Boiler shows F.91, hot water temperature control unstable on combi models

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

Sticking DHW flow turbine, Limescale or debris in DHW inlet, Faulty DHW flow sensor, Wiring fault to sensor

Safety: Isolate power and water supply before removing any DHW components.

Clean inlet filter: On many Vaillant combis, there is a filter at the cold water inlet. Isolate cold supply, remove and clean the filter mesh.
Check flow rate: Ensure adequate mains water pressure and flow to the boiler.
Engineer service: A technician can remove and clean or replace the DHW flow turbine/sensor assembly if it is sticking due to scale.

Medium 3/5

DHW Flow Sensor / Turbine

Check Price on Amazon

We may earn a commission from links on this page.