fix(water-heating): derive combi keep-hot from the PCDB record, default no-keep-hot

SAP 10.2 Table 3a (PDF p.160) additional combi loss (61)m. Two coupled
defects, both surfaced by simulated case 49 (000565 + gas combi, U985
"Combi keep hot type = None") sitting at SAP 71.43 vs the worksheet's 72:

1. The cascade defaulted EVERY non-PCDB combi to the flat keep-hot
   time-clock row (600 × n/365). A combi WITHOUT a keep-hot facility uses
   row 1 (600 × fu × n/365, fu = V_d/100 when daily HW < 100 L/day) —
   over-counting (61)m for the no-keep-hot cohort. `water_heating_from_
   cert` now defaults to the "without keep-hot" row.

2. `pcdb_combi_loss_override` returned None for keep_hot_facility=1/
   timer=1, leaning on the OLD flat-600 default. So flipping the default
   silently turned 190 corpus PCDB keep-hot-time-clock combis into
   no-keep-hot. Fixed to return the flat keep-hot row EXPLICITLY.

Key insight (the Summary is the input echo; the U985 keep-hot line is a
computed OUTPUT, so it must be derivable): keep-hot rides on the PCDB
boiler record (Table 105 keep_hot_facility/timer), resolved by
`pcdb_combi_loss_override`. A generic SAP-code combi with no PCDB record
(case 49, PCDF ref 0) has no keep-hot by construction → row 1. So the
default is not a guess — it is the spec-correct value for non-PCDB combis.

Worksheet-proven: case 49 → cost £726.696, SAP 72 — matching the
accredited worksheet to the digit (continuous 71.6945 = the worksheet's
own 71.6945). 000516 (keep-hot None) also exact (£860.716, SAP 63);
000490 (PCDB 10328, keep_hot_facility=1/timer=1) keeps its flat-600 via
the PCDB path. Masked until now because every prior combi-loss worksheet
fixture was keep-hot (000490/000474/000480 time-clock) or had V_d >= 100
every month (001431, rows coincide); case 49 is the first no-keep-hot one.

Corpus within-0.5 72.7% -> 73.3%, MAE 0.781 -> 0.774, PE 3.5 -> 3.4;
ratcheted _MAX_SAP_MAE 0.785 -> 0.775, _MAX_PE_PER_M2_MAE 3.6 -> 3.5.

Note: pyright strict type gate not run locally (pyright not installed).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

domain/sap10_calculator/rdsap/cert_to_inputs.py

@@ -200,6 +200,7 @@ from domain.sap10_calculator.worksheet.water_heating import (
     PIPEWORK_INSULATED_UNINSULATED,
     TABLE_J1_TCOLD_FROM_MAINS_C,
     WaterHeatingResult,
+    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,
@@ -5385,10 +5386,11 @@ def pcdb_combi_loss_override(
             )
         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 3: 600 × n_m / 365 (keep-hot, time
+                # clock). Returned EXPLICITLY — the cascade default is now
+                # the "without keep-hot" row, so the keep-hot value must be
+                # delivered here rather than leaned on the default.
+                return combi_loss_monthly_kwh_table_3a_keep_hot_time_clock()
             # 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
@@ -6540,11 +6542,14 @@ def _water_heating_worksheet_and_gains(
         main
     ):
         # SAP 10.2 §4 line 7702 fallback: non-combi main heating → (61)m
-        # = 0. Without this gate the cascade falls through to `combi_
-        # loss_monthly_kwh_table_3a_keep_hot_time_clock()` (600 kWh/yr)
-        # on every cert lacking a PCDB Table 105 boiler record —
-        # including all heat pump certs.
+        # = 0. Without this gate the cascade falls through to the Table 3a
+        # "without keep-hot" default on every cert lacking a PCDB Table
+        # 105 boiler record — including all heat pump certs.
         combi_loss_override = zero_monthly
+    # A non-PCDB combi (override still None here) has no PCDB-declared
+    # keep-hot facility → `water_heating_from_cert` applies the Table 3a
+    # "without keep-hot" row (600 × fu × n/365). Keep-hot rides only on the
+    # PCDB boiler record, resolved above by `pcdb_combi_loss_override`.
     # SAP 10.2 §4 lines 7670-7693 + Tables 2/2a/2b — cylinder storage loss
     # (56)m. Spec p.135 instructs entering 0 in (47) for instantaneous /
     # combi systems, so the override is only built when the cert explicitly

domain/sap10_calculator/worksheet/water_heating.py

@@ -960,7 +960,9 @@ def water_heating_from_cert(
     combi = (
         combi_loss_monthly_kwh_override
         if combi_loss_monthly_kwh_override is not None
-        else combi_loss_monthly_kwh_table_3a_keep_hot_time_clock()
+        else combi_loss_monthly_kwh_table_3a_row_1_no_keep_hot(
+            daily_hot_water_monthly_l_per_day=daily_total,
+        )
     )
     zero12 = (0.0,) * 12
     solar_storage = (

tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py

@@ -3865,6 +3865,38 @@ def test_mvhr_system_values_fall_back_to_table_4g_defaults_without_pcdb_index()
     assert abs(values.in_use_sfp_w_per_l_per_s - 5.0) <= 1e-9
 
 
+def test_pcdb_combi_loss_keep_hot_time_clock_returns_explicit_flat_600() -> None:
+    # Arrange — SAP 10.2 Table 3a (PDF p.160): a PCDB combi declaring a
+    # keep-hot facility on a time clock (keep_hot_facility=1, keep_hot_
+    # timer=1) uses the flat 600 × n/365 row. The keep-hot status rides on
+    # the PCDB boiler record, NOT a separate cert field — a non-PCDB combi
+    # has no keep-hot and falls to the "without keep-hot" 600 × fu default.
+    # pcdb_combi_loss_override must return this row EXPLICITLY (it must not
+    # lean on the cascade default, which is now "without keep-hot").
+    # Vaillant Ecotec Pro (PCDB 10328) is the 000490 boiler.
+    from domain.sap10_calculator.rdsap.cert_to_inputs import pcdb_combi_loss_override
+    from domain.sap10_calculator.tables.pcdb import gas_oil_boiler_record
+    from domain.sap10_calculator.worksheet.water_heating import (
+        combi_loss_monthly_kwh_table_3a_keep_hot_time_clock,
+    )
+
+    record = gas_oil_boiler_record(10328)
+    assert record is not None
+    assert record.keep_hot_facility == 1
+    assert record.keep_hot_timer == 1
+    monthly = (200.0,) * 12  # energy content / daily HW unused for this row
+
+    # Act
+    override = pcdb_combi_loss_override(
+        record,
+        energy_content_monthly_kwh=monthly,
+        daily_hot_water_monthly_l_per_day=monthly,
+    )
+
+    # Assert — explicit flat keep-hot row, not None.
+    assert override == combi_loss_monthly_kwh_table_3a_keep_hot_time_clock()
+
+
 def test_api_mechanical_ventilation_unmapped_code_raises() -> None:
     # Arrange — an out-of-range `mechanical_ventilation` integer is a
     # spec-coverage gap: raise rather than silently default to NATURAL
@@ -5599,16 +5631,15 @@ 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 row 3 (600 × n/365) — whenever the PCDB record lodges
-    keep-hot with a time clock but has insufficient EN 13203 lab data,
-    or sits in a storage / FGHRS row (Table 3b/3c rows 2-5, deferred
-    until a fixture exercises them).
+    """The override gate dispatches the PCDB record's keep-hot fields to
+    the matching Table 3a row, or returns None for a storage / FGHRS row
+    (Table 3b/3c rows 2-5, deferred until a fixture exercises them).
 
-    Per Slice S0380.21: keep_hot_facility ∈ {None, 0} dispatches to
-    Table 3a row 1 (`600 × fu × n/365`), keep_hot_facility=1 + no
-    timer dispatches to row 4 (`900 × n/365`). Only the electric
-    keep-hot variants (keep_hot_facility ∈ {2, 3}) now raise
+    keep_hot_facility ∈ {None, 0} → Table 3a row 1 (`600 × fu × n/365`,
+    without keep-hot); keep_hot_facility=1 + timer=1 → row 3 (`600 ×
+    n/365`, EXPLICIT — the cascade default is now "without keep-hot");
+    keep_hot_facility=1 + no timer → row 4 (`900 × n/365`). Only the
+    electric keep-hot variants (keep_hot_facility ∈ {2, 3}) raise
     `UnresolvedPcdbCombiLoss` — Table 3a Note 2 fuel-split deferred."""
     # Arrange — a minimal record skeleton, mutated per scenario via
     # dataclasses.replace.
@@ -5635,7 +5666,7 @@ def test_pcdb_combi_loss_override_returns_none_or_raises_for_untested_or_storage
         loss_factor_f1_kwh_per_day=0.5,
         loss_factor_f2_kwh_per_day=0.001,
         rejected_factor_f3_per_litre=0.00014,
-        keep_hot_facility=1,  # lodges keep-hot → cascade default 600 is correct
+        keep_hot_facility=1,  # lodges keep-hot, time clock → flat 600 row
         keep_hot_timer=1,
         raw=(),
     )
@@ -5649,17 +5680,19 @@ def test_pcdb_combi_loss_override_returns_none_or_raises_for_untested_or_storage
         )
         is None
     )
-    # separate_dhw_tests=0 + keep_hot_facility=1 + timer=1 → None (no
-    # 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),
-            energy_content_monthly_kwh=energy_content,
-            daily_hot_water_monthly_l_per_day=daily_hw,
-        )
-        is None
+    # separate_dhw_tests=0 + keep_hot_facility=1 + timer=1 → Table 3a row 3
+    # (600 × n/365, keep-hot time clock) returned EXPLICITLY. The cascade
+    # default is now "without keep-hot" (600 × fu), so the keep-hot row
+    # must be delivered here, not leaned on the default.
+    from domain.sap10_calculator.worksheet.water_heating import (
+        combi_loss_monthly_kwh_table_3a_keep_hot_time_clock,
     )
+
+    assert pcdb_combi_loss_override(
+        replace(base, separate_dhw_tests=0),
+        energy_content_monthly_kwh=energy_content,
+        daily_hot_water_monthly_l_per_day=daily_hw,
+    ) == combi_loss_monthly_kwh_table_3a_keep_hot_time_clock()
     # separate_dhw_tests=0 + keep_hot_facility=None → Table 3a row 1
     # (600 × fu × n/365) — Slice S0380.21 dispatch.
     row_1 = pcdb_combi_loss_override(

tests/domain/sap10_calculator/worksheet/_elmhurst_worksheet_000490.py

@@ -42,7 +42,10 @@ from domain.sap10_ml.tests._fixtures import (
 )
 from domain.sap10_calculator.worksheet.solar_gains import RoofWindowInput, RooflightInput
 from domain.sap10_calculator.worksheet.ventilation import MechanicalVentilationKind
-from domain.sap10_calculator.worksheet.water_heating import TABLE_J1_TCOLD_FROM_MAINS_C
+from domain.sap10_calculator.worksheet.water_heating import (
+    TABLE_J1_TCOLD_FROM_MAINS_C,
+    combi_loss_monthly_kwh_table_3a_keep_hot_time_clock,
+)
 
 from tests.domain.sap10_calculator.worksheet._elmhurst_fixtures import (
     SECTION_8C_ALL_ZERO_MONTHLY,
@@ -284,9 +287,12 @@ MIXER_SHOWER_FLOW_RATES_L_PER_MIN: tuple[float, ...] = (7.0,)
 COLD_WATER_TEMPS_C: tuple[float, ...] = TABLE_J1_TCOLD_FROM_MAINS_C
 LOW_WATER_USE: bool = False
 # Vaillant Ecotec Pro combi, "Combi keep hot type = Gas/Oil, time clock":
-# the orchestrator's default (Table 3a row "time-clock keep-hot")
-# reproduces this fixture exactly. No override needed.
-COMBI_LOSS_OVERRIDE: Optional[tuple[float, ...]] = None
+# SAP 10.2 Table 3a flat keep-hot row (600 × n/365). The orchestrator's
+# default is now "without keep-hot" (600 × fu × n/365, the modal lodging),
+# so this keep-hot fixture supplies the override explicitly.
+COMBI_LOSS_OVERRIDE: Optional[tuple[float, ...]] = (
+    combi_loss_monthly_kwh_table_3a_keep_hot_time_clock()
+)
 ELECTRIC_SHOWER_OVERRIDE: Optional[tuple[float, ...]] = None
 
 # §4 Water heating energy requirements

tests/domain/sap10_calculator/worksheet/test_water_heating.py

@@ -970,13 +970,17 @@ def test_water_heating_from_cert_matches_elmhurst_worksheet_000490() -> None:
     # Arrange
     epc = _w000490.build_epc()
 
-    # Act
+    # Act — 000490 lodges "Combi keep hot type = time clock" (U985 §15),
+    # so the (61)m combi loss is the Table 3a flat keep-hot row, not the
+    # "without keep-hot" 600 × fu default that `water_heating_from_cert`
+    # now applies when no override is supplied.
     result = water_heating_from_cert(
         epc=epc,
         mixer_shower_flow_rates_l_per_min=(7.0,),
         has_bath=True,
         cold_water_temps_c=TABLE_J1_TCOLD_FROM_MAINS_C,
         low_water_use=False,
+        combi_loss_monthly_kwh_override=combi_loss_monthly_kwh_table_3a_keep_hot_time_clock(),
     )
 
     # Assert — annual output equals the days-month-weighted sum of (64)m

tests/infrastructure/epc_client/test_sap_accuracy_corpus.py

@@ -215,9 +215,20 @@ _MIN_WITHIN_HALF_SAP = 0.72
 # (7a)=0 is explicit): case 49's (8) openings line is 0.0000 — our default had
 # added 20 m3/h (0.0723 ach), inflating (22b)/(25)/(38) and demand (SAP 70.90
 # -> 71.43; the worksheet's own continuous SAP is 71.69 -> rounds to 72).
-_MAX_SAP_MAE = 0.785
+# Then 0.781 -> 0.774 (within-0.5 72.7% -> 73.3%, PE 3.5 -> 3.4) via the combi
+# keep-hot fix: SAP 10.2 Table 3a (PDF p.160) — keep-hot rides on the PCDB
+# boiler record (Table 105 keep_hot_facility/timer), NOT a separate cert field.
+# (a) A non-PCDB / generic-SAP-code combi has no keep-hot → "without keep-hot"
+# row 600 × fu × n/365 (fu = V_d/100 when V_d < 100), now the cascade default;
+# the cascade had wrongly defaulted every non-PCDB combi to the flat keep-hot
+# 600 × n/365 row. (b) `pcdb_combi_loss_override` now returns the keep-hot row
+# EXPLICITLY for keep_hot_facility=1/timer=1 (was returning None to lean on the
+# old flat default — so flipping the default had silently turned 190 PCDB
+# keep-hot combis into no-keep-hot). Closes case 49 EXACTLY (cost £726.696,
+# SAP 72 = the worksheet to the digit).
+_MAX_SAP_MAE = 0.775
 _MAX_CO2_MAE_TONNES = 0.09      # t CO2 / yr vs co2_emissions_current
-_MAX_PE_PER_M2_MAE = 3.6        # kWh / m2 / yr vs energy_consumption_current
+_MAX_PE_PER_M2_MAE = 3.5        # kWh / m2 / yr vs energy_consumption_current
 
 
 def _load_corpus() -> list[dict[str, Any]]: