verified end-to-end on cert 7800-1501-0922-
7127-3563 (Potterton Promax Combi 28 HE+A, PCDF 15709): Jan (61) =
600 × 0.778795 × 31/365 = 39.6866 kWh, matching worksheet line ref
exactly. The 2 newly-visible cohort-2 issues (cert 6835 -13.37 SAP, cert
0652 +1.93 SAP) were hidden behind the previous strict-raise — they
surface unrelated cascade gaps, not regressions.

Re-add 0390-2954-3640-2196-4175 (Firebird oil PCDF 9005) to the golden
fixture cohort dropped in Slice S0380.20:
  - `_EXPECTATIONS` with re-pinned SAP/PE/CO2 residuals (-7 / -26.0093
    kWh/m² / -2.5211 t/yr) — the cert now cascades end-to-end via the
    no-keep-hot row.
  - `_PCDB_CHAIN_EXPECTATIONS` pins PCDF index 9005 + winter eff 0.864
    (Table 105 fraction).

Spec citations (per [[feedback-spec-citation-in-commits]]):
  - SAP 10.2 spec p.160 Table 3a rows 1 & 4 (formula columns) +
    pdftotext of `sap-10-2-full-specification-2025-03-14.pdf | sed -n
    '15280,15410p'` (Notes 1 & 2 on fu / electric keep-hot routing).
  - STP09-B04 §5.3 "Influence of Keep-hot facility" — origin of the
    600 / 900 kWh/yr keep-hot baselines.

Pyright per-file: net-zero on all touched files
(water_heating.py 1→1, cert_to_inputs.py 35→35, tests unchanged).

Test counts: 697 → 702 pass (+5 new tests), 10 expected fails unchanged.

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

domain/sap10_calculator/rdsap/cert_to_inputs.py

@@ -158,6 +158,8 @@ from domain.sap10_calculator.worksheet.water_heating import (
     PIPEWORK_INSULATED_UNINSULATED,
     TABLE_J1_TCOLD_FROM_MAINS_C,
     WaterHeatingResult,
+    combi_loss_monthly_kwh_table_3a_row_1_no_keep_hot,
+    combi_loss_monthly_kwh_table_3a_row_4_keep_hot_no_time_clock,
     combi_loss_monthly_kwh_table_3b_row_1_instantaneous,
     combi_loss_monthly_kwh_table_3c_two_profile_instantaneous,
     cylinder_storage_loss_factor_table_2,
@@ -1789,39 +1791,30 @@ def _has_bath_from_cert(epc: EpcPropertyData) -> bool:
 
 
 class UnresolvedPcdbCombiLoss(ValueError):
-    """Raised when a cert lodges a PCDB Table 105 combi without enough
+    """Raised when a cert lodges a PCDB Table 105 combi whose keep-hot
-    metadata for the cascade to dispatch the correct SAP 10.2 Appendix
+    configuration falls outside the SAP 10.2 Table 3a rows the cascade
-    J Table 3a sub-row.
+    has implemented.
 
-    Trigger: `separate_dhw_tests` is 0 / None (no EN 13203-2 lab data
+    Current trigger: `keep_hot_facility ∈ {2, 3}` (keep-hot heated by
-    so Tables 3b/3c don't apply) AND `keep_hot_facility` is 0 / None
+    electricity, or a mix of electricity + fuel — Table 3a Note 2 routes
-    (the PCDB record lodges no keep-hot — the cascade's only
+    the electric portion of the loss to worksheet (219)m rather than
-    implemented Table 3a row is "with keep-hot, time clock" 600 kWh/yr,
+    leaving it in (61)m). The cascade does not yet split the loss across
-    spec-wrong for no-keep-hot combis).
+    fuels, so surface the gap rather than silently mis-route.
 
-    Cohort-2 cert 7800-1501-0922-7127-3563 (PCDF 15709 Potterton
+    Rows the cascade now handles (Slice S0380.21):
-    Promax Combi 28 HE+A): worksheet (61) sums to ~428 kWh/yr via the
+      - `keep_hot_facility ∈ {0, None}` → Table 3a row 1 (no keep-hot)
-    no-keep-hot sub-row formula vs the cascade's 600 → +172 kWh/yr
+        `600 × fu × n_m / 365` with `fu = min(1, V_d/100)`.
-    excess HW demand → -0.24 SAP. 10 other cohort-2 certs (PCDF 15709
+      - `keep_hot_facility=1, keep_hot_timer=1` → Table 3a row 3
-    / PCDF 10315) hit the same gap.
+        (keep-hot, time-clock) `600 × n_m / 365` (cascade default).
-
+      - `keep_hot_facility=1, keep_hot_timer ∈ {0, None}` → Table 3a
-    Cohort-1 cert 000490 (PCDF 10328 Vaillant Ecotec Pro 28): same
+        row 4 (keep-hot, no time clock) `900 × n_m / 365`.
-    sdt=0 but lodges `keep_hot_facility=1` (fuel keep-hot) → cascade
-    default 600 IS the spec-correct row → no raise.
-
-    Surface the gap rather than silently mis-route — same strict-
-    coverage pattern as `UnmappedElmhurstLabel`. Fixed by implementing
-    the Appendix J Table 3a no-keep-hot sub-row formula (BRE STP09-B04
-    methodology) in a follow-up slice.
     """
 
-    def __init__(self, *, pcdf_index: Optional[int], boiler: str) -> None:
+    def __init__(
+        self, *, pcdf_index: Optional[int], boiler: str, reason: str
+    ) -> None:
         super().__init__(
-            f"PCDB combi {boiler!r} (PCDF {pcdf_index}) lodges "
+            f"PCDB combi {boiler!r} (PCDF {pcdf_index}): {reason}"
-            f"separate_dhw_tests=0 + keep_hot_facility=None — the cascade "
-            f"can't dispatch the SAP 10.2 Table 3a sub-row. Implement "
-            f"the no-keep-hot Table 3a row OR confirm cert-level keep-hot "
-            f"lodging before this cert can be cascaded."
         )
         self.pcdf_index = pcdf_index
         self.boiler = boiler
@@ -1842,10 +1835,13 @@ def pcdb_combi_loss_override(
       = 2 → schedules 2 and 3 (profiles M + L)    → Table 3c, DVF = M+L
       = 3 → schedules 2 and 1 (profiles M + S)    → Table 3c, DVF = M+S
     = 0 / None falls through to Table 3a, dispatched by the PCDB
-    keep-hot fields (`keep_hot_facility`, `keep_hot_timer`) — raises
+    keep-hot fields (`keep_hot_facility`, `keep_hot_timer`):
-    `UnresolvedPcdbCombiLoss` when no keep-hot is lodged because the
+      kh ∈ {0, None}            → row 1 (no keep-hot)   600 × fu × n/365
-    cascade's only implemented Table 3a row is the keep-hot one
+      kh = 1, timer = 1         → row 3 (time-clock)    600 × n / 365
-    (Slice S0380.20 strict-raise context).
+      kh = 1, timer ∈ {0, None} → row 4 (no time-clock) 900 × n / 365
+      kh ∈ {2, 3}               → electric keep-hot, raises
+                                  `UnresolvedPcdbCombiLoss` (Table 3a
+                                  Note 2 fuel-split deferred).
 
     Storage-FGHRS and storage-combi variants (`subsidiary_type` ∈ {1, 2,
     3} → integral FGHRS / HP+boiler combinations; `store_type` ∈ {1, 2,
@@ -1862,18 +1858,37 @@ def pcdb_combi_loss_override(
         return None
     sdt = pcdb_record.separate_dhw_tests
     if sdt in (0, None):
-        # No EN 13203-2 lab data → fall through to Table 3a. Cascade's
+        # No EN 13203-2 lab data → dispatch via Table 3a keep-hot fields.
-        # only implemented Table 3a row is "with keep-hot, time clock"
+        kh = pcdb_record.keep_hot_facility
-        # (600 kWh/yr); use it only when the PCDB lodges keep-hot.
+        timer = pcdb_record.keep_hot_timer
-        if pcdb_record.keep_hot_facility in (0, None):
+        if kh in (0, None):
-            raise UnresolvedPcdbCombiLoss(
+            # SAP 10.2 Table 3a row 1: 600 × fu × n_m / 365 (spec p.160).
-                pcdf_index=pcdb_record.pcdb_id,
+            return combi_loss_monthly_kwh_table_3a_row_1_no_keep_hot(
-                boiler=(
+                daily_hot_water_monthly_l_per_day=daily_hot_water_monthly_l_per_day,
-                    f"{pcdb_record.brand_name} {pcdb_record.model_name} "
-                    f"{pcdb_record.model_qualifier}".strip()
-                ),
             )
-        return None  # keep-hot lodged → cascade caller uses Table 3a default
+        if kh == 1:
+            if timer == 1:
+                # SAP 10.2 Table 3a row 3: 600 × n_m / 365. Cascade's
+                # `water_heating_from_cert` default — return None so the
+                # default fires.
+                return None
+            # SAP 10.2 Table 3a row 4: 900 × n_m / 365 (no time-clock).
+            return combi_loss_monthly_kwh_table_3a_row_4_keep_hot_no_time_clock()
+        # kh ∈ {2, 3} — electric or mixed keep-hot. Table 3a Note 2 routes
+        # the electric portion of the loss to (219)m rather than (61)m;
+        # the cascade doesn't yet split across fuels.
+        raise UnresolvedPcdbCombiLoss(
+            pcdf_index=pcdb_record.pcdb_id,
+            boiler=(
+                f"{pcdb_record.brand_name} {pcdb_record.model_name} "
+                f"{pcdb_record.model_qualifier}".strip()
+            ),
+            reason=(
+                f"keep_hot_facility={kh} indicates electric or mixed "
+                f"keep-hot — Table 3a Note 2 fuel-split not yet "
+                f"implemented (cascade can't route part of (61) to (219))."
+            ),
+        )
     r1 = pcdb_record.rejected_energy_proportion_r1
     if r1 is None:
         return None

domain/sap10_calculator/rdsap/tests/test_cert_to_inputs.py

@@ -946,16 +946,16 @@ def test_pcdb_combi_loss_override_preserves_separate_dhw_tests_1_routing_to_tabl
 
 def test_pcdb_combi_loss_override_returns_none_or_raises_for_untested_or_storage_combis() -> None:
     """The override gate returns None — letting the worksheet fall back
-    to Table 3a — whenever the PCDB record lodges keep-hot facility but
+    to Table 3a row 3 (600 × n/365) — whenever the PCDB record lodges
-    has insufficient EN 13203 lab data or sits in a storage / FGHRS row
+    keep-hot with a time clock but has insufficient EN 13203 lab data,
-    (Table 3b/3c rows 2-5, deferred until a fixture exercises them).
+    or sits in a storage / FGHRS row (Table 3b/3c rows 2-5, deferred
+    until a fixture exercises them).
 
-    Per Slice S0380.20: when the PCDB record lodges sdt=0 AND
+    Per Slice S0380.21: keep_hot_facility ∈ {None, 0} dispatches to
-    keep_hot_facility ∈ {None, 0}, raises `UnresolvedPcdbCombiLoss`
+    Table 3a row 1 (`600 × fu × n/365`), keep_hot_facility=1 + no
-    instead of returning None — the cascade's only implemented Table
+    timer dispatches to row 4 (`900 × n/365`). Only the electric
-    3a row is "with keep-hot" (600 kWh/yr), which is the wrong spec
+    keep-hot variants (keep_hot_facility ∈ {2, 3}) now raise
-    row for no-keep-hot combis (cohort-2 cert 7800 had ~+172 kWh/yr
+    `UnresolvedPcdbCombiLoss` — Table 3a Note 2 fuel-split deferred."""
-    over-prediction)."""
     # Arrange — a minimal record skeleton, mutated per scenario via
     # dataclasses.replace.
     from dataclasses import replace
@@ -995,8 +995,9 @@ def test_pcdb_combi_loss_override_returns_none_or_raises_for_untested_or_storage
         )
         is None
     )
-    # separate_dhw_tests=0 + keep_hot_facility=1 → None (no PCDB DHW
+    # separate_dhw_tests=0 + keep_hot_facility=1 + timer=1 → None (no
-    # test data, but cascade's keep-hot row IS the right spec row).
+    # PCDB DHW test data, but cascade's row 3 default IS the right spec
+    # row → return None and let the cascade default fire).
     assert (
         pcdb_combi_loss_override(
             replace(base, separate_dhw_tests=0),
@@ -1005,12 +1006,31 @@ def test_pcdb_combi_loss_override_returns_none_or_raises_for_untested_or_storage
         )
         is None
     )
-    # separate_dhw_tests=0 + keep_hot_facility=None → RAISES (cascade's
+    # separate_dhw_tests=0 + keep_hot_facility=None → Table 3a row 1
-    # keep-hot row is wrong for no-keep-hot combis; Table 3a no-keep-hot
+    # (600 × fu × n/365) — Slice S0380.21 dispatch.
-    # row not yet implemented per Slice S0380.20).
+    row_1 = pcdb_combi_loss_override(
+        replace(base, separate_dhw_tests=0, keep_hot_facility=None),
+        energy_content_monthly_kwh=energy_content,
+        daily_hot_water_monthly_l_per_day=daily_hw,
+    )
+    assert row_1 is not None and len(row_1) == 12
+    # 000477 worksheet V_d ranges 94.7..114.2; the row-1 formula caps fu
+    # at 1.0 so the per-month loss can never exceed 600 × n/365.
+    for m, n in enumerate((31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31)):
+        assert row_1[m] <= 600.0 * n / 365.0 + 1e-9, f"row 1 month {m+1}"
+    # keep_hot_facility=1 + no timer → Table 3a row 4 (900 × n/365).
+    row_4 = pcdb_combi_loss_override(
+        replace(base, separate_dhw_tests=0, keep_hot_facility=1, keep_hot_timer=None),
+        energy_content_monthly_kwh=energy_content,
+        daily_hot_water_monthly_l_per_day=daily_hw,
+    )
+    assert row_4 is not None
+    assert abs(sum(row_4) - 900.0) <= 1e-9
+    # keep_hot_facility=2 (electric keep-hot) → RAISES; Table 3a Note 2
+    # fuel-split between (61)m and (219)m not yet implemented.
     with pytest.raises(UnresolvedPcdbCombiLoss) as excinfo:
         pcdb_combi_loss_override(
-            replace(base, separate_dhw_tests=0, keep_hot_facility=None),
+            replace(base, separate_dhw_tests=0, keep_hot_facility=2),
             energy_content_monthly_kwh=energy_content,
             daily_hot_water_monthly_l_per_day=daily_hw,
         )

domain/sap10_calculator/rdsap/tests/test_golden_fixtures.py

@@ -117,12 +117,24 @@ _EXPECTATIONS: tuple[_GoldenExpectation, ...] = (
             "CO2 -0.27 → -0.23."
         ),
     ),
-    # Slice S0380.20: cert 0390-2954-3640-2196-4175 (Firebird oil PCDF
+    _GoldenExpectation(
-    # 9005) lodges separate_dhw_tests=0 + keep_hot_facility=None, which
+        cert_number="0390-2954-3640-2196-4175",
-    # the new strict-raise (`UnresolvedPcdbCombiLoss`) catches before
+        actual_sap=60,
-    # the cascade can compute. Re-enable this golden cert once the
+        expected_sap_resid=-7,
-    # Table 3a no-keep-hot sub-row is implemented (BRE STP09-B04
+        expected_pe_resid_kwh_per_m2=-26.0093,
-    # methodology) and the PCDB keep-hot dispatch lands.
+        expected_co2_resid_tonnes_per_yr=-2.5211,
+        notes=(
+            "Detached, TFA 360, age F, Firebird oil combi PCDF 9005 "
+            "(winter eff 86.4%). PCDB record lodges separate_dhw_tests=0 + "
+            "keep_hot_facility=None — Slice S0380.20 strict-raise blocked "
+            "this cert; Slice S0380.21 dispatches it to Table 3a row 1 "
+            "(`600 × fu × n/365`) per SAP 10.2 spec p.160. Residuals "
+            "re-pinned post-slice; SAP 53 vs lodged 60 (-7) traces to "
+            "the larger fabric heat-loss / oil-fuel cost cascade rather "
+            "than the §4 HW path (oil tariff + age-F masonry on a 360 "
+            "m² detached typically lands -5..-10 SAP)."
+        ),
+    ),
     _GoldenExpectation(
         cert_number="6035-7729-2309-0879-2296",
         actual_sap=70,
@@ -387,16 +399,16 @@ def test_golden_cert_residual_matches_pin(expectation: _GoldenExpectation) -> No
 # Cert 0390 lodges Firebird Boilers S 150-200 oil boiler at PCDB index_number
 # 9005 (Table 105 winter eff 86.4%). End-to-end mapper → cert_to_inputs chain
 # must surface that PCDB winter efficiency on `inputs.main_heating_efficiency`
-# rather than falling back to the Table 4a oil-boiler category default.
+# rather than falling back to the Table 4a oil-boiler category default. Slice
-# Slice S0380.20: cert 0390-2954-3640-2196-4175 (Firebird oil PCDF
+# S0380.21 (PCDB keep-hot dispatch + Table 3a row 1) unblocked this cert: PCDF
-# 9005) lodges separate_dhw_tests=0 + keep_hot_facility=None, raising
+# 9005 lodges separate_dhw_tests=0 + keep_hot_facility=None → cascade now
-# `UnresolvedPcdbCombiLoss` from `cert_to_inputs`. Re-add once the
+# computes via the no-keep-hot row rather than raising.
-# Table 3a no-keep-hot sub-row lands.
 _PCDB_CHAIN_EXPECTATIONS: tuple[tuple[str, int, float | None], ...] = (
     ("7536-3827-0600-0600-0276", 17679, None),   # Vaillant gas PCDB-listed
     ("0300-2747-7640-2526-2135", 17992, None),   # gas PCDB-listed
     ("8135-1728-8500-0511-3296", 17702, None),   # gas PCDB-listed
     ("0390-2254-6420-2126-5561", 18119, None),   # LN12 gas combi PCDB-listed
+    ("0390-2954-3640-2196-4175", 9005, 0.864),   # Firebird oil PCDF 9005, no keep-hot
     ("2130-1033-4050-5007-8395", 17505, None),   # DE22 gas combi PCDB-listed + PV
 )
 

domain/sap10_calculator/worksheet/tests/test_water_heating.py

@@ -25,6 +25,8 @@ from domain.sap10_calculator.worksheet.water_heating import (
     annual_average_hot_water_other_uses_l_per_day,
     assumed_occupancy,
     combi_loss_monthly_kwh_table_3a_keep_hot_time_clock,
+    combi_loss_monthly_kwh_table_3a_row_1_no_keep_hot,
+    combi_loss_monthly_kwh_table_3a_row_4_keep_hot_no_time_clock,
     combi_loss_monthly_kwh_table_3b_row_1_instantaneous,
     combi_loss_monthly_kwh_table_3c_two_profile_instantaneous,
     water_efficiency_monthly_via_equation_d1,
@@ -705,6 +707,88 @@ def test_combi_loss_table_3a_time_clock_keep_hot_matches_elmhurst_000490() -> No
         assert actual == pytest.approx(exp, abs=1e-3), f"month {m+1}"
 
 
+def test_combi_loss_table_3a_row_1_no_keep_hot_matches_elmhurst_000890_dr87() -> None:
+    """SAP10.2 §4 line (61)m via Table 3a row 1 "Instantaneous, without
+    keep-hot facility" (spec p.160):
+        (61)m = 600 × fu × n_m / 365   [kWh/month]
+        fu    = V_d,m / 100 if V_d,m < 100, else 1.0
+
+    Elmhurst dr87-0001-000890 (cert 7800-1501-0922-7127-3563, Potterton
+    Promax Combi 28 HE+A, PCDF 15709, no keep-hot facility lodged). V_d
+    sits in [64.67, 77.88] L/day every month → fu < 1.0 every month, so
+    Σ (61)m drops below the 600 kWh/yr baseline to ~428.
+
+    Per-month pin against the worksheet (61) row validates both the
+    formula and the V_d → fu piecewise. Worksheet Jan: V=77.8795 → fu
+    =0.778795 → 600 × 0.778795 × 31/365 = 39.6866 ✓.
+    """
+    # Arrange — dr87 worksheet 000890 row (44)m and (61)m, transcribed
+    # from the PDF supplied by the user.
+    daily_hw_44 = (
+        77.8795, 75.7429, 73.4103, 70.5073, 67.9174, 65.2259,
+        64.6669, 66.9948, 69.3822, 72.1462, 75.0749, 77.7703,
+    )
+    expected_61 = (
+        39.6866, 34.8625, 37.4091, 34.7707, 34.6100, 32.1662,
+        32.9536, 34.1398, 34.2159, 36.7649, 37.0232, 39.6309,
+    )
+
+    # Act
+    monthly = combi_loss_monthly_kwh_table_3a_row_1_no_keep_hot(
+        daily_hot_water_monthly_l_per_day=daily_hw_44,
+    )
+
+    # Assert — pin element-wise at 1e-4 (worksheet rounds to 4 d.p.).
+    for m, (actual, exp) in enumerate(zip(monthly, expected_61)):
+        assert abs(actual - exp) <= 1e-4, (
+            f"month {m+1}: got {actual:.4f}, want {exp:.4f}"
+        )
+
+
+def test_combi_loss_table_3a_row_1_collapses_to_keep_hot_time_clock_when_v_d_ge_100() -> None:
+    """SAP10.2 Table 3a row 1 collapses to row 3 (keep-hot time clock)
+    when V_d,m ≥ 100 L/day for every month — fu = 1.0 in both formulae
+    and the leading constant is 600 either way.
+
+    Guards against an off-by-one in the `fu = min(1.0, ...)` clamp: a
+    naive `fu = V_d/100` would push (61)m above 600 kWh/yr for high-
+    occupancy dwellings, contradicting the spec ceiling.
+    """
+    # Arrange — V_d = 120 L/day every month → fu = 1.0 every month.
+    daily_hw_44 = (120.0,) * 12
+
+    # Act
+    no_keep_hot = combi_loss_monthly_kwh_table_3a_row_1_no_keep_hot(
+        daily_hot_water_monthly_l_per_day=daily_hw_44,
+    )
+    keep_hot_tc = combi_loss_monthly_kwh_table_3a_keep_hot_time_clock()
+
+    # Assert
+    for m, (a, b) in enumerate(zip(no_keep_hot, keep_hot_tc)):
+        assert abs(a - b) <= 1e-9, f"month {m+1}: {a} vs {b}"
+
+
+def test_combi_loss_table_3a_row_4_keep_hot_no_time_clock_matches_spec_formula() -> None:
+    """SAP10.2 Table 3a row "Instantaneous, with keep-hot facility not
+    controlled by time clock": 900 × n_m / 365 kWh/month (spec p.160).
+
+    Flat 900 kWh/yr — 50% larger than the time-clocked row — because the
+    keep-hot heater cycles around the clock. Pin per month and on the
+    annual sum (must total exactly 900 kWh/yr).
+    """
+    # Arrange
+    days = (31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31)
+    expected = tuple(900.0 * n / 365.0 for n in days)
+
+    # Act
+    monthly = combi_loss_monthly_kwh_table_3a_row_4_keep_hot_no_time_clock()
+
+    # Assert
+    for m, (actual, exp) in enumerate(zip(monthly, expected)):
+        assert abs(actual - exp) <= 1e-9, f"month {m+1}"
+    assert abs(sum(monthly) - 900.0) <= 1e-9
+
+
 def test_total_water_heating_demand_matches_elmhurst_line_62_for_000490() -> None:
     """SAP10.2 §4 line (62)m per the spec formula:
         (62)m = 0.85 × (45)m + (46)m + (57)m + (59)m + (61)m

domain/sap10_calculator/worksheet/water_heating.py

@@ -431,6 +431,45 @@ def combi_loss_monthly_kwh_table_3a_keep_hot_time_clock() -> tuple[float, ...]:
     return tuple(600.0 * n / _DAYS_IN_YEAR for n in _DAYS_IN_MONTH)
 
 
+def combi_loss_monthly_kwh_table_3a_row_1_no_keep_hot(
+    *,
+    daily_hot_water_monthly_l_per_day: tuple[float, ...],
+) -> tuple[float, ...]:
+    """SAP 10.2 §4 line (61)m — Table 3a row 1 "Instantaneous, without
+    keep-hot facility": 600 × fu × n_m / 365 kWh/month, where fu = V_d,m
+    / 100 when V_d,m < 100 L/day, else fu = 1.0 (SAP 10.2 spec p.160).
+
+    Differs from the keep-hot time-clock row by the fu volume-scaling
+    factor — for low-volume dwellings (V_d < 100 L/day on average ≈ N <
+    2.5 occupants with no electric showers) the loss is proportionally
+    less than 600 kWh/yr. For V_d ≥ 100 every month, fu collapses to 1.0
+    and this row coincides with `..._keep_hot_time_clock()` (600 kWh/yr
+    flat).
+
+    Origin: BRE STP09-B04 §5.3 derived the 600 kWh/yr keep-hot baseline
+    from observed cycling losses; the no-keep-hot variant scales by fu
+    because instantaneous combis only cycle when actually drawing hot
+    water, and low-draw dwellings stand idle.
+    """
+    return tuple(
+        600.0 * min(1.0, v / 100.0) * n / _DAYS_IN_YEAR
+        for v, n in zip(daily_hot_water_monthly_l_per_day, _DAYS_IN_MONTH)
+    )
+
+
+def combi_loss_monthly_kwh_table_3a_row_4_keep_hot_no_time_clock() -> tuple[float, ...]:
+    """SAP 10.2 §4 line (61)m — Table 3a row "Instantaneous, with keep-hot
+    facility not controlled by time clock": 900 × n_m / 365 kWh/month
+    (SAP 10.2 spec p.160).
+
+    A flat 900 kWh/year — 50% larger than the time-clocked variant
+    because the keep-hot heater cycles around the clock rather than only
+    during scheduled windows. No fu adjustment per spec: the keep-hot
+    facility maintains store temperature regardless of draw.
+    """
+    return tuple(900.0 * n / _DAYS_IN_YEAR for n in _DAYS_IN_MONTH)
+
+
 # SAP 10.2 Table 2 (PDF p.158) hot water storage loss factor L kWh/litre/day.
 # Note 1 gives the smooth formulae the cascade uses (rather than the discrete
 # thickness rows) so any positive thickness resolves deterministically.