BoilerFaultCodesYour boiler has stopped working because the water isn't moving through the system fast enough, causing the internal temperature to rise too quickly.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Water circulation fault (Differential between flow and return thermistors too great)
What causes this fault?
This fault occurs when the boiler detects that water is heating up too quickly because it isn't being pushed around your radiators fast enough. The most common reasons are a pump that has seized or failed, a blockage caused by sludge in the pipework, or air trapped within the system preventing flow.
This issue is most common during the autumn when heating systems are turned back on after being idle all summer, which can cause internal pump parts to stick.
DIY fix possible
Check that all radiator valves are fully openEnsure the boiler pressure is between 1.0 and 1.5 barBleed all radiators to remove trapped air from the system
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 F23 — no call-out fees, no surprises.
Affiliate links — we may earn a commission at no extra cost to you.
Glow-worm Ultracom 24cx 30cx 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.