Replacing stc9004c and Digistat SCR

7WD

Need advice on how to replace the STC9004C and Digistat SCR currently connected to my system through the Honeywell Sundial wiring system.
I did replace the stc9004C and the hot water worked but not the CH, as I feel wiring available from.Digistat ScR needs to be connected to Tado, but not sure how to as did t wanted to short any systems in the cold weather conditions.
Need Tado help here...

wateroakley

@7WD Tado should give you wiring instructions when you start the install. Replacing the Honeywell ST9400C with a Tado Wireless Receiver should be a straightforward swap of the wiring from the ST9400C backplate onto the Tado Wireless Receiver. You'll probably need to add two purple links (in the starter kit) from 'L' to CH 'COM' and HW 'COM'.

You should not need to fiddle around the inside the Honeywell Sundial wiring box. FYI, if you do inadvertently short something out, the Honeywell box has a 5x20mm 3A fuse inside - it's not the standard size you'll find in a 13A household plug.

Looking at the Drayton documents ... the Digistat SCR (wired version) should be a link (use a Wago connector in the Tado starter kit) between the wire on 'COM' to the wire on 'On' (call for heat). For the Digistat RF wireless version, link the receiver wires on 1 and 3 with a Wago connector. If you've not touched the Digistat, you should be able to set it to maximum to 'call for heat'. https://www.draytoncontrols.co.uk/sites/default/files/Digistat%2B%20and%20Digistat%2BRF%20Installation%20Guide.pdf

7WD

Thanks. But I know I don't need to or shouldn't require to touch the Sundial wiring. BTW my sundial has only one fuse.

Having replaced the st4900c as per instructions, my CH won't work as the Digistat ScR will need to be isolated as the wires connect to the CH valve via the sundial wiring. So I believe the wires from to scr back plate need to be extended to the Tado receiver and that's the diagram or instructions I am looking for. Is that what you tried to tell me using the Wago connector?

Currently instructions are only for either the stc9004C replacement or SCR replacement Individually and not replacing both together.
Currently I have replaced the scr only with the tado receiver but want to replace the stc9004c also as unable to use the HW control feature...and if I replace stc9004C only there is no valve control for CH.

Would be great if I can get help fromm Tado.

wateroakley

For help, it's best to to post pictures and details of your existing setup and wiring.

Assuming the wiring is S-Plan or Y-Plan ... you'll need to rewire both the controller and room stat ...

A) Take a photo of the ST9400C backplate and wiring. Label the wires: L; N; HW_ON; CH_ON; Earth. B) replace the ST9400C Backplate with the wireless receiver. C) Connect the ST9400C Backplate wires to the new wireless receiver connections as follows: ST9400C Backplate N to wireless receiver N; L to L; 3 (HW ON) to HW NO; 4 (CH ON) to CH NO. Earth to the Earth terminal. Add two purple wire links on the receiver: from L to CH COM and to HW COM

D) Join two wires on the Digistat SCR connections, COM and On, with the Wago connector. If you've got an N wire, terminate it safely. That could be leaving the N wire on the SCR terminal.

If in any doubt, best to consult a professional.

7WD

Thanks @wateroakley. I also have live on that plate which I feel I will need to terminate.

In the Digistat SCR, wires are connected on N, L, 1 & 3 which are going into the sundial wiring where the L is to the Boiler connector on the sundial and the other to the Room STAT. See the below picture. The four wires are connected within the Honeywell Sundial.

Are you suggesting me terminate safely Blue, and use the wago connector to connect the grey and black? And then of course connect the STC9400C as in the Tado doc while leaving only the plate of the SCR safely.Also terminate Brown safely as blue.

Correct me if I am wrong.

If I do that then the COM will be normally closed with Heat on...giving a continuous signal to the valve via the Honeywell sundial to remain on; won't it?

Appreciate your patience in understanding my needs.

wateroakley

@7WD assuming you have the digitstat rf version …

on the scr receiver, terminate the L and N safely. That could be to leave the L and N wires on the scr terminals.

Yes, join the wires on 1 and 3 with a wago connector. That means the thermostat is always ‘calling for heat’.

Whenever the Tado app schedule tells the wireless receiver (that replaced the ST9400) to ‘call for heat’ thru the existing sundial wiring, the boiler/pump should fire up.

7WD

Hi @wateroakley

My Heating works now and able to control. But my hot water is not able to be controlled or even the test for hot water doesn't work?

Any insights?

Monit

7WD

The test seems to work, but triggering Hot water ON/OFF from the Tado Temperature Sensor or the APP doesn't.

Strangely. DO I need to sync anything?

7WD

OK...all working now. Had to reset the Tado receiver.

Thanks for you help.

wateroakley

Glad to hear it’s all working now.