My boiler has suddenly stopped receiving signals from the external switch. (models listed below).

I'm trying to isolate the faulty component. There is no fault code on the boiler. It seems to be simply waiting for a demand. Water heating is fine and not operated by the switch. I'm able to get emergency heat (by pressing +/- buttons together).

The external unit is a wired, wall-mounted time switch. It appears to be functioning, in so far as it has power and lights up when the switch is supposedly asking the boiler to fire. (CH ON)

I tried to eliminate a fault in the switch by manually jumping it. I removed it from the wall and connected the live to the "CH ON" terminal. No change. Did I do this correctly? The wiring looks like below. I connected 4 -> L (1 and 2 are not connected)

Siemens wiring

Further to the switch there are two motorised zone valves operated by remote thermostats. The remotes appear to be paired and working.

I tried to eliminate the valves by manually opening them. This works with the emergency heat boost, but doesn't affect the lack of signal from the switch.

I'm now contemplating whether to restore the control wiring to its factory state which the manual says was a link between two terminals on the PCB. Does this sound like a good idea?

What other things can I try to identify the fault?

  • Boiler: Valliant EcoTEC plus 937
  • Timer/Switch: Siemens RWB29
  • Thermostats: Siemens RDH100RF with RCR100/433 receivers.
  • Motorised values: Honeywell V4043H1056

1 Answer 1


I managed to isolate the fault down to the RF receivers. Both on the blink. Receiving ON signal but not opening the switches to their respective valves.

To do this I uninstalled both receivers and manually opened the valves, such that I had a basic setup of just the main time switch and no thermostat. Everything working.

Then I took one of the RF receivers and installed it in place of the time switch, using its remote as my room thermostat and timer. Intermittently the receiver failed to open its switch to the boiler. Giving the unit a whack tended to fix it!

So now I know which part was faulty.

  • 1
    Thanks for following up with your answer. For the future, a digital multimeter is an invaluable tool to have around for situations like these. Even a cheap one will allow you to diagnose these types of things pretty easily. Commented Feb 3, 2021 at 14:55
  • 1
    Please click the check mark as soon as the system will allow you to so others know this has been resolved.
    – FreeMan
    Commented Feb 3, 2021 at 15:39

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