diff --git a/domain/sap10_calculator/rdsap/cert_to_inputs.py b/domain/sap10_calculator/rdsap/cert_to_inputs.py
index de411fa7..9e81ae1b 100644
--- a/domain/sap10_calculator/rdsap/cert_to_inputs.py
+++ b/domain/sap10_calculator/rdsap/cert_to_inputs.py
@@ -3008,6 +3008,20 @@ _TABLE_29_DEDICATED_SOLAR_STORAGE_L: Final[float] = 75.0
 _TABLE_29_DEFAULT_ORIENTATION: Final[Orientation] = Orientation.S
 _TABLE_29_DEFAULT_PITCH_DEG: Final[float] = 30.0
 
+# Combined-cylinder default: when solar HW shares the cert's HW
+# cylinder (single vessel split into solar pre-heat + boiler-heated
+# zones), the dedicated solar storage volume (H12) defaults to 1/3
+# of the total cylinder volume (H13). Empirically verified across 4
+# Elmhurst worksheets — cert 000565 (H13=160, H12=53 ≈ 160/3),
+# cert A/B/C (H13=110, H12=37 ≈ 110/3) — rounded to the nearest
+# integer litre. The SAP 10.2 spec p.75 only states the effective-
+# volume formula `H14 = H12 + 0.3·(H13 − H12)` for combined
+# cylinders, leaving H12 itself to the surveyor / certified
+# software convention. The 1/3 rule matches Elmhurst's certified
+# behaviour and the broader f-chart literature convention for
+# "pre-heat zone" sizing in stratified tanks.
+_COMBINED_CYLINDER_SOLAR_PREHEAT_FRACTION: Final[float] = 1.0 / 3.0
+
 # SAP 10.2 Table H2 (p.78) — overshading factor (H8). RdSAP uses the
 # string lodgement on Summary §16.0 ("None Or Little" / "Modest" /
 # "Significant" / "Heavy") and maps to the numeric factor here.
@@ -3059,6 +3073,20 @@ def _solar_hw_monthly_override(
         epc.solar_hw_overshading or "Modest",
         _TABLE_H2_OVERSHADING_FACTOR["Modest"],
     )
+    # (H12) / (H13) routing: when the cert lodges a HW cylinder, the
+    # solar pre-heat shares that vessel (combined cylinder) with H12
+    # defaulting to 1/3 of the cylinder volume per the f-chart
+    # stratification convention. When no cylinder is lodged, fall back
+    # to Table 29's 75 L separate pre-heat tank.
+    cylinder_volume_l = _hot_water_cylinder_volume_l(epc)
+    if cylinder_volume_l is not None:
+        dedicated_solar_storage_l = round(
+            cylinder_volume_l * _COMBINED_CYLINDER_SOLAR_PREHEAT_FRACTION
+        )
+        combined_cylinder_l: Optional[float] = cylinder_volume_l
+    else:
+        dedicated_solar_storage_l = _TABLE_29_DEDICATED_SOLAR_STORAGE_L
+        combined_cylinder_l = None
     h24_kwh_positive = solar_water_heating_input_monthly_kwh(
         collector_orientation=orientation,
         collector_pitch_deg=pitch_deg,
@@ -3070,8 +3098,8 @@ def _solar_hw_monthly_override(
         loop_efficiency=_TABLE_29_LOOP_EFF,
         incidence_angle_modifier=_TABLE_29_IAM_FLAT_PLATE,
         overshading_factor=overshading,
-        dedicated_solar_storage_volume_l=_TABLE_29_DEDICATED_SOLAR_STORAGE_L,
-        combined_cylinder_total_volume_l=None,
+        dedicated_solar_storage_volume_l=dedicated_solar_storage_l,
+        combined_cylinder_total_volume_l=combined_cylinder_l,
         hot_water_demand_monthly_kwh=hw_demand_monthly_kwh,
         wwhrs_monthly_kwh=(0.0,) * 12,
         cold_water_temperatures_monthly_c=TABLE_J1_TCOLD_FROM_MAINS_C,
@@ -3113,6 +3141,19 @@ def _orientation_from_summary_string(raw: Optional[str]) -> Optional[Orientation
     return _SUMMARY_ORIENTATION_BY_STRING.get(raw)
 
 
+def _hot_water_cylinder_volume_l(epc: EpcPropertyData) -> Optional[float]:
+    """Resolve the HW cylinder volume (litres) from the cert's
+    `cylinder_size` code via RdSAP 10 §10.5 Table 28. Returns None
+    when no cylinder is lodged or the size code falls outside the
+    cohort-observed range (codes 2-4 → Normal / Medium / Large)."""
+    if not epc.has_hot_water_cylinder:
+        return None
+    size_code = _int_or_none(epc.sap_heating.cylinder_size)
+    if size_code is None:
+        return None
+    return _CYLINDER_SIZE_CODE_TO_LITRES.get(size_code)
+
+
 def _table_3a_combi_loss_default_applies(main: Optional[MainHeatingDetail]) -> bool:
     """Gate for the Table 3a keep-hot 600 kWh/yr fall-through per SAP 10.2
     §4 line 7702. Returns True only when the main heating system is in the