F.90
FAULT CODE
Vaillant
Water Flow Rate Too Low
Medium Severity
Engineer Required
Insufficient water flow through the primary heat exchanger — caused by an airlocked or undersized pump, closed valves, or heavy sludge deposits.
Step-by-Step Fix Guide
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Step 1
Check system pressure and bleed radiators
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Step 2
Ensure all radiator valves and zone valves are open
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Step 3 Engineer only
If the fault persists after bleeding, a Gas Safe engineer should check pump flow rate and system design
Gas Safe Reminder
Work on gas appliances must be carried out by a Gas Safe registered engineer. The steps above cover safe checks a homeowner can perform - any internal component replacement requires a qualified professional. If in doubt, always call an engineer.