BoilerFaultCodesYour boiler has detected that water is returning much colder than it left, usually because it isn't moving through your radiators properly.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical 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.
DIY fix possible
Check that all radiator valves (TRVs) are fully openEnsure your central heating pump is running and hasn't seizedBleed all radiators to remove trapped air that might be blocking water flow
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Protect yourself from future repair bills
A boiler cover plan would cover faults like E11 — no call-out fees, no surprises.
Affiliate links — we may earn a commission at no extra cost to you.
Remeha Avanta 18V is discontinued — time for a new one?
Parts for discontinued models can be hard to find and expensive. A new A-rated boiler could save you money on both repairs and energy bills.
Affiliate links — we may earn a commission at no extra cost to you.