Glow-worm Control stuck on / thermostat failure

Furnace runs continuously and overheats the house, even when thermostat is turned down

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

Stuck relay on Glow-worm PCB, Room thermostat contacts welded closed, Faulty external programmer, Incorrect wiring bypassing controls

Safety: Overheating can stress the system. Turn down the boiler flow temperature as a temporary measure.

User steps:

Turn the room thermostat fully down and set programmer to OFF; if the furnace continues to run, switch off at the isolator.

Technician steps:

Test room thermostat and programmer outputs; replace if stuck on.
Check wiring for any permanent links across call-for-heat terminals.
Inspect PCB relays controlling the burner and pump; replace PCB if relays are welded closed.

Hard 4/5

Room Thermostat / Main PCB

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