Flashing LED on thermostat

[john4703]

I’ve just rewired a central heating system in a church. There are 4 zones, each with a motorised valve.

I used Sangamo electronic room thermostats. I installed 4 of them. They all work well but on two of them the LED flashes most of the time. The LED flashes when there is no power to the thermostat. I’ve wired them in this way. Main supply then time switch then thermostat then relay then boiler and pump and motorised valve.

Once the time switch supplies power to the thermostat the led becomes continuous until the room reaches the right temperature and then goes out (exactly what I would expect).

Has anyone any idea why on two of them the LED flashes when there is no supply to the stat. They are all wired using 6243Y 3 core and earth cable.

 

[stem]

I seem to remember hearing of a similar situation some years ago, oddly enough also in a church with a rogue flashing LED on some electrical equipment (sound equipment though, not heating related) from memory I seem to think that it had something to do with the building lights being wired in single cable rather than twin, and it turned out that the single cables were inducing a voltage on the wires to the equipment, unlike twin cables that contain both L & N which cancel the effects of induction.

You could try turning off all of the other circuits at the consumer unit, except the heating and see if it stops.