BoilerFaultCodesWhat the manual says
Manufacturer manual270:exc temp diff h’exch Excessive max temperature difference across the heat exchanger during 5 minutes or excessive limit temperature difference. If temperature difference fell below (threshold - switching differential boiler): automatic reset
From this model's manual
Verified against the Navien/Navien Ncm 1130hh installation & service manual.
Engineer required
This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Related fault codes
All 77 fault codes on the NCM 1130HH
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