diff --git a/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000474.py b/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000474.py
index 6bb99857..1dafc14d 100644
--- a/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000474.py
+++ b/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000474.py
@@ -197,3 +197,21 @@ LINE_46_M_DISTRIBUTION_LOSS_KWH: tuple[float, ...] = (
     26.1600, 23.0055, 24.1612, 20.6307, 19.5714, 17.1754,
     16.6394, 17.5728, 18.0628, 20.6883, 22.6596, 25.7985,
 )
+LINE_56_M_STORAGE_LOSS_KWH: tuple[float, ...] = (0.0,) * 12  # combi, no cylinder
+LINE_57_M_SOLAR_STORAGE_KWH: tuple[float, ...] = (0.0,) * 12  # no solar HW
+LINE_59_M_PRIMARY_LOSS_KWH: tuple[float, ...] = (0.0,) * 12   # combi, no primary circuit
+# (61)m combi loss values from worksheet — derived from Table 3b (PCDB-
+# tested boiler). Not yet computable by our cascade; lodged for assertion.
+LINE_61_M_COMBI_LOSS_KWH: tuple[float, ...] = (
+    28.7238, 25.9337, 28.6905, 27.7191, 28.6040, 27.6419,
+    28.5422, 28.5649, 27.6693, 28.6326, 27.7530, 28.7178,
+)
+LINE_62_M_TOTAL_WH_KWH: tuple[float, ...] = (
+    203.1238, 179.3035, 189.7652, 165.2572, 159.0798, 142.1443,
+    139.4718, 145.7166, 148.0876, 166.5545, 178.8167, 200.7079,
+)
+LINE_63A_M_WWHRS_KWH: tuple[float, ...] = (0.0,) * 12
+LINE_63B_M_PV_DIVERTER_KWH: tuple[float, ...] = (0.0,) * 12
+LINE_63C_M_SOLAR_KWH: tuple[float, ...] = (0.0,) * 12
+LINE_63D_M_FGHRS_KWH: tuple[float, ...] = (0.0,) * 12
+LINE_64_M_OUTPUT_FROM_WH_KWH: tuple[float, ...] = LINE_62_M_TOTAL_WH_KWH
diff --git a/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000490.py b/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000490.py
index 5a297793..0956a0c6 100644
--- a/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000490.py
+++ b/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000490.py
@@ -196,3 +196,8 @@ LINE_62_M_TOTAL_WH_KWH: tuple[float, ...] = (
     238.8196, 211.2342, 224.4651, 197.4680, 191.5045, 172.6544,
     170.4499, 177.1525, 179.0276, 199.5259, 212.0383, 236.2237,
 )
+LINE_63A_M_WWHRS_KWH: tuple[float, ...] = (0.0,) * 12         # no WWHRS
+LINE_63B_M_PV_DIVERTER_KWH: tuple[float, ...] = (0.0,) * 12   # no PV diverter
+LINE_63C_M_SOLAR_KWH: tuple[float, ...] = (0.0,) * 12         # no solar HW
+LINE_63D_M_FGHRS_KWH: tuple[float, ...] = (0.0,) * 12         # no FGHRS
+LINE_64_M_OUTPUT_FROM_WH_KWH: tuple[float, ...] = LINE_62_M_TOTAL_WH_KWH  # no reductions
diff --git a/packages/domain/src/domain/sap/worksheet/tests/test_water_heating.py b/packages/domain/src/domain/sap/worksheet/tests/test_water_heating.py
index a3517d70..db8b87d4 100644
--- a/packages/domain/src/domain/sap/worksheet/tests/test_water_heating.py
+++ b/packages/domain/src/domain/sap/worksheet/tests/test_water_heating.py
@@ -26,6 +26,7 @@ from domain.sap.worksheet.water_heating import (
     hot_water_baths_monthly_l_per_day,
     hot_water_mixer_showers_monthly_l_per_day,
     hot_water_other_uses_monthly_l_per_day,
+    output_from_water_heater_monthly_kwh,
     total_hot_water_monthly_l_per_day,
     total_water_heating_demand_monthly_kwh,
 )
@@ -503,6 +504,56 @@ def test_total_water_heating_demand_matches_elmhurst_line_62_for_000490() -> Non
         assert actual == pytest.approx(exp, abs=1e-3), f"month {m+1}"
 
 
+@pytest.mark.parametrize("fixture", (_w000474, _w000490), ids=("000474", "000490"))
+def test_output_from_water_heater_matches_elmhurst_line_64(fixture) -> None:  # type: ignore[no-untyped-def]
+    """SAP10.2 §4 line (64)m:
+        (64)m = max(0, (62)m + (63a)m + (63b)m + (63c)m + (63d)m)
+
+    The (63 a-d) WWHRS / PV-diverter / Solar / FGHRS inputs are entered
+    as negative quantities (heat displaced FROM the water heater). The
+    max-clamp guards against the worksheet text "if (64)m < 0 then set
+    to 0" — a renewable-heavy system would otherwise show negative
+    delivered heat for the warmest months.
+
+    Both fixtures have all four inputs zero (no WWHRS, no PV diverter,
+    no solar HW, no FGHRS), so (64)m = (62)m for every month.
+    """
+    # Arrange / Act
+    monthly = output_from_water_heater_monthly_kwh(
+        total_demand_monthly_kwh=fixture.LINE_62_M_TOTAL_WH_KWH,
+        wwhrs_monthly_kwh=fixture.LINE_63A_M_WWHRS_KWH,
+        pv_diverter_monthly_kwh=fixture.LINE_63B_M_PV_DIVERTER_KWH,
+        solar_monthly_kwh=fixture.LINE_63C_M_SOLAR_KWH,
+        fghrs_monthly_kwh=fixture.LINE_63D_M_FGHRS_KWH,
+    )
+
+    # Assert
+    for m, (actual, exp) in enumerate(zip(monthly, fixture.LINE_64_M_OUTPUT_FROM_WH_KWH)):
+        assert actual == pytest.approx(exp, abs=1e-3), f"month {m+1}"
+
+
+def test_output_from_water_heater_clamps_to_zero_when_renewables_exceed_demand() -> None:
+    """(64)m floor at zero per spec: a solar HW system that contributes
+    more in July than the dwelling demands shouldn't show negative
+    delivered heat for that month."""
+    # Arrange — 100 kWh demand, -150 kWh solar input (overshoots)
+    demand = tuple(100.0 for _ in range(12))
+    solar = tuple(-150.0 for _ in range(12))
+    zero = tuple(0.0 for _ in range(12))
+
+    # Act
+    monthly = output_from_water_heater_monthly_kwh(
+        total_demand_monthly_kwh=demand,
+        wwhrs_monthly_kwh=zero,
+        pv_diverter_monthly_kwh=zero,
+        solar_monthly_kwh=solar,
+        fghrs_monthly_kwh=zero,
+    )
+
+    # Assert
+    assert all(v == pytest.approx(0.0, abs=1e-9) for v in monthly)
+
+
 def test_assumed_occupancy_floor_at_n_eq_1_for_small_dwellings() -> None:
     """Appendix J piecewise definition: TFA ≤ 13.9 m² → N=1 exactly. A
     tiny studio flat at the boundary is the most common trigger."""
diff --git a/packages/domain/src/domain/sap/worksheet/water_heating.py b/packages/domain/src/domain/sap/worksheet/water_heating.py
index 4b20a1ed..9a20560d 100644
--- a/packages/domain/src/domain/sap/worksheet/water_heating.py
+++ b/packages/domain/src/domain/sap/worksheet/water_heating.py
@@ -272,6 +272,36 @@ def total_water_heating_demand_monthly_kwh(
     )
 
 
+def output_from_water_heater_monthly_kwh(
+    *,
+    total_demand_monthly_kwh: tuple[float, ...],
+    wwhrs_monthly_kwh: tuple[float, ...],
+    pv_diverter_monthly_kwh: tuple[float, ...],
+    solar_monthly_kwh: tuple[float, ...],
+    fghrs_monthly_kwh: tuple[float, ...],
+) -> tuple[float, ...]:
+    """SAP 10.2 §4 line (64)m:
+        (64)m = max(0, (62)m + (63a)m + (63b)m + (63c)m + (63d)m)
+
+    Output from the water heater after subtracting renewable / heat-
+    recovery contributions. The four reduction inputs are entered as
+    negative quantities (heat displaced FROM the boiler/cylinder), so
+    the formula uses + not -. Spec note "if (64)m < 0 then set to 0"
+    floors the result month-by-month — a renewable-heavy system can't
+    show negative delivered heat for the warmest months.
+    """
+    return tuple(
+        max(0.0, t + w + pv + s + f)
+        for t, w, pv, s, f in zip(
+            total_demand_monthly_kwh,
+            wwhrs_monthly_kwh,
+            pv_diverter_monthly_kwh,
+            solar_monthly_kwh,
+            fghrs_monthly_kwh,
+        )
+    )
+
+
 def _days_weighted_average(monthly: tuple[float, ...]) -> float:
     """Σ value[m] × n_m / 365 — used by Appendix J equations J4 and J9."""
     return sum(v * d for v, d in zip(monthly, _DAYS_IN_MONTH)) / _DAYS_IN_YEAR