Saunier Duval
Boiler Firing Without Thermostat Control
Boiler runs continuously and overheats rooms even when thermostat is set low or off
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Safety Warning
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.
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Possible Causes
Room thermostat terminals bridged permanently at boiler, Stuck relay in external RF receiver, Thermostat configured in manual override or frost mode, Incorrect wiring bypassing thermostat
How to Fix / Troubleshooting
Safety: Turn off boiler power before accessing its terminals or any external receiver.
- Check thermostat mode: Ensure the thermostat is not in manual "ON" or party mode. Set it to automatic schedule or OFF and see if the boiler stops.
- Inspect boiler terminals: At the boiler, verify that the room thermostat contacts are not permanently bridged with a link wire if a smart thermostat is installed.
- Check RF receiver: If you have a Saunier Duval RF receiver, observe its status LED. If it shows permanent demand, reset the receiver by pressing the reset button as per manual.
- Test without thermostat: Temporarily disconnect the thermostat cable at the boiler. If the boiler still runs, the fault is in the boiler or internal wiring, not the thermostat.
If the boiler continues to fire with no demand signal, call a heating engineer; the boiler PCB or gas valve control may be faulty.
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Repair Difficulty
Hard
4/5
Required Part
Saunier Duval RF Receiver or Boiler Room Thermostat Link
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