BoilerFaultCodesTechnical description
Delta T (Return - Flow) too large
What causes this fault?
This fault occurs when the water isn't flowing through your heating system fast enough, causing a large temperature gap between the outgoing and incoming pipes. The most common reasons are a failing internal pump that can't push the water around or a blockage in the system, such as a build-up of sludge or a closed valve. When the boiler detects this imbalance, it shuts down to prevent the heat exchanger from overheating.
This issue often comes to light during the first cold spell of autumn when the heating is turned on after being dormant all summer, as pumps are prone to seizing 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.