Advice on Wiring for Nest 3rd Gen Learning Thermostat

[stem]

Excellent that's very helpful, I think we are getting somewhere now. I can confirm that the existing room thermostat cable is the third from the left connected to wiring centre terminals 2 (Blue), 3 (Yellow) and 4 (Red). If you can just physically check that first, that would be great.

If the above is correct, we have correctly identified the thermostat cable, and you can proceed to wire the Heatlink as per your last post above.

To decommission the old thermostat, remove the cable completely, and move the yellow wire from the MOMO valve presently in 3, to terminal 4 where red wire to the thermostat came from.

Once this cable is removed from the wiring centre you could use it to connect the Heatlink (if you can extend it there) to the Nest Thermostat. T1 & T2

What was confusing me earlier is the MOMO open and closed signal wires, I have found more information on line and found that Yellow is open and White closed. The opposite way around to what I originally had thought. This was confusing me until you explained where the MOMO was connected.

 

[AlphaLimaGolf]

If you can just physically check that first, that would be great.

Checked, and that is all correct.

I understand moving the yellow cable from MoMo (terminal 3) and putting it in terminal 4, but where you say 'To decommission the old thermostat, remove the cable completely', which cable do you mean? Do you mane the red cable in terminal 4? Just remove it and blank it off?

 

[stem]

Disconnect the third cable from the left (the one going to the existing room thermostat) by disconnecting all of its wires completely and remove it from the wiring centre.

Move the yellow wire from the MoMo Valve from terminal 3 to 4

 

[AlphaLimaGolf]

Got ya. Think that is everything.

Really appreciate the time and help you have given me.

 

[AlphaLimaGolf]

One final question. The current programmer, has 2 earth wires attached to it (see attachment). The new Heat Link doesn't have a place for earths, can I just blank these off, or do they need to be earthed somewhere?

 

[stem]

Heatlink earth terminal third from right.

Post back how you get on, you never know someone else with a MOMO valve may find this post in the future.

 

[AlphaLimaGolf]

Heatlink earth terminal third from right.

Not sure how I missed that, it's been a long day!

Post back how you get on, you never know someone else with a MOMO valve may find this post in the future.

Yes, I will do. As I can't extend the thermostat wires down to the kitchen (where the boiler and current programmer are), at least not without lifting floorboards etc, my plan is to move the Heat Link to the upstairs airing cupboard so I can connect up the thermostat wires from there (t1 and t2). Am fairly sure I know which cables (going to test the properly first) run down to the programmer and need to be cut short so they terminate upstairs where I want to put the Heat Link.

The only other option is to run a 12V DC converter where the thermostat is currently wired in to the 230v and use this to send 12V down to the Nest Thermostat. I presume the Nest doesn't care if it gets 12V from somewhere other than the t1 and t2 terminals on the Heat Link.

Will hopefully get it done later in the week or next weekend and will update then.

 

[AlphaLimaGolf]

All installed

Took me about 2 hours to do.

First I tested the Heat Link connected in the original programmer's position and the Thermostat powered by USB cable. Once confirmed hot water and central heating worked fine, I then traced the wires up to the airing cupboard, cut them here and wired up the Heat Link and it now sits next to the wiring centre all tucked away.

I then moved the yellow MOMO cable from terminal 3 to 4 as you explained and removed the thermostat cable. Finally I used two of the thermostat wires in the T1 and T2 terminals of the Heat Link, and used these to power the Nest Thermosat which is now sat nicely on the wall where the old stat used to be (see image below). I had to use the included blanking plate to cover the unwallpapered area left visible form the old stat.

All in all, very pleased and a big thanks to Stem for all his help and advice. Having everything figured out in advance made it a breeze, I just took my time and followed the steps required.

 

[AlphaLimaGolf]

Although the installation went well, I have noticed an issue with the Nest.

Central heating works fine and the boost for hot water works fine too, but the hot water is not being switched on when scheduled.

If I set it to come on from 4-5pm it shows a message that next hot water time is at 4pm, then when I check at 4.10pm it is not on and now says it is scheduled to come on at 5pm (the off time). At 5pm nothing happens and now it says it is next scheduled to come on the next morning, which again it doesn’t.

Seems like a software problem with the Nest, as it heats the water when I tell it to boost it. Has anyone else had this issue.

 

[AlphaLimaGolf]

OK, finally found the issue.

Although it was scheduled to come on, the hot water was not set to ‘on’. There was a setting hidden within the app to switch it on.

You’d think it would tell you there is another setting, not just the timer.