Honeywell ST6400C Programmer reboots itself when CH+HW programs turn on

[rumbln]

Hi - I have inherited an old ST6400C when we moved in 7 years ago. I'd guess its 15 years old looking at the agr of the boiler etc. Generally been working OK but I recently replaced the mechanical dial wired thermostat ( see pic ) with a meross mts200b ( see pic ).

Since then ( it may only be a coincident ), fairly regularly, but not all the time, the programmable timer (ST6400C) will just reboot ( continuously) when CH+HW go live.

I believe the mts200b wiring is correct - the switched live goes live when the thermostat drops below temperature.

Have a just got a dodgy ST6400C or is my wiring of the thermostat causing it?

Any help greatly appreciated!

Thanks

 

[Stuckinarut]

If it’s worked before you changed the wiring, then it’s suggests this could be the fault. Have you tried putting the Honeywell T6360 back in to confirm? I think your Meross needs a permanent live and permanent neutral. Your dial stat (T6360) could have been wired incorrectly/back to front too.

 

[rumbln]

Ah good points Stuckinarut,

Unfortunately the T6360 was thrown away. The meross unit does require a permanent live but I've turned the CH on permanently so I'm assuming that effectively the same thing. The wires may be connected wrongky but I ran a multimeter over N,L and S/L and all behaved as I expected.

I was wondering if I could take the meross unit out of the equation and replacing it with a wiring block I could discount the meross unit ?

 

[Stuckinarut]

I was wondering if I could take the meross unit out of the equation and replacing it with a wiring block I could discount the meross unit ?

That would work.

 

[RandomGrinch]

is my wiring of the thermostat causing it?

Your wiring to the Meross appears to be correct.

I was wondering if I could take the meross unit out of the equation and replacing it with a wiring block I could discount the meross unit ?

Personally, I would take the ST6400C out of the equation.
It already has the C/H turned to permanently on; why not bypass it completely?
If you can show us the wiring behind the ST6400C, we can advise you how to bridge it out.

 

[rumbln]

Thanks Stuckinarut, I think I've isolated the problem to the ST6400C. Whilst this was continuously rebooting I disconnected all the wiring of the meross - S/L is no longer live. The rebooting continued. I then turned off the HW on the ST6400C and it stopped rebooting - leaving the CH circuit still on but not drawing any heat from the boiler as I had disconnected the meross. I then connected the L and S/L that had previously been connected to the meross and the boiler started to draw heat.

In my mind this proves there's a fault with the ST6400C.

Is this sound logic?

 

[Stuckinarut]

Sounds good to me and good bit of fault finding.

 

[rumbln]

Stuckinarut I would like to permanently connect the merros and bypass the CH timer - here's the backplate of the ST6400C- can you explain the changes I need please ?View attachment 363738

 

[Stuckinarut]

Stuckinarut I would like to permanently connect the merros and bypass the CH timer - here's the backplate of the ST6400C- can you explain the changes I need please ?View attachment 363738

Attachment link doesn’t work. Usually just a case of linking the live and switched live. Are you keeping the hot water side of it?

 

[rumbln]

Thanks - yes - planning to keep th HW.

 

[RandomGrinch]

Thanks - yes - planning to keep th HW.

View attachment 364007

With the power safely isolated; either move the red wire in terminal 4 of the wiring centre to join all the other wires in terminal 1.
Or (probably a little simpler), move the grey wire from terminal 4 on the ST6400C backplate to join with the Brown wire in the L terminal.