diff --git a/domain/sap10_calculator/rdsap/cert_to_inputs.py b/domain/sap10_calculator/rdsap/cert_to_inputs.py
index dd32a9d3..a873e5f9 100644
--- a/domain/sap10_calculator/rdsap/cert_to_inputs.py
+++ b/domain/sap10_calculator/rdsap/cert_to_inputs.py
@@ -588,6 +588,23 @@ def _water_heating_main(
     return details[0]
 
 
+def _water_heating_fuel_code(epc: EpcPropertyData) -> Optional[int]:
+    """Fuel code for water heating per the cert's WHC routing. Prefers
+    an explicitly-lodged `water_heating_fuel`; otherwise falls back to
+    the fuel of whichever main system services DHW (Main 2 for WHC
+    914, Main 1 otherwise — see `_water_heating_main`).
+
+    Replaces the pattern `epc.sap_heating.water_heating_fuel or
+    main_fuel` that defaulted to Main 1 unconditionally; for cert
+    000565 the explicit fuel is None and Main 1 is a heat pump with
+    no fuel_type lodged, so the old fallback resolved to None and CO2/
+    PE/cost lookups returned defaults instead of the gas-combi values.
+    """
+    if epc.sap_heating.water_heating_fuel:
+        return epc.sap_heating.water_heating_fuel
+    return _main_fuel_code(_water_heating_main(epc))
+
+
 def _main_heating_efficiency(epc: EpcPropertyData) -> float:
     """SAP 10.2 (206) main system 1 efficiency as a 0..1 fraction.
 
@@ -1605,7 +1622,7 @@ def environmental_section_from_cert(
     main = _first_main_heating(epc)
     main_fuel = _main_fuel_code(main)
     main_factor = co2_factor_kg_per_kwh(main_fuel)
-    water_fuel = epc.sap_heating.water_heating_fuel or main_fuel
+    water_fuel = _water_heating_fuel_code(epc)
     water_factor = co2_factor_kg_per_kwh(water_fuel)
 
     # Compute per-end-use CO2. For electricity end-uses, monthly Table 12d
@@ -1712,7 +1729,7 @@ def primary_energy_section_from_cert(
     main = _first_main_heating(epc)
     main_fuel = _main_fuel_code(main)
     main_pe = primary_energy_factor(main_fuel)
-    water_fuel = epc.sap_heating.water_heating_fuel or main_fuel
+    water_fuel = _water_heating_fuel_code(epc)
     water_pe = primary_energy_factor(water_fuel)
     main_1 = er.main_1_fuel_kwh_per_yr * main_pe
     main_2 = er.main_2_fuel_kwh_per_yr * main_pe
@@ -2765,7 +2782,7 @@ def _fuel_cost(
         return _ZERO_FUEL_COST_FOR_OFF_PEAK
 
     main_fuel_code = _main_fuel_code(main)
-    water_heating_fuel_code = epc.sap_heating.water_heating_fuel
+    water_heating_fuel_code = _water_heating_fuel_code(epc)
 
     # Std electricity for all single-row end-uses (pumps/fans, lighting,
     # cooling). Table 32 code 30.
@@ -2776,7 +2793,7 @@ def _fuel_cost(
         table_32_unit_price_p_per_kwh(main_fuel_code) * _PENCE_TO_GBP
     )
     water_high_rate_gbp_per_kwh = (
-        table_32_unit_price_p_per_kwh(water_heating_fuel_code or main_fuel_code)
+        table_32_unit_price_p_per_kwh(water_heating_fuel_code)
         * _PENCE_TO_GBP
     )
     # Secondary fuel cost: route through the cert's `secondary_fuel_type`
@@ -3286,7 +3303,7 @@ def cert_to_inputs(
             _STANDARD_ELECTRICITY_FUEL_CODE,
         ),
         hot_water_co2_factor_kg_per_kwh=co2_factor_kg_per_kwh(
-            epc.sap_heating.water_heating_fuel or main_fuel
+            _water_heating_fuel_code(epc)
         ),
         pumps_fans_co2_factor_kg_per_kwh=_effective_monthly_co2_factor(
             _days_in_month_proportioned(pumps_fans_kwh, _DAYS_IN_MONTH),
@@ -3346,7 +3363,7 @@ def cert_to_inputs(
         ),
         space_heating_primary_factor=primary_energy_factor(main_fuel),
         hot_water_primary_factor=primary_energy_factor(
-            epc.sap_heating.water_heating_fuel or main_fuel
+            _water_heating_fuel_code(epc)
         ),
         other_primary_factor=primary_energy_factor(30),  # standard electricity
         # SAP 10.2 Table 12e (p.195) per-end-use effective PE factors. Same