diff --git a/domain/sap10_calculator/rdsap/cert_to_inputs.py b/domain/sap10_calculator/rdsap/cert_to_inputs.py
index 92158613..58a08efd 100644
--- a/domain/sap10_calculator/rdsap/cert_to_inputs.py
+++ b/domain/sap10_calculator/rdsap/cert_to_inputs.py
@@ -659,6 +659,19 @@ _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
 
+# 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
+# range cooker with boiler for water heating only. Each is a heat
+# generator feeding the cylinder through a primary loop, so SAP 10.2
+# Table 3 (PDF p.160) row 1 primary circuit loss applies — independent
+# of the space-heating system (which for these certs is a separate main,
+# e.g. electric storage heaters). 941 (electric HP for water only) is
+# excluded: HP DHW vessels follow the Table 3 integral-vessel rules.
+_WATER_HEATING_BOILER_CIRCULATOR_CODES: Final[frozenset[int]] = frozenset(
+    {911, 912, 913} | set(range(921, 932))
+)
+
 
 # SAP 10.2 Appendix M equation (M1): EPV = 0.8 × kWp × S × ZPV, summed
 # per array. The module efficiency constant (0.8), orientation-dependent
@@ -5110,6 +5123,18 @@ def _primary_loss_applies(
     # kWh/yr primary loss to a system with no primary circuit at all.
     if water_heating_code == _WHC_ELECTRIC_IMMERSION:
         return False
+    # SAP 10.2 Table 3 (PDF p.160) row 1 — a dedicated "boiler/circulator
+    # for water heating only" (WHC 911 gas / 912 liquid / 913 solid /
+    # 921-931 range cooker with boiler) is a heat generator feeding the
+    # cylinder through a primary loop, so the loss applies regardless of
+    # the space-heating main. Checked off `water_heating_code` (not
+    # `main`) because for these certs the resolved DHW `main` is the
+    # SPACE main (e.g. an electric storage heater, SAP code 402) — the
+    # gas/oil water boiler isn't a `main_heating_detail`. Simulated case
+    # 19 (storage main + WHS 911 + 210 L cylinder): worksheet (59) = 676.68
+    # kWh/yr — zero before this branch.
+    if water_heating_code in _WATER_HEATING_BOILER_CIRCULATOR_CODES:
+        return True
     if main.main_heating_category == 4:
         if hp_record is None:
             # No PCDB record → assume separate-vessel (conservative; the
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 69afd8dc..fc95112c 100644
--- a/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
+++ b/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
@@ -60,6 +60,7 @@ from domain.sap10_calculator.rdsap.cert_to_inputs import (
     _is_off_peak_meter,  # pyright: ignore[reportPrivateUsage]
     _main_floor_u_value,  # pyright: ignore[reportPrivateUsage]
     _other_fuel_cost_gbp_per_kwh,  # pyright: ignore[reportPrivateUsage]
+    _primary_loss_applies,  # pyright: ignore[reportPrivateUsage]
     _rdsap_extract_fans_default,  # pyright: ignore[reportPrivateUsage]
     _pv_overshading_factor,  # pyright: ignore[reportPrivateUsage]
     _pv_pitch_deg,  # pyright: ignore[reportPrivateUsage]
@@ -1956,6 +1957,46 @@ 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_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)
+    # connected to a hot water storage vessel via insulated or
+    # uninsulated pipes". The dedicated "boiler/circulator for water
+    # heating only" water-heating codes (Table 4a hot-water section, PDF
+    # p.166): 911 gas, 912 liquid fuel, 913 solid fuel, 921-931 range
+    # cooker with boiler — each is a boiler feeding the cylinder through a
+    # primary loop, so the loss applies regardless of what the SPACE
+    # heating system is. Simulated case 19 pairs electric storage heaters
+    # (SAP code 402) for space with a WHS 911 gas boiler/circulator for
+    # water: `_water_heating_main` resolves to the code-402 storage main
+    # (electric, no primary loop), so before this slice every dedicated-
+    # boiler branch missed the cylinder's primary circuit and (59)m went
+    # to zero — dropping the worksheet's 676.68 kWh/yr (59) and inflating
+    # HW fuel (219).
+    storage_heater_main = MainHeatingDetail(
+        has_fghrs=False,
+        main_fuel_type=30,  # electricity
+        heat_emitter_type="",
+        emitter_temperature=1,
+        main_heating_control=2402,
+        main_heating_category=None,
+        sap_main_heating_code=402,  # electric storage heaters (space)
+    )
+
+    # Act
+    applies_with_cylinder = _primary_loss_applies(
+        storage_heater_main, True, None, water_heating_code=911,
+    )
+    applies_without_cylinder = _primary_loss_applies(
+        storage_heater_main, False, None, water_heating_code=911,
+    )
+
+    # Assert — WHS 911 + cylinder → primary loss applies; no cylinder →
+    # no primary circuit, no loss.
+    assert applies_with_cylinder is True
+    assert applies_without_cylinder is False
+
+
 def test_water_efficiency_uses_table_4a_water_column_for_heat_pumps_per_sap_10_2() -> None:
     # Arrange — SAP 10.2 Table 4a (PDF p.163-164) gives heat pumps two
     # efficiency columns: "space" and "water". For low-temperature