fix(hw): direct-acting electric boiler (191) → zero primary circuit loss

SAP 10.2 Table 3 (PDF p.160) names "Direct-acting electric boiler"
verbatim in the primary-loss zero list (alongside electric immersion,
combi, CPSU, integral-vessel heat pump). RdSAP 10 §12 (p.62) classifies
SAP code 191 as the direct-acting electric boiler. Its cylinder is
immersion-heated with no primary pipework, so no primary circuit loss
applies — but `_primary_loss_applies` had no 191 branch, so a 191 main
(main_heating_category 2, "Boiler and radiators, electric") fell through
to the cat-{1,2} boiler branch and accrued ~1177 kWh/yr of phantom
primary loss on the electric-flat segment.

Validated against the cert-2474 worksheet: §4 (59) primary loss = 0,
(64) HW output 1760 (cylinder) + (64a) shower 581. Cert 2474 HW kWh
3585 → 2408; SAP 64.66 → 70.35 (the residual to the lodged 78 is an
Unknown-meter data-fidelity artifact — the register recorded meter_type=3
"Unknown" but the lodged rating used an 18-hour off-peak meter, per RdSAP
§12 / the example worksheets).

Eval mean|err| 1.720 → 1.708 (headline 45.0%, flat ±1 cert — the
electric-flat segment is dominated by the meter data-fidelity artifact).
Regression green (2448 pass incl. golden 6035 + ASHP cohort 1e-4);
pyright net-zero.

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

domain/sap10_calculator/rdsap/cert_to_inputs.py

@@ -663,6 +663,11 @@ _INSTANTANEOUS_WATER_CODES: Final[frozenset[int]] = frozenset({907, 909})
 # zero-loss list, so primary loss is zero whenever this code is lodged.
 _WHC_ELECTRIC_IMMERSION: Final[int] = 903
 
+# SAP 10.2 Table 4a "direct-acting electric boiler" (RdSAP 10 §12 p.62).
+# Named in the SAP 10.2 Table 3 (PDF p.160) primary-loss zero list, so a
+# 191 main feeding a cylinder incurs no primary circuit loss.
+_DIRECT_ACTING_ELECTRIC_BOILER_CODE: Final[int] = 191
+
 # Water-heating codes for a dedicated "boiler/circulator for water
 # heating only" — SAP 10.2 Table 4a hot-water section (PDF p.166):
 # 911 gas, 912 liquid fuel, 913 solid fuel boiler/circulator; 921-931
@@ -5307,6 +5312,16 @@ def _primary_loss_applies(
     # kWh/yr — zero before this branch.
     if water_heating_code in _WATER_HEATING_BOILER_CIRCULATOR_CODES:
         return True
+    # SAP 10.2 Table 3 (PDF p.160) zero-loss list names "Direct-acting
+    # electric boiler" verbatim. RdSAP 10 §12 (p.62) classifies SAP code
+    # 191 as the direct-acting electric boiler: its cylinder is immersion-
+    # heated with no primary pipework, so no primary loss — even though it
+    # lodges as main_heating_category 2 ("Boiler and radiators, electric")
+    # and would otherwise hit the cat-{1,2} boiler branch below. Checked
+    # before that branch so the electric-flat segment (cert 2474: WHC 901
+    # + code 191 + cylinder) no longer accrues ~1177 kWh/yr phantom loss.
+    if main.sap_main_heating_code == _DIRECT_ACTING_ELECTRIC_BOILER_CODE:
+        return False
     if main.main_heating_category == 4:
         if hp_record is None:
             # No PCDB record → assume separate-vessel (conservative; the

tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py

@@ -2164,6 +2164,36 @@ def test_secondary_electric_off_peak_bills_at_table_12a_direct_acting_high_rate(
     assert abs(secondary_rate_gbp_per_kwh - 0.1529) <= 1e-6
 
 
+def test_sap_table_3_primary_loss_zero_for_direct_acting_electric_boiler() -> None:
+    # Arrange — SAP 10.2 Table 3 (PDF p.160) names "Direct-acting electric
+    # boiler" verbatim in the primary-loss zero list (alongside electric
+    # immersion, combi, CPSU, integral-vessel heat pump). RdSAP 10 §12
+    # (p.62) classifies SAP code 191 as the "direct-acting electric
+    # boiler", so a 191 main feeding a cylinder (WHC 901, "from main
+    # system") incurs NO primary circuit loss — the DHW is immersion-
+    # heated, with no primary pipework. The cat-{1,2} branch in
+    # `_primary_loss_applies` mis-fires here (main_heating_category=2),
+    # returning True and adding ~1177 kWh/yr of phantom primary loss to
+    # the cat-2 electric-flat segment (cert 2474 worksheet (59) = 0).
+    electric_boiler_main = MainHeatingDetail(
+        has_fghrs=False,
+        main_fuel_type=29,  # electricity
+        heat_emitter_type=1,
+        emitter_temperature="NA",
+        main_heating_control=2106,
+        main_heating_category=2,  # "Boiler and radiators, electric"
+        sap_main_heating_code=191,  # direct-acting electric boiler
+    )
+
+    # Act — cylinder present, WHC 901 (HW from the electric boiler).
+    applies = _primary_loss_applies(
+        electric_boiler_main, True, None, water_heating_code=901,
+    )
+
+    # Assert — direct-acting electric boiler → Table 3 zero list → no loss.
+    assert applies is False
+
+
 def test_sap_table_3_primary_loss_applies_to_dedicated_water_heating_boiler_circulator() -> None:
     # Arrange — SAP 10.2 Table 3 (PDF p.160) row 1: primary circuit loss
     # applies when "hot water is heated by a heat generator (e.g. boiler)