ATAG Return Temperature Rise Too Fast

Furnace displays F9, often with rapid cycling on and off

This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.

If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.

Possible Causes

Bypass valve stuck open, Very low system water volume, Incorrect system design causing short-cycling

Safety first: Turn off power before adjusting any internal valves.

Check that most radiators or heating circuits are open; do not run the boiler with only a tiny load.
Inspect any automatic bypass valve and ensure it is correctly set, not fully open.
If a system filter or balancing valves are present, verify they are not excessively restricting flow.
Persistent F9 may indicate a system design issue; consult a heating engineer to review pipe sizing and load.

Hard 4/5

ATAG automatic bypass valve

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