fix(water-heating): assume cylinder thermostat present for electric/immersion/heat-network DHW (SAP 9.4.9)

SAP 10.2 §9.4.9 (PDF p.32) verbatim: "A cylinder thermostat should be
assumed to be present when the domestic hot water is obtained from a heat
network, an immersion heater, a thermal store, a combi boiler or a CPSU."
RdSAP 10 Table 29 (p.56) points the no-access default at this rule.

The storage-loss Table 2b temperature factor previously read only the
lodged `cylinder_thermostat` ("Y") — so an unlodged thermostat always took
the ×1.3 absent-penalty, over-stating storage loss by 30%. New
`_cylinder_thermostat_present` assumes it present when DHW is from a heat
network, WHC 903 (immersion), or a direct-acting electric boiler (SAP code
191 — electric-resistance, immersion-equivalent).

Found via the worksheet-folder harness: cert 2474-3059-4202-4496-3200
(Summary path: WHC 901, main SAP 191, electric, no lodged cylinder stat)
diverged −1.86 from its dr87 worksheet. The worksheet lodges (53)
temperature factor 0.6000 (present) and "add cylinder thermostat (SAP
increase too small)" — already assumed present. Fix lands HW output (64)
2701.99 → 2323.88, EXACT to the worksheet; 2474 −1.86 → −0.87 (residual is
a separate space-demand fabric thread). No other worksheet in the 47-cert
harness moved.

API eval within-0.5 56.9% → 57.6%; mean|err| 1.197 → 1.185; signed
−0.202 → −0.165. Regression green (only pre-existing fails); goldens +
heating corpus unaffected.

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

domain/sap10_calculator/rdsap/cert_to_inputs.py

@@ -6174,6 +6174,42 @@ def _primary_loss_override(
     return base
 
 
+def _cylinder_thermostat_present(
+    epc: EpcPropertyData, main: Optional[MainHeatingDetail],
+) -> bool:
+    """Whether a cylinder thermostat is present for the Table 2b temperature
+    factor (absent → ×1.3 penalty on the storage loss).
+
+    A lodged "Y" wins. Otherwise SAP 10.2 §9.4.9 (PDF p.32) verbatim: "A
+    cylinder thermostat should be assumed to be present when the domestic
+    hot water is obtained from a heat network, an immersion heater, a
+    thermal store, a combi boiler or a CPSU." RdSAP 10 Table 29 (PDF p.56)
+    points the no-access default at this rule. So a cylinder heated by an
+    immersion (WHC 903), a direct-acting electric boiler (SAP code 191 —
+    electric-resistance, immersion-equivalent), or a heat network gets the
+    base Table 2b factor (no absent-thermostat ×1.3).
+
+    Cert 2474 (Summary path: WHC 901, main SAP 191, electric, no lodged
+    cylinder thermostat) is the case: the dr87 worksheet lodges (53)
+    temperature factor 0.6000 (thermostat present), and the "add cylinder
+    thermostat" recommendation reads "SAP increase too small" because it
+    is already assumed present. Without this the cascade applied ×1.3 and
+    over-stated storage loss by ~378 kWh/yr (SAP −1.86)."""
+    if epc.sap_heating.cylinder_thermostat == "Y":
+        return True
+    dhw_main = _water_heating_main(epc)
+    if _is_heat_network_main(dhw_main):
+        return True
+    if epc.sap_heating.water_heating_code == _WHC_ELECTRIC_IMMERSION:
+        return True
+    if (
+        dhw_main is not None
+        and dhw_main.sap_main_heating_code == _DIRECT_ACTING_ELECTRIC_BOILER_CODE
+    ):
+        return True
+    return False
+
+
 def _cylinder_storage_loss_override(
     epc: EpcPropertyData,
     main: Optional[MainHeatingDetail],
@@ -6217,7 +6253,7 @@ def _cylinder_storage_loss_override(
         volume_l=volume_l,
         insulation_type=insulation_label,
         thickness_mm=float(thickness_mm),
-        has_cylinder_thermostat=sh.cylinder_thermostat == "Y",
+        has_cylinder_thermostat=_cylinder_thermostat_present(epc, main),
         # SAP 10.2 Table 2b note b (PDF p.159) verbatim restricts the
         # ×0.9 multiplier to boiler / warm-air / heat-pump systems —
         # community heating excluded. Gate via the dedicated helper so

tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py

@@ -49,6 +49,7 @@ from domain.sap10_calculator.exceptions import (
 from domain.sap10_calculator.rdsap.cert_to_inputs import (
     SAP_10_2_SPEC_PRICES,
     _apply_heat_network_hiu_default_store,  # pyright: ignore[reportPrivateUsage]
+    _cylinder_thermostat_present,  # pyright: ignore[reportPrivateUsage]
     _has_suspended_timber_floor_per_spec,  # pyright: ignore[reportPrivateUsage]
     _heat_network_code_302_effective_factor,  # pyright: ignore[reportPrivateUsage]
     _heat_network_community_fuel_code,  # pyright: ignore[reportPrivateUsage]
@@ -365,6 +366,60 @@ def test_heat_network_primary_loss_uses_p1_h3_all_months_per_table_3() -> None:
     assert abs(sum(wh_result.primary_loss_monthly_kwh) - expected_primary_kwh) <= 1e-6
 
 
+def test_cylinder_thermostat_assumed_present_per_sap_9_4_9() -> None:
+    # Arrange — SAP 10.2 §9.4.9 (PDF p.32): "A cylinder thermostat should be
+    # assumed to be present when the domestic hot water is obtained from a
+    # heat network, an immersion heater, a thermal store, a combi boiler or
+    # a CPSU." A direct-acting electric boiler (SAP code 191) heats the
+    # cylinder by electric resistance (immersion-equivalent) → assumed
+    # present even with no lodged cylinder thermostat. Reproduces cert
+    # 2474-3059-4202-4496-3200 (Summary path: WHC 901, main SAP 191).
+    direct_electric = MainHeatingDetail(
+        has_fghrs=False,
+        main_fuel_type=30,  # standard electricity
+        heat_emitter_type=0,
+        emitter_temperature="NA",
+        main_heating_control=2106,
+        main_heating_category=2,
+        sap_main_heating_code=191,
+    )
+    part = make_building_part(construction_age_band="D")
+    epc_191 = make_minimal_sap10_epc(
+        total_floor_area_m2=_TYPICAL_TFA_M2,
+        habitable_rooms_count=4,
+        country_code="ENG",
+        sap_building_parts=[part],
+        sap_heating=make_sap_heating(
+            main_heating_details=[direct_electric],
+            water_heating_code=901,  # from main, no lodged cylinder stat
+        ),
+    )
+    # A gas boiler is NOT in the §9.4.9 list — its cylinder keeps the
+    # absent-thermostat default (Table 2b ×1.3) when none is lodged.
+    gas_boiler = MainHeatingDetail(
+        has_fghrs=False,
+        main_fuel_type=26,
+        heat_emitter_type=1,
+        emitter_temperature=1,
+        main_heating_control=2106,
+        main_heating_category=2,
+        sap_main_heating_code=102,
+    )
+    epc_gas = make_minimal_sap10_epc(
+        total_floor_area_m2=_TYPICAL_TFA_M2,
+        habitable_rooms_count=4,
+        country_code="ENG",
+        sap_building_parts=[part],
+        sap_heating=make_sap_heating(
+            main_heating_details=[gas_boiler], water_heating_code=901,
+        ),
+    )
+
+    # Act / Assert — 191 electric DHW assumes present; gas boiler does not.
+    assert _cylinder_thermostat_present(epc_191, direct_electric) is True
+    assert _cylinder_thermostat_present(epc_gas, gas_boiler) is False
+
+
 def test_heat_network_distribution_electricity_per_sap_10_2_appendix_c_3_2() -> None:
     # Arrange — heat-network main (Table 4a code 301 = community heating,
     # category 6). SAP 10.2 Appendix C §C3.2 (PDF p.51): distribution