BoilerFaultCodesTechnical description
Delta T (Flow - Return) too large
What causes this fault?
This fault occurs when water is unable to move through the boiler fast enough to carry heat away, causing a dangerous temperature gap between the outgoing and incoming pipes. The most common reasons are a failing internal pump, a build-up of sludge blocking the pipework, or air trapped within the heat exchanger. Essentially, the boiler is producing heat but has nowhere to send it, so it shuts down to prevent internal damage.
This fault is more frequent in autumn when the heating is first turned on after summer, as the pump can seize or 'stick' during the months 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.