BoilerFaultCodesTechnical description
Different flow/return temperatures
What causes this fault?
This fault occurs when the boiler detects that heat is not being carried away from the unit quickly enough, often due to a pump failure or a blockage in the pipework. It physically happens because the water exiting the boiler is significantly hotter than the water coming back, suggesting the flow has slowed down or stopped entirely. The most common reasons are a seized circulation pump or a buildup of sludge and debris within the main heat exchanger.
This issue often surfaces at the start of autumn when heating systems are turned back on after sitting idle all summer, which can cause internal pump components to seize.
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.