Updating Honeywell ST6100A to ST9400C

[mmigs]

Hi all,

Hoping someone may be able to help. My old St6100A was looking a bit battered so I wanted to replace it. Naively bought the ST9400C because same brand, same back plate, thought I might just be able to replace the unit and not the back plate. Unplug one and plug in the other.

My setup was this - I had the ST6100A set to continuous CH, with a Nest E thermostat then managing the temperature of the house.

I tried replacing as per the above and the unit itself works fine, however the Nest E is seemingly no longer in control of the heating when in continuous heating mode.

Is there something that I need to do with the wiring? Have uploaded photos of the existing wiring and both units diagrams

Any help much appreciated.

 

[CBW]

It appears your ST6100A is a single channel timer and your replacement ST9400C is a twin channel heating and hot water programmer. It’s probably best for you to swap for a single channel timer, otherwise it would need a few alterations and a non operational hot water function

 

[RandomGrinch]

Hopefully your system isn't using zero voltage switching (it's probably mains switching) - your previous controller was connecting terminals 1 and 4 together, and using terminal 3 as a spare terminal (probably to link in the Nest E).
Putting the new controller on the backplate could have energised any of those terminals with mains voltage (depending on your choice of setting), hence giving you some weird results!

Don't put the ST9400C back on the baseplate.

However, and this is the big question - why do you need a controller, if you have the Nest E?

Just put the wire currently in terminal 1, into terminal 4 (or vice versa), and the ST6100A should be completely linked out. Or, trace the wires back and remove the ST6100A completely.

 

[ericmark]

If you look at the wiring for that back plate there are three versions

they all use the same back plate, but single channel is volt free, twin channel has off contacts needed with some systems, and a triple channel only has on controls. I am showing the Drayton wiser since that has all three versions, but hive and Honeywell use same back plates.

What I can't understand is with a single channel why was it left in place when Nest e was fitted. I did the same with my old house, the simple thermostat was replaced with a programmable thermostat when we moved to non tungsten lights, as without the radiant heat from the bulbs, we needed the air temperature to raise in the evening. And when fitted the time clock was removed.