diff --git a/domain/sap10_calculator/rdsap/cert_to_inputs.py b/domain/sap10_calculator/rdsap/cert_to_inputs.py
index dd22c0f0..dd32a9d3 100644
--- a/domain/sap10_calculator/rdsap/cert_to_inputs.py
+++ b/domain/sap10_calculator/rdsap/cert_to_inputs.py
@@ -556,6 +556,38 @@ def _first_main_heating(epc: EpcPropertyData) -> Optional[MainHeatingDetail]:
     return details[0] if details else None
 
 
+# Elmhurst RdSAP water-heating codes that route DHW to a non-Main-1
+# system. RdSAP code 914 = "from second main system" — DHW is
+# serviced by Main 2 (typically a gas combi providing DHW only) while
+# Main 1 handles space heat (e.g. cert 000565: HP Main 1 + gas combi
+# Main 2 + WHC 914). The water-heating cascade reads Main 2's PCDB
+# record / SAP code / fuel when this routing applies.
+_WATER_FROM_SECOND_MAIN_CODES: Final[frozenset[int]] = frozenset({914})
+
+
+def _water_heating_main(
+    epc: EpcPropertyData,
+) -> Optional[MainHeatingDetail]:
+    """The `MainHeatingDetail` that services DHW per the cert's
+    `water_heating_code` routing. WHC 914 ("from second main system")
+    returns Main 2 when present; otherwise returns Main 1.
+
+    The water-heating cascade (Table 4a / Appendix D2.1 summer
+    efficiency, water-heating fuel cost / CO2 / PE) keys off this
+    helper rather than `_first_main_heating` so the right system's
+    efficiency and fuel propagate to DHW.
+    """
+    details = epc.sap_heating.main_heating_details if epc.sap_heating else []
+    if not details:
+        return None
+    if (
+        epc.sap_heating.water_heating_code in _WATER_FROM_SECOND_MAIN_CODES
+        and len(details) >= 2
+    ):
+        return details[1]
+    return details[0]
+
+
 def _main_heating_efficiency(epc: EpcPropertyData) -> float:
     """SAP 10.2 (206) main system 1 efficiency as a 0..1 fraction.
 
@@ -2877,14 +2909,24 @@ def cert_to_inputs(
     # `main_fuel_kwh = q_useful × DLF = q_generated`, matching the spec's
     # "unit prices per kWh of heat generated" convention.
     eff = _main_heating_efficiency(epc)
-    if pcdb_main is not None and pcdb_main.summer_efficiency_pct is not None:
-        water_eff = pcdb_main.summer_efficiency_pct / 100.0
+    # Water-heating efficiency reads from the main that ACTUALLY services
+    # DHW per the cert's `water_heating_code` routing (Elmhurst WHC 914
+    # = "from second main system" → Main 2). For single-main certs and
+    # WHC 901/902 this resolves to Main 1, matching the prior behaviour.
+    water_main = _water_heating_main(epc)
+    water_pcdb_main = (
+        gas_oil_boiler_record(water_main.main_heating_index_number)
+        if water_main is not None and water_main.main_heating_index_number is not None
+        else None
+    )
+    if water_pcdb_main is not None and water_pcdb_main.summer_efficiency_pct is not None:
+        water_eff = water_pcdb_main.summer_efficiency_pct / 100.0
     else:
         water_eff = _water_efficiency_with_category_inherit(
             water_heating_code=epc.sap_heating.water_heating_code,
-            main_code=main_code,
-            main_category=main_category,
-            main_fuel=main_fuel,
+            main_code=water_main.sap_main_heating_code if water_main is not None else None,
+            main_category=water_main.main_heating_category if water_main is not None else None,
+            main_fuel=_main_fuel_code(water_main),
         )
     # SAP 10.2 Appendix N3.6 + N3.7(a) — when an HP cert lodges a PCDB
     # Table 362 record, the cascade replaces the Table 4a defaults with