Honeywell ST9400

[keldar]

My Honeywell St9400 stopped working and I can't get the replacement to work.

Stupidly I forgot to take a picture of the wires before and one of the wires has slipped out!!!! Currently struggling to get a plumber to come due to demand so need some advice:

1. If i Plug the spare wire into 1 the boiler calls and turns on central heating - but if I put heating on the boiler goes off.
2. If i Plug the spare wire into 2 the boiler calls and turns off hot water - but if I put central heating on the boiler goes off.
3. If i leave the wire free nothing happens.

Attach a photo and any advice is appreciated. Not sure how to tell if I have an s, y or w plan heating system.

Cheers in advance

 

[Hugh Jaleak]

Don't rely on guesswork, you need to trace the loose wire back, and find out where it goes. Wiring diagram tells you what needs to connect where, but with no idea of what the other end is connected to you risk blowing something expensive.

Also, ensure power is switched off before removing/refitting programmer, or you risk blowing that as well.

 

[RandomGrinch]

My Honeywell St9400 stopped working and I can't get the replacement to work.

and one of the wires has slipped out!!!!

After you changed the controller, and before the wire slipped out - did the controller work correctly, or are we chasing an earlier issue?

It appears as if it could be a Y plan system - do you have a 3 port valve?

It would be very unlikely for any wire to be connected to terminal 2.

 

[keldar]

Thanks for the reply, how do ai know if I have a 3 port valve

 

[RandomGrinch]

Thanks for the reply, how do ai know if I have a 3 port valve

Well...
It's a valve and it has three ports on it!

 

[keldar]

Yes I have one of those.

It’s very strange that plugging the wire in 1 or 2 makes things work

 

[RandomGrinch]

Yes I have one of those.

It’s very strange that plugging the wire in 1 or 2 makes things work

With a Y plan there should be wires in terminals 1,, 3 and 4 of the controller.
And as it doesn't work with the loose wire in 1, that leads to the question...

After you changed the controller, and before the wire slipped out - did the controller work correctly, or are we chasing an earlier issue?

Y plan explainer here...

Y Plan central heating system

Y-plan central heating system operation and wiring diagram

flameport.com

 

[keldar]

Yes it works in 1, but the hot water turns off. Might be problem with the 3 port switch

 

[keldar]

Turns out the wire was needed in live!!! Works okay for now lol