Salus st620RF flickering on/off when switched on

7 Dec 2023
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United Kingdom

I have a Salus ST620RF Thermostat which is used as an on/off switch for a water pump for our radiator water loop when the water is being heated.

Normal behaviour is that we use it as a manual switch (Manual Setting) and manually flick it between on and off as we need the radiators running.

A few days ago it started "clicking" when switched on and the pump doesn't properly start, at the same time the bottom (On, Cycle, Off) button flickers between red and green, the top switch light (Manual/Automatic - set to Manual) is blue/purple. It flickers quickly for a while and the pump doesn't start, though it sounds like its starting to on each "flicker" as if the relay is engaging and "completing the circuit" very briefly before opening it again. This continues with the "flickering" slowing gradually until it stops, then the light stays green, and the pump comes on and proceeds to work correctly.

Video of it happening:

Theres nothing in the manual that covers this combination of lights, so my best guess (via my amateur understanding of electronics) is that the relay isn't firing properly, or that theres a problem on the device PCB which is causing a strange error state. I've opened the receiver and theres is nothing obviously failing inside. Very minor age related corrosion but nothing significant.

I very much suspect this is going to be a case of getting the thermostat/switch replaced, but I'm curious as to what this could be! It seems that this could effectively be replaced with a normal on off switch.
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