Advice on Wiring for Nest 3rd Gen Learning Thermostat

[AlphaLimaGolf]

Hello,

I am after some advice on the wiring for the Heat Link for the Nest 3rd Gen. I have a S system with two separate 2 port valves for hot water tank and central heating.

Please see attached photos for the wiring of my current programmer (acl Lifestyle Model LP522) and the S system wiring diagram for the nest.

As far as I can tell, I obviously need to put the live and neutral in the L and N ports on the Heat Link, then put the heating (currently port 4) into port 3 on the heat link and hot water (currently port 3) into port 6 on the heat link. Finally I need to bridge from live to port 2 and then to port 5 on the Nest Heat Link. Is this correct?

What is confusing me, is that there are currently two cables in the live port on my current programmer and a cable in port 2 (CH OFF).

Finally, I currently have a Sunvic TLX thermostat on the wall in the hall which I'd like to wire into ports T1 and T2 on the Heat Link, but have no idea where these cables come out. Maybe it is best to just power the Nest Thermostat by USB and let it connect wirelessly only.

If you need anymore info, please let me know.

Cheers
Andy

 

[stem]

As far as I can tell, I obviously need to put the live and neutral in the L and N ports on the Heat Link, then put the heating (currently port 4) into port 3 on the heat link and hot water (currently port 3) into port 6 on the heat link. Finally I need to bridge from live to port 2 and then to port 5 on the Nest Heat Link. Is this correct?

Yes that is all correct.

What is confusing me, is that there are currently two cables in the live port on my current programmer

That's fine, they both need to connect to the live terminal of the Heatlink. If it's difficult to get them both into L with the link to 2 & 5 also being there, put one of them in 5 as it is connected to the L via the links anyway.

and a cable in port 2 (CH OFF).

Very very unusual, only occasionally seen with an extremely rare type of motorised valve known as a MOMO. It may be that the terminal has been used just to park a redundant wire. But the only way to know for sure is to trace the other end and see where it goes. Or you could try disconnecting & isolating it to see if the system still works as it should, or even put it in the heating satisfied terminal (1) of the Heatlink which would be replicating exactly what you have now, and if it works now..... Having said that I would want to know what it actually did.

Finally, I currently have a Sunvic TLX thermostat on the wall in the hall which I'd like to wire into ports T1 and T2 on the Heat Link, but have no idea where these cables come out. Maybe it is best to just power the Nest Thermostat by USB and let it connect wirelessly only.

Because the Nest carries out the functions of the room thermostat, the old wired thermostat needs to be decommissioned properly to prevent it overriding the Nest. It can't simply be disconnected as this would leave the heating wiring 'open circuit' and it will not operate. The thermostat can be decommissioned in three ways, in order of preference (or professionalism) they are.

1. Find the origin of the thermostat cable and disconnect it. Then link the terminals where the two switching wires were connected to together

2. Remove the existing thermostat and replace it with a junction box, connect the two switching wires together and isolate the neutral (if there is one)

3. Leave the existing room thermostat in place and fix it so that it is set to the highest setting. (Or put both of the switching wires in 'L' terminal, and isolate the Neutral)

If you want to use the cable to connect T1 & T2 you will have to use method 1 and completely disconnect it from the mains wiring. before connecting it to T1 and T2 which are only 12v

 

[AlphaLimaGolf]

Thank you for the comprehensive reply.

The two 2 port valves I have are a Honeywell (hot water) and Sachwell (central heating). I think I will put the wire (CH OFF) in to terminal 1 of the heat link if that is the equivalent terminal.

For the Thermostat, I'd like to connect it to T1 and T2 and power the nest thermostat this way rather than use the USB cable provided. But I will need to use the current wiring there (not able to run new wires) to connect it to T1 and T2, the problem is finding where it currently terminates. Am not sure if it is at the boiler/programmer which is in the kitchen or at the junction box which is upstairs in the airing cupboard. From the diagram attached below it seems to suggest they will most likely terminate at the junction box in the airing cupboard (although in this diagram, this is also connected to port 4 of the programmer - CH ON).

Is this the right way to go about it?

Thanks

 

[stem]

Unfortunately, there are quite a few ways to wire up 'S Plan' systems that achieve the same result electrically speaking. The thermostat is generally wired back to the wiring centre (as per your posted drawing. The terminals numbered 1 to 10 represent the wiring centre) but not always. Sometimes it is wired directly from the programmer without going through the wiring centre, or a separate junction box may even be added elsewhere. It won't go to the boiler (the motorised valves control the boiler) It all depends upon the personal preferences of the original installer, any existing wiring that may have been used, and the layout of the various components in your property.

What I can tell you is that one of the thermostat wires will be eventually connected to the brown wire of the central heating motorised valve So, I usually trace it back from that.

If you want to post a photo of your wiring centre and indicate which is the cable that goes to the central heating motorised valve, I should be able to advise further.

 

[AlphaLimaGolf]

I have attached 2 images of the wiring centre below, but it has a lot more terminals than the standard 10. For the 2nd image I have drawn on where the wires terminate.

Cable 1 is hot water valve, 2 is central heating valve and 3 goes down to the kitchen where the boiler and programmer are.

All 3 red wires from these cables terminate in terminal 9, the two blue wires from the cables going to the HW and CW port valves terminate in the same unnumbered terminal.

I have a feeling this is too much of a wiring mess for you to make sense of. But thanks for taking a look.

 

[AlphaLimaGolf]

Am guessing the red cable which goes down to the kitchen where the boiler is switches the boiler on when either of the two 2 port valves are switched on, like you said above.

Think this also means it is going to be tricky to wire up the new nest thermostat using the old thermostat's wires.

I either need to move the new Heat Link to the airing cupboard so it can be wired to the thermostat wires (need to work out which ones they are), but this means all the cables which go to the current programmer downstairs will need to be pulled up so they are available there.

or

Remove the current thermostat wires from the wiring centre and run them down to the kitchen so I can place the Heat Link next to the boiler where the current programmer is.

Or is there a better way?

 

[stem]

It's a bit difficult to see in the photo, but the cable from the heating motorised valve (2) would normally have, brown, blue, orange and grey wires. Is that correct? and if so, what terminal number does the brown wire go to?

 

[AlphaLimaGolf]

I have 5 wires coming from the central heating valve cable (2): Yellow, green/yellow (earth), orange (which I thought was red), blue and white (maybe grey, but looks more white).

Yellow goes to terminal 3, green/yellow goes to the bottom terminal of vertically aligned terminals, orange goes to terminal 9, blue goes to 2nd top of vertically aligned terminals and white goes to terminal 12.

So no brown cable I'm afraid.

Does that help at all?

 

[stem]

It does. thanks. I think you probably do have a MOMO valve. Unlike the usual motorised valves that are electrically opened and pulled closed by a a spring. MOMO valves are electrically opened and closed, which would explain the existing wire to terminal (2) ch off. But they are rare. It's a few years since I last saw one.



Can you confirm what wires you have at the existing Sunvic TLX thermostat, it will also have a connection to CH off, or heating satisfied. If it is.

with a MOMO, it will be the motorised valves white wire that is one of the connections to the existing room thermostat.

 

[AlphaLimaGolf]

Ok, so in that case, (2) CH Off will definitely need to go into terminal 1 of the Heat Link in order to switch the heating off?

Attached is the thermostat and wiring diagram for it. As far as I can see, it only has two wires in it, a blue one in the top left and a red one in the bottom right. Also what looks like a resistor in the top right, which I believe is to help with temp response to help shutting off at the correct temp.

EDIT: The red wire also terminates in the top left, a little below the blue wire (see the left side view image).

 

[AlphaLimaGolf]

Also, attached is the central heating motorised valved, which doesn't have a sprung manual arm on the side like the one for the hot water does. So looks like it is a MoMo as you thought.

 

[stem]

I cans see the wires, as they are connected under the plastic cover at the bottom. The wires would be

3. = Live
1. = Switched live, Heating call for heat
2. = Switched live, Heating satisfied.

Here is a diagram of a TLX 2356 and it can be used with a MOMO. Here's a diagram from the instructions. You will see that there is a connection to 2 Switched live, Heating satisfied. If you could check this that would be helpful. You can still use the Nest, no problem, but the wiring will be a bit different from your first post.

 

[AlphaLimaGolf]

I have taken a better image (there is also a blue wire in the top left which is hard to see). Hopefully the rest is a lot clearer.

So yes it uses terminals 1-3 with 4 unused, by the looks of it.

 

[stem]

OK, MOMO it is.

So the easiest way would probably to wire the MOMO valve to the Heatlink as follows:

MOMO valve White wire = Energise to open = Heatlink 'Heating Call for Heat' terminal (3)
MOMO valve Yellow wire = Energise to close = Heatlink 'Heating Satisfied' terminal (1)

Then the existing thermostat cable can be removed. I can't follow the wires in your photo to determine where each wire goes (it's not the neatest installation) but
1) The yellow wire from the thermostat should be connected to the white wire of the MOMO valve, [Maybe wiring centre terminal 15?]
2) The Blue wire from the existing thermostat should be connected to the yellow wire of the MOMO. [Maybe wiring centre 13 & 2, which are then joined by a red link at the top]

I assume that the Honeywell valve for the hot water isn't a MOMO as there isn't presently a connection to your existing programmer terminal 1 'HW off', but I can see a second white wire in the photo of your wiring centre. Maybe it was originally a MOMO valve and was changed at some point and the old cable retained.

 

[AlphaLimaGolf]

Ok, so my original wiring (top post) for the Heat Link is fine, I just need to also put the CH OFF (from current programmer), into terminal 1 (heating satisfied) on the Heat Link, which will tell the MoMo Valve to close when the Heating Temp is reached/satisfied.

So think I know how to wire the Heat Link now, just not how to decommision the old thermostat. As I have no wires in place to run the thermostat connection in the wire centre (which is upstairs) down to the kitchen where the Heat Link will be, I am guessing I will have to run the Nest Thermostat via USB power. Which is a shame, as I wanted to mount it where the old thermostat is and use the cables there (they are currently 230v and will just blow the Nest if I use these as is).

So just need to clear up the thermostat wiring, this is what I have from your two points above:

1) The white wire from the MoMo goes to terminal 12 (not 15) and also has 2 red cables terminating here, but no yellow wire.
2) The Yellow wire of the MoMo goes to terminal 3, which has a 2nd yellow wire there too, but no red wire joining it to another terminal at the top.


Finally, yes, the honeywell valve for hot water is spring loaded and not a MoM, buy possible that it was a MoMo before. The honeywell is a much newer valve, so am guessing they change the type when replacing it.