diff --git a/domain/sap10_calculator/rdsap/cert_to_inputs.py b/domain/sap10_calculator/rdsap/cert_to_inputs.py
index d35be43b..9c6cc278 100644
--- a/domain/sap10_calculator/rdsap/cert_to_inputs.py
+++ b/domain/sap10_calculator/rdsap/cert_to_inputs.py
@@ -5223,6 +5223,15 @@ _CYLINDER_SIZE_EXACT: Final[int] = 6
 _CYLINDER_INACCESSIBLE_DUAL_IMMERSION_L: Final[float] = 210.0
 _CYLINDER_INACCESSIBLE_SOLID_FUEL_L: Final[float] = 160.0
 _CYLINDER_INACCESSIBLE_DEFAULT_L: Final[float] = 110.0
+# RdSAP 10 §10.5 (PDF p.55): "If the actual size is not determined, the size
+# of a hot-water cylinder is taken as according to Table 28." For a cylinder
+# present but with no size descriptor lodged (size code 0 / absent), the
+# baseline Table 28 default is the "Normal" row (110 L) — the same value
+# §10.7 instantiates as the first-row default. The context-dependent
+# Inaccessible 210/160 values are NOT applied here: they are tied to the
+# explicit "Inaccessible" descriptor (code 5) the assessor lodges
+# deliberately, not to a merely-unpopulated size field.
+_CYLINDER_SIZE_NOT_DETERMINED_L: Final[float] = 110.0
 
 
 def _cylinder_inaccessible_volume_l(epc: EpcPropertyData) -> float:
@@ -6163,11 +6172,21 @@ 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 — Normal/Medium/Large
     (codes 2/3/4), Inaccessible (5, context-dependent) and Exact (6, lodged
-    measured volume). Returns None when no cylinder is lodged or no size
-    code resolves."""
+    measured volume). Returns None only when no cylinder is lodged.
+
+    RdSAP 10 §10.5 (PDF p.55): "If the actual size is not determined, the
+    size of a hot-water cylinder is taken as according to Table 28." When a
+    cylinder IS present but no size descriptor resolves (size code 0 /
+    absent, or Exact with no measured volume), fall back to the Table 28
+    baseline "Normal" default (110 L). Without this the cylinder resolved
+    to None, silently dropping BOTH its storage loss and the Table 13
+    high-rate fraction, over-rating unsized-cylinder electric dwellings."""
     if not epc.has_hot_water_cylinder:
         return None
-    return _cylinder_volume_l_from_code(epc)
+    volume_l = _cylinder_volume_l_from_code(epc)
+    if volume_l is not None:
+        return volume_l
+    return _CYLINDER_SIZE_NOT_DETERMINED_L
 
 
 def _immersion_is_single(epc: EpcPropertyData) -> Optional[bool]:
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 9d7971cf..a862c238 100644
--- a/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
+++ b/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
@@ -561,6 +561,25 @@ def test_cylinder_size_exact_code_6_uses_lodged_measured_volume() -> None:
     assert volume_l is not None and abs(volume_l - 150.0) <= 1e-9
 
 
+def test_cylinder_present_but_size_not_determined_defaults_to_normal_110l() -> None:
+    # Arrange — RdSAP 10 §10.5 (PDF p.55): "If the actual size is not
+    # determined, the size of a hot-water cylinder is taken as according to
+    # Table 28." A cylinder IS present but no size descriptor resolves (gov
+    # API lodges `cylinder_size=0`) → the Table 28 baseline "Normal" default
+    # of 110 L, NOT None (which silently dropped the cylinder's storage loss
+    # AND the Table 13 high-rate fraction, over-rating the dwelling).
+    from domain.sap10_calculator.rdsap.cert_to_inputs import (
+        _hot_water_cylinder_volume_l,  # pyright: ignore[reportPrivateUsage]
+    )
+    epc = _cylinder_epc(cylinder_size=0)
+
+    # Act
+    volume_l = _hot_water_cylinder_volume_l(epc)
+
+    # Assert
+    assert volume_l is not None and abs(volume_l - 110.0) <= 1e-9
+
+
 def test_cylinder_size_inaccessible_code_5_solid_fuel_boiler_uses_160l() -> None:
     # Arrange — RdSAP 10 §10.5 Table 28: an "Inaccessible" cylinder (code 5)
     # heated "from a solid fuel boiler" uses 160 litres.
diff --git a/tests/infrastructure/epc_client/test_sap_accuracy_corpus.py b/tests/infrastructure/epc_client/test_sap_accuracy_corpus.py
index e6c3be1e..378b6b99 100644
--- a/tests/infrastructure/epc_client/test_sap_accuracy_corpus.py
+++ b/tests/infrastructure/epc_client/test_sap_accuracy_corpus.py
@@ -41,10 +41,11 @@ _CORPUS = Path(
 )
 
 # Measured floors/ceilings over the fixed corpus at HEAD (1000 certs, 0 skips).
-# Current: SAP within-0.5 = 66.1%, SAP MAE = 1.128 (Table 12a Grid 1
-# integrated-storage code-408 0.20 high-rate fraction, this slice: sap408
-# over-rate +14.6/+12.9/+12.7 -> +7.1/+5.1/+3.4; prior slice was the
-# heat-network Table 4c(3) flat-rate charging factor).
+# Current: SAP within-0.5 = 66.1%, SAP MAE = 1.124 (RdSAP 10 §10.5 present-
+# but-unsized cylinder -> Table 28 Normal 110 L default, this slice — a
+# correctness fix: 7 certs that silently dropped storage loss + Table 13;
+# marginal on the headline. Prior slices: Table 12a code-408 0.20 storage
+# high-rate fraction; heat-network Table 4c(3) flat-rate charging factor).
 # CO2 MAE = 0.28 t/yr (signed +0.17 — a systematic over-estimate, see below).
 # PE  MAE = 14.7 kWh/m2/yr (signed +9.1).
 #