diff --git a/domain/sap10_calculator/rdsap/cert_to_inputs.py b/domain/sap10_calculator/rdsap/cert_to_inputs.py
index 6cec1313..13a7cf62 100644
--- a/domain/sap10_calculator/rdsap/cert_to_inputs.py
+++ b/domain/sap10_calculator/rdsap/cert_to_inputs.py
@@ -1479,6 +1479,35 @@ def _water_heating_main(
     return details[0]
 
 
+def _water_heating_main_space_fraction(
+    epc: EpcPropertyData, secondary_fraction: float
+) -> float:
+    """Fraction of TOTAL space heating provided by the DHW boiler — the
+    SAP 10.2 Appendix D §D2.1(2) Equation D1 Q_space weight.
+
+    Eq D1's monthly water-heater efficiency blends η_winter / η_summer by
+    the ratio of the boiler's space-heating load to its water load. On a
+    single-main / WHC-901 cert that load is the whole main share,
+    (202) = 1 − (201). On a dual-main cert the DHW boiler does ONLY its
+    own share — (204) for Main 1, (205) for Main 2 — so feeding it the
+    dwelling total over-weights η_winter and under-states HW fuel
+    (simulated case 6: Main 1 serves DHW + 51% of space heat; using 100%
+    of demand gave HW −78 kWh vs the worksheet)."""
+    details = epc.sap_heating.main_heating_details if epc.sap_heating else []
+    main_fraction = 1.0 - secondary_fraction  # (202)
+    if len(details) < 2:
+        return main_fraction
+    main_2 = details[1]
+    main_2_of_main = (
+        main_2.main_heating_fraction / 100.0
+        if main_2.main_heating_fraction is not None
+        else 0.0
+    )
+    if _water_heating_main(epc) is details[1]:
+        return main_fraction * main_2_of_main  # (205) — DHW from Main 2
+    return main_fraction * (1.0 - main_2_of_main)  # (204) — DHW from Main 1
+
+
 def _rdsap_tariff(epc: EpcPropertyData) -> Tariff:
     """Resolve the cert's Table 12a tariff column via RdSAP 10 §12
     Rules 1-4 (page 62). Consults BOTH main heating systems — §12
@@ -6329,8 +6358,14 @@ def cert_to_inputs(
     # Q_space (kWh/month) per spec = (98c)m × (204) = (98c)m × (1 −
     # sec_frac) for single-main fixtures.
     if wh_result is not None:
+        # Eq D1 Q_space is the DHW boiler's OWN space-heating load — its
+        # (204)/(205) share of total — not the dwelling total (202). See
+        # `_water_heating_main_space_fraction`.
+        water_main_space_fraction = _water_heating_main_space_fraction(
+            epc, secondary_fraction_value
+        )
         space_heating_monthly_useful_kwh = tuple(
-            q * (1.0 - secondary_fraction_value)
+            q * water_main_space_fraction
             for q in space_heating_result.total_space_heating_monthly_kwh
         )
         hw_kwh = _apply_water_efficiency(
diff --git a/tests/domain/sap10_calculator/rdsap/test_golden_fixtures.py b/tests/domain/sap10_calculator/rdsap/test_golden_fixtures.py
index 381895b5..b17cc6f5 100644
--- a/tests/domain/sap10_calculator/rdsap/test_golden_fixtures.py
+++ b/tests/domain/sap10_calculator/rdsap/test_golden_fixtures.py
@@ -83,8 +83,8 @@ _EXPECTATIONS: tuple[_GoldenExpectation, ...] = (
         cert_number="0240-0200-5706-2365-8010",
         actual_sap=73,
         expected_sap_resid=-1,
-        expected_pe_resid_kwh_per_m2=+1.6893,
-        expected_co2_resid_tonnes_per_yr=+0.0815,
+        expected_pe_resid_kwh_per_m2=+1.8687,
+        expected_co2_resid_tonnes_per_yr=+0.0907,
         notes=(
             "Detached house, TFA 118, age J, oil boiler PCDB-listed + PV + "
             "RR on BP[0]. Mapper DOES extract sap_room_in_roof.room_in_roof_"
@@ -176,7 +176,14 @@ _EXPECTATIONS: tuple[_GoldenExpectation, ...] = (
             "two-control blend. Lowers MIT ~0.037 °C → space-heating demand "
             "falls → PE +2.1519 → +1.6893, CO2 +0.1051 → +0.0815 (both "
             "closer to zero; SAP integer 72 unchanged). Verified 1e-4 "
-            "against the case-6 worksheet (87)/(90)/(98c)."
+            "against the case-6 worksheet (87)/(90)/(98c). "
+            "Slice S0380.206 fed Eq D1 the DHW boiler's OWN (204) space "
+            "share (Main 1 = 51%) instead of the dwelling total (202) — "
+            "the worksheet-validated case-6 fix that lands its (219) HW "
+            "exact. For 0240 this raises HW fuel slightly → PE +1.6893 → "
+            "+1.8687, CO2 +0.0815 → +0.0907 (SAP 72 unchanged). The lodged "
+            "73 carries Elmhurst's own residual; case 6 is the spec "
+            "authority per [[feedback-worksheet-not-api-reference]]."
         ),
     ),
     _GoldenExpectation(
diff --git a/tests/domain/sap10_calculator/worksheet/_elmhurst_worksheet_001431_case6.py b/tests/domain/sap10_calculator/worksheet/_elmhurst_worksheet_001431_case6.py
index 95e4824f..0ef2714e 100644
--- a/tests/domain/sap10_calculator/worksheet/_elmhurst_worksheet_001431_case6.py
+++ b/tests/domain/sap10_calculator/worksheet/_elmhurst_worksheet_001431_case6.py
@@ -92,6 +92,12 @@ LINE_231_PUMPS_FANS_KWH: Final[float] = 356.0
 LINE_211_MAIN_1_FUEL_KWH: Final[float] = 7741.6458
 LINE_213_MAIN_2_FUEL_KWH: Final[float] = 6995.3106
 
+# Worksheet (219) water-heating fuel (kWh/yr). The DHW boiler is Main 1
+# (WHC 901), which provides only 51% of space heating, so SAP 10.2
+# Appendix D Eq D1 weights η_winter by Main 1's (204) share — not the
+# dwelling total — when blending the monthly water-heater efficiency.
+LINE_219_HOT_WATER_FUEL_KWH: Final[float] = 4902.8601
+
 # Worksheet (70) "Pumps, fans" internal-gain (W), heating-season only
 # (Jun-Sep = 0). = 10 W = the two-main-system central-heating-pump pair
 # per SAP 10.2 Table 5a note a): Main 1 ("2013 or later" → 3 W) + Main 2
diff --git a/tests/domain/sap10_calculator/worksheet/test_section_cascade_pins.py b/tests/domain/sap10_calculator/worksheet/test_section_cascade_pins.py
index 9ec6975f..2835eb16 100644
--- a/tests/domain/sap10_calculator/worksheet/test_section_cascade_pins.py
+++ b/tests/domain/sap10_calculator/worksheet/test_section_cascade_pins.py
@@ -301,6 +301,23 @@ def test_case6_main_2_emitter_and_control_extracted() -> None:
     assert main_2.main_heating_control == 2110
 
 
+def test_section_4_hot_water_fuel_case6_match_pdf() -> None:
+    """(219) water-heating fuel for simulated case 6. The DHW boiler (Main
+    1, WHC 901) provides only 51% of space heating, so SAP 10.2 Appendix D
+    §D2.1(2) Equation D1 must weight η_winter by Main 1's (204) share, not
+    the dwelling total (202). Pre-S0380.206 the cascade fed Eq D1 the full
+    dwelling space load → over-weighted η_winter → HW −78 kWh."""
+    # Arrange / Act — real cascade (the §2.4 helper skips the cylinder gate).
+    ci = cert_to_inputs(_w001431_case6.build_epc())
+
+    # Assert
+    _pin(
+        ci.hot_water_kwh_per_yr,
+        _w001431_case6.LINE_219_HOT_WATER_FUEL_KWH,
+        "§4 (219) case6",
+    )
+
+
 def test_section_9a_per_system_fuel_case6_match_pdf() -> None:
     """(211)/(213) per-system space-heating fuel for simulated case 6. The
     dual oil boiler heats different parts (Main 1 radiators/2106 living,