BoilerFaultCodesGrant Flow Fault Code
Also written as FLOW — same fault.
The Grant Flow fault code means: your boiler is not detecting that a hot tap has been turned on, which is preventing it from heating your water.. It appears on 3 Grant models in our database.
Technical description: If the boiler is not lit when a hot tap is opened this could mean there is a lack of water flow or possible flow switch problem
What causes the Grant Flow fault?
This fault occurs when the boiler fails to detect water moving through the pipes when you turn on a hot tap. The most common reason is that the internal sensor used to trigger the burner has become stuck or damaged, preventing the boiler from knowing there is a demand for hot water. In some cases, a perished rubber diaphragm or a blockage within the unit is stopping the mechanical parts from moving correctly.
This fault is slightly more common in winter when incoming mains water is much colder, which can cause internal rubber components to become less flexible and more prone to failure.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Grant models that show the Flow fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Grant Flow fault code information on this page is sourced and cross-checked against the official Grant service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026