BoilerFaultCodesTechnical description
Return temperature > flow temperature + 40 K (60 K for solar).
What causes this fault?
This fault occurs when the boiler detects that the water coming back from your radiators is much hotter than the water it just sent out, which shouldn't happen during normal operation. The most common reason is a lack of water flow caused by a blockage, trapped air, or a failing pump, but it can also be triggered if the electronic sensors that measure temperature become faulty and provide 'garbage' readings to the control board.
This fault is more common in autumn when heating systems are turned back on after a long summer break, as internal pump parts or radiator valves can seize up from disuse.
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.