diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index e17b43033..22d9227fb 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -620,8 +620,13 @@ class EpcPropertyDataMapper:
                     MainHeatingDetail(
                         has_fghrs=d.has_fghrs == "Y",
                         main_fuel_type=d.main_fuel_type,
-                        boiler_flue_type=None,
-                        fan_flue_present=None,
+                        # Preserve the lodged flue/pump/PCDB fields, matching the
+                        # 19.0/20.0/… paths. The 17.0 schema *does* lodge all of
+                        # these (RdSapSchema17_0.MainHeatingDetail); hardcoding
+                        # them to None silently dropped a PCDB main-heating index
+                        # (efficiency lookup) and the flue/pump SAP adjustments.
+                        boiler_flue_type=d.boiler_flue_type,
+                        fan_flue_present=d.fan_flue_present == "Y",
                         heat_emitter_type=d.heat_emitter_type,
                         emitter_temperature=d.emitter_temperature,
                         main_heating_number=d.main_heating_number,
@@ -629,9 +634,9 @@ class EpcPropertyDataMapper:
                         main_heating_category=d.main_heating_category,
                         main_heating_fraction=d.main_heating_fraction,
                         sap_main_heating_code=d.sap_main_heating_code,
-                        central_heating_pump_age=None,
+                        central_heating_pump_age=d.central_heating_pump_age,
                         main_heating_data_source=d.main_heating_data_source,
-                        main_heating_index_number=None,
+                        main_heating_index_number=d.main_heating_index_number,
                     )
                     for d in schema.sap_heating.main_heating_details
                 ],
@@ -642,7 +647,15 @@ class EpcPropertyDataMapper:
                 water_heating_fuel=schema.sap_heating.water_heating_fuel,
                 immersion_heating_type=schema.sap_heating.immersion_heating_type,
                 cylinder_insulation_type=schema.sap_heating.cylinder_insulation_type,
-                cylinder_thermostat=None,
+                # Preserve the lodged cylinder thermostat + insulation thickness,
+                # matching the 19.0/20.0/… paths. Both are lodged by the 17.0
+                # schema; hardcoding them to None (a) read as "no cylinder
+                # thermostat" so the water-heating worksheet over-applied its
+                # ×1.3 penalty and `recommend_cylinder_thermostat` could fire on
+                # a dwelling that already has one, and (b) discarded the measured
+                # insulation thickness so the cylinder-loss cascade fell back to
+                # age-band defaults.
+                cylinder_thermostat=schema.sap_heating.cylinder_thermostat,
                 # Preserve the lodged secondary heating from the 17.0 sap_heating
                 # block, matching every other API path (19.0/20.0/…). Hardcoding
                 # these to None silently dropped a lodged FIXED secondary (SAP
@@ -654,7 +667,7 @@ class EpcPropertyDataMapper:
                     schema.sap_heating.secondary_heating_type,
                 ),
                 secondary_heating_type=schema.sap_heating.secondary_heating_type,
-                cylinder_insulation_thickness_mm=None,
+                cylinder_insulation_thickness_mm=schema.sap_heating.cylinder_insulation_thickness,
             ),
             # ADR-0028: 990/1000 omit sap_windows -> synthesised from the
             # glazed_area band + TFA via the shared reduced-field core. The 10
diff --git a/tests/datatypes/epc/domain/test_mapper_sap_heating_17_0_drops.py b/tests/datatypes/epc/domain/test_mapper_sap_heating_17_0_drops.py
new file mode 100644
index 000000000..ee7fc7afe
--- /dev/null
+++ b/tests/datatypes/epc/domain/test_mapper_sap_heating_17_0_drops.py
@@ -0,0 +1,77 @@
+"""Regression: the RdSAP-Schema-17.0 mapper path must preserve the whole lodged
+`sap_heating` block, like every other API path (19.0/20.0/…).
+
+`from_rdsap_schema_17_0` was an older copy that hardcoded a cluster of lodged
+`sap_heating` fields to None while the 19.0+ paths read them from
+`schema.sap_heating`:
+
+  * MainHeatingDetail.boiler_flue_type / fan_flue_present / central_heating_pump_age
+    / main_heating_index_number  (the last is the PCDB efficiency index)
+  * SapHeating.cylinder_thermostat            — gates the water-heating ×1.3 penalty
+                                                 and `recommend_cylinder_thermostat`
+  * SapHeating.cylinder_insulation_thickness  — drives the cylinder-loss cascade
+
+The 17.0 schema lodges all of them (see RdSapSchema17_0), so the hardcode silently
+degraded both the SAP water-heating figure and the hot-water recommendations for
+17.0 certs. This guard makes sure the block stays mapped. (The `secondary_*_type`
+pair of the same block is covered by its own dedicated secondary-heating test.)
+"""
+
+from __future__ import annotations
+
+import json
+from pathlib import Path
+from typing import Any
+
+from datatypes.epc.domain.mapper import EpcPropertyDataMapper
+
+# A real RdSAP-Schema-17.0 cert that lodges a cylinder thermostat, a measured
+# cylinder-insulation thickness and flue/pump fields on its main heating.
+_FIXTURE = Path("tests/fixtures/epc_prediction/PE71NT/cert-26619d8e7f8e.json")
+
+# A PCDB main-heating index the fixture happens not to carry; injected to prove
+# the field flows through (it used to be dropped to None unconditionally).
+_MAIN_HEATING_INDEX = 4321
+
+
+def _load_17_0_cert() -> dict[str, Any]:
+    data: dict[str, Any] = json.loads(_FIXTURE.read_text())
+    assert data["schema_type"] == "RdSAP-Schema-17.0"
+    return data
+
+
+def test_17_0_path_preserves_lodged_cylinder_fields() -> None:
+    # Arrange — the real cert lodges a cylinder thermostat + measured thickness.
+    data = _load_17_0_cert()
+    assert data["sap_heating"]["cylinder_thermostat"] == "Y"
+    assert data["sap_heating"]["cylinder_insulation_thickness"] == 25
+
+    # Act
+    epc = EpcPropertyDataMapper.from_api_response(data)
+
+    # Assert — both survive the 17.0 path now (they used to be dropped to None),
+    # so the water-heating worksheet and cylinder-thermostat recommendation see
+    # the real dwelling.
+    assert epc.sap_heating.cylinder_thermostat == "Y"
+    assert epc.sap_heating.cylinder_insulation_thickness_mm == 25
+
+
+def test_17_0_path_preserves_lodged_main_heating_detail_fields() -> None:
+    # Arrange — the fixture lodges flue/pump on the main heating; inject a PCDB
+    # index (absent on this cert) to cover the efficiency-lookup field too.
+    data = _load_17_0_cert()
+    detail = data["sap_heating"]["main_heating_details"][0]
+    assert detail["boiler_flue_type"] == 2
+    assert detail["fan_flue_present"] == "Y"
+    assert detail["central_heating_pump_age"] == 1
+    detail["main_heating_index_number"] = _MAIN_HEATING_INDEX
+
+    # Act
+    epc = EpcPropertyDataMapper.from_api_response(data)
+
+    # Assert — the whole MainHeatingDetail block carries through (was all None).
+    mapped = epc.sap_heating.main_heating_details[0]
+    assert mapped.boiler_flue_type == 2
+    assert mapped.fan_flue_present is True
+    assert mapped.central_heating_pump_age == 1
+    assert mapped.main_heating_index_number == _MAIN_HEATING_INDEX