BoilerFaultCodesTechnical description
ALTERNATING 'H' '3' GO TO FRAME 92
What causes this fault?
This fault occurs when the water inside the heat exchanger or the pipework gets too hot, triggering a safety shut-off to protect the boiler. The most common reasons are a failed pump that isn't moving water through the system fast enough, or a blockage caused by a build-up of sludge and debris. It can also be caused by a sensor that has become faulty and is sending the wrong temperature reading to the main control board.
This fault is often seen at the start of winter when the heating is turned on for the first time after summer, as pumps can seize up during long periods of inactivity.
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.