Replacing Centaurplus C17 with nest thermostat setup

[shonik]

Hi all,

I've recently moved home and am looking at replacing our centaurplus C17 with a google nest setup. We have a combi-boiler (independent C30) going into the centaurplus, and a single room thermostat (honeywell). I've attached pics of all of these.

Below you can see the wiring of the centaurplus C17.

From reading around, I think the wiring would go as follows (left numbers are heat link):
E, N, L - the same (assuming the green/yellow is earth).
2 (common) -> L
3 -> 4 (grey wire)

Then the wiring for the thermostat is below:

Looks like from this, I would only need to attach
T1 -> 1
T2 -> 2

Does the above sound correct? Any help would be appreciated!

 

[Adam_151]

Hi all,


From reading around, I think the wiring would go as follows (left numbers are heat link):
E, N, L - the same (assuming the green/yellow is earth).
2 (common) -> L
3 -> 4 (grey wire)

I'm not sure if I'm reading you right, but to two, you just want the black with the brown oversleeving on, no jumper/link (just as there isn't atm), Yes put the grey to 3 (and fit brown oversleeving to that too)

Basically 2 on heatlink is the same as 2 on the C17. 1 On the heatlink is the same as 3 on the C17. 3 On the heatlink is the same as 4 on the C17.
4+5+6 On the heatlink are the hotwater channel that the C17 doesn't have. and 1 on the C17 is not connected to anything inside

Then the wiring for the thermostat is below:
Looks like from this, I would only need to attach
T1 -> 1
T2 -> 2

Partly , and this is very important, you can see the other end of the thermostat cable at the RHS of the boiler, it will be connected to a pair of thermostat connections inside the boiler controls box, if you leave it there, the nest expecting 5v will let all the smoke out and it won't work again unless you can find a way to get it back in..... So yes thats how you would connect it at the nest end, but you also need to get the other end of the cable into the heatlink, where it connects to the T1 and T2 on there too. The cable connected to the nest, must only have its other end connected to the heatlink, not to anything else. After disconnecting the cable from the boiler, you will have to link L and SL together to bypass the stat, but make sure neutral is dis-connected.

 

[shonik]

I'm not sure if I'm reading you right, but to two, you just want the black with the brown oversleeving on, no jumper/link (just as there isn't atm), Yes put the grey to 3 (and fit brown oversleeving to that too)

Basically 2 on heatlink is the same as 2 on the C17. 1 On the heatlink is the same as 3 on the C17. 3 On the heatlink is the same as 4 on the C17.
4+5+6 On the heatlink are the hotwater channel that the C17 doesn't have. and 1 on the C17 is not connected to anything inside

Partly , and this is very important, you can see the other end of the thermostat cable at the RHS of the boiler, it will be connected to a pair of thermostat connections inside the boiler controls box, if you leave it there, the nest expecting 5v will let all the smoke out and it won't work again unless you can find a way to get it back in..... So yes thats how you would connect it at the nest end, but you also need to get the other end of the cable into the heatlink, where it connects to the T1 and T2 on there too. The cable connected to the nest, must only have its other end connected to the heatlink, not to anything else. After disconnecting the cable from the boiler, you will have to link L and SL together to bypass the stat, but make sure neutral is dis-connected.

I see, thanks very much, I would definitely have got this wrong had you not mentioned the last part... So essentially I'll need to open the boiler to remove the cables connected to the thermostat, and connect these to the heatlink?

 

[Adam_151]

Its looking that way, can you trace the grey cable we can see at the side of the boiler? does that go into the boiler? And can you see it going to the stat the other end?

The alternatives arre firstly to identfy switched live and live in that old stat and either joining them there, or shorten the cable box and put a box on there, and then to either:

A) put your nest on its stand somewhere like the sideboard and use a USB power supply
B) Run a new cable between the heatlink and the thermostat location (or a new location of your choosing)

 

[shonik]

Its looking that way, can you trace the grey cable we can see at the side of the boiler? does that go into the boiler? And can you see it going to the stat the other end?

The alternatives arre firstly to identfy switched live and live in that old stat and either joining them there, or shorten the cable box and put a box on there, and then to either:

A) put your nest on its stand somewhere like the sideboard and use a USB power supply
B) Run a new cable between the heatlink and the thermostat location (or a new location of your choosing)

OK managed to open the boiler and trace the grey wire to where it meets the white output from the c17:

Seems like the grey wire (4) from the C17 goes to the brown wire from the right grey cable via a jumper? The black wire from the C17 (2) goes straight into the boiler (which is going to be common on the nest). The black wire from the right grey cable is joined with the blue live wire from the C17, while the grey wire from the right grey cable goes to the boiler.

So from this looks like the black cable goes to 1 (L) on the thermostat, grey goes to 3 and brown goes to 2? What do you think? I can't trace these with my multimeter unfortunately as the boiler and thermostat are at opposite ends of the house, and the cable goes into the ceiling so not possible to trace directly...

 

[shonik]

Its looking that way, can you trace the grey cable we can see at the side of the boiler? does that go into the boiler? And can you see it going to the stat the other end?

The alternatives arre firstly to identfy switched live and live in that old stat and either joining them there, or shorten the cable box and put a box on there, and then to either:

A) put your nest on its stand somewhere like the sideboard and use a USB power supply
B) Run a new cable between the heatlink and the thermostat location (or a new location of your choosing)

OK another update - double checked the thermostat and am sure the grey wire from the boiler is going to it - they full grey wire is in the trunking to the thermostat, where the wire is split and the three (grey, brown and black) go into the thermostat while earth is kept disconnected outside.

This has got me a bit confused though, as the brown is actually going into 1, but according to the diagram on the back of the honeywell this should be live, but it is actually connected to 4 in the c17, while the black wire going to 2 is the one crossed with the blue live wire from the c17? Would appreciate any words of wisdom

 

[Adam_151]

This has got me a bit confused though, as the brown is actually going into 1, but according to the diagram on the back of the honeywell this should be live, but it is actually connected to 4 in the c17, while the black wire going to 2 is the one crossed with the blue live wire from the c17? Would appreciate any words of wisdom

All seems pretty normal to me, the brown is (switched live from the programmer) to the stat. The black is neutral for the accelerator resistor within the stat.


Take a step back and look at it, what you have is a brown and blue all the way from the fused spur to the boiler, they are looped via the supply to the C17, and in two separate cables, these supply the boiler. Now the boiler will come on when the two terminals on the far left are linked, now on yours you can see it actually provides a direct link to the far left one from the live in with the brown link wires, this isn't always the case, and sometimes it'll be via the boiler PCB and a fuse. It then expects to see live back on the second to left terminal to make it fire up. So we send it out to the programmer on the black core on terminal 2 in the C17, now when the programmer calls for heat, it switches that onto the grey on terminal four, this runs into the boiler terminal box, where it is connected onto the brown of the cable to the stat where it goes to live (1) , when the stat calls for heat, it switches this onto the grey in switched live (3) which returns to the boiler and goes to the second from left terminal. So it goes from the boiler, through the programmer, through the stat and back to the boiler, so only if both the programmer and stat are saying heat is needed then will the boiler come on. The grey provides a neutral to the stat for own use.

So as well as replacing the existing programmer with the heat link, you need to get rid of the stat from the picture and use its wiring for the heatlink. The grey and earth can be disconnected from their respective terminals without disturbing the other wires in them. The Grey also gets disconnected, and you replace that with the other grey thats comming from the programmer, this way was in the connector with the black from the stat, so this is also disconnected now. Take the grey cable completely out of the boiler, and this is to be run into the heatlink, you might need a joint box and a bit of cable to extend it. Pick one colour and connect it to T1 on the heatlink at one end, and T1 on the nest at the other, Pick another and use for the T2 connection. I suppose arguably, you should use brown T1 and Grey T2, but if you used brown and black then it wouldnt matter. Do not use the earth for either of these. A lot of folk, would for tidyness sake connect the earth on this cable to earth at the heatlink (it saves having a core floating around), but strictly speaking its unnessary as this is a SELV (separated extra low voltage) connection.

 

[shonik]

All seems pretty normal to me, the brown is (switched live from the programmer) to the stat. The black is neutral for the accelerator resistor within the stat.


Take a step back and look at it, what you have is a brown and blue all the way from the fused spur to the boiler, they are looped via the supply to the C17, and in two separate cables, these supply the boiler. Now the boiler will come on when the two terminals on the far left are linked, now on yours you can see it actually provides a direct link to the far left one from the live in with the brown link wires, this isn't always the case, and sometimes it'll be via the boiler PCB and a fuse. It then expects to see live back on the second to left terminal to make it fire up. So we send it out to the programmer on the black core on terminal 2 in the C17, now when the programmer calls for heat, it switches that onto the grey on terminal four, this runs into the boiler terminal box, where it is connected onto the brown of the cable to the stat where it goes to live (1) , when the stat calls for heat, it switches this onto the grey in switched live (3) which returns to the boiler and goes to the second from left terminal. So it goes from the boiler, through the programmer, through the stat and back to the boiler, so only if both the programmer and stat are saying heat is needed then will the boiler come on. The grey provides a neutral to the stat for own use.

So as well as replacing the existing programmer with the heat link, you need to get rid of the stat from the picture and use its wiring for the heatlink. The grey and earth can be disconnected from their respective terminals without disturbing the other wires in them. The Grey also gets disconnected, and you replace that with the other grey thats comming from the programmer, this way was in the connector with the black from the stat, so this is also disconnected now. Take the grey cable completely out of the boiler, and this is to be run into the heatlink, you might need a joint box and a bit of cable to extend it. Pick one colour and connect it to T1 on the heatlink at one end, and T1 on the nest at the other, Pick another and use for the T2 connection. I suppose arguably, you should use brown T1 and Grey T2, but if you used brown and black then it wouldnt matter. Do not use the earth for either of these. A lot of folk, would for tidyness sake connect the earth on this cable to earth at the heatlink (it saves having a core floating around), but strictly speaking its unnessary as this is a SELV (separated extra low voltage) connection.

OK I see, brilliant, yeah I got a bit lost and confused the neutral connection with live on the c17, that actually all makes sense to the point I'll be able to re-wire this myself. Just to make sure I've understood, counting terminals from left on boiler:

First terminal: Black cable in terminal 1 will go to heatlink common (2)
Second terminal: Remove grey cable coming from stat (3) to second terminal on boiler and replace with grey cable from c17 (4), heatlink (3) (i.e. heatlink 3 goes to boiler terminal 2). Put grey stat cable to T2, brown stat cable to t1.
Third terminal: Brown live in third boiler terminal will go to L on heatlink
Fourth terminal:
- Remove black cable from stat
- Blue will go to N on heatlink
Fifth terminal:
- Earth from stat will be removed (no earth on nest).
- Other earth will go to earth on heatlink
Sixth terminal:
- I guess this stays the same as it loops the live into common on the heatlink?

 

[shonik]

All seems pretty normal to me, the brown is (switched live from the programmer) to the stat. The black is neutral for the accelerator resistor within the stat.


Take a step back and look at it, what you have is a brown and blue all the way from the fused spur to the boiler, they are looped via the supply to the C17, and in two separate cables, these supply the boiler. Now the boiler will come on when the two terminals on the far left are linked, now on yours you can see it actually provides a direct link to the far left one from the live in with the brown link wires, this isn't always the case, and sometimes it'll be via the boiler PCB and a fuse. It then expects to see live back on the second to left terminal to make it fire up. So we send it out to the programmer on the black core on terminal 2 in the C17, now when the programmer calls for heat, it switches that onto the grey on terminal four, this runs into the boiler terminal box, where it is connected onto the brown of the cable to the stat where it goes to live (1) , when the stat calls for heat, it switches this onto the grey in switched live (3) which returns to the boiler and goes to the second from left terminal. So it goes from the boiler, through the programmer, through the stat and back to the boiler, so only if both the programmer and stat are saying heat is needed then will the boiler come on. The grey provides a neutral to the stat for own use.

So as well as replacing the existing programmer with the heat link, you need to get rid of the stat from the picture and use its wiring for the heatlink. The grey and earth can be disconnected from their respective terminals without disturbing the other wires in them. The Grey also gets disconnected, and you replace that with the other grey thats comming from the programmer, this way was in the connector with the black from the stat, so this is also disconnected now. Take the grey cable completely out of the boiler, and this is to be run into the heatlink, you might need a joint box and a bit of cable to extend it. Pick one colour and connect it to T1 on the heatlink at one end, and T1 on the nest at the other, Pick another and use for the T2 connection. I suppose arguably, you should use brown T1 and Grey T2, but if you used brown and black then it wouldnt matter. Do not use the earth for either of these. A lot of folk, would for tidyness sake connect the earth on this cable to earth at the heatlink (it saves having a core floating around), but strictly speaking its unnessary as this is a SELV (separated extra low voltage) connection.

Hi again, just wanted to say thank you so much for your help! Thermostat arrived this morning and managed to install it as per your guidance - looks like everything is working as expected!

 

[Taylortwocities]

Glad it’s working. now you need take some time to do the job properly.

In your first picture, all cables most go inside the Nest box. You cannot leave the inner wires hanging out like that. And you must connect the earth on the right hand cable to the other earths.

 

[shonik]

Glad it’s working. now you need take some time to do the job properly.

In your first picture, all cables most go inside the Nest box. You cannot leave the inner wires hanging out like that. And you must connect the earth on the right hand cable to the other earths.

For the right cable the earth and other loose wire isn’t actually connected on the other side - should the earth still be connected to the earth anyway? Yeah I need to figure out how to tidy the cables up, only reason I left them out like that was because the outer white layer was too restrictive as wires from the same cable were going to opposite ends of the heat link ... would covering the individual wires with electrical tape be sufficient?

 

[Taylortwocities]

Yes the earth must be connected on all cables. Even if (whatever it is on) the other end does not need an earth.

The outer sheath provides a physical protection for the inner conductors and provides a double layer of insulation. sticky tape doesn’t provide either.
Also it comes undone leaving a sticky mess after a year or two.

If the cables aren’t quite long enough, moving the Nest a cm or two will probably give you the extra length you need.

 

[ericmark]

I know the problem I also have installed Nest Gen 3, there is not much room to connect the wires, and I do realise the cable to right is only 12 volt, however Nest instructions actually say the earth is only required if the thermostat is hard wired, so yes earth is required, it is called PELV (protective extra-low voltage). An extra-low voltage system which is not electrically separated from Earth, but which otherwise satisfies all the requirements for SELV. Not a clue why, but not ours to reason why.

 

[shonik]

Yes the earth must be connected on all cables. Even if (whatever it is on) the other end does not need an earth.

The outer sheath provides a physical protection for the inner conductors and provides a double layer of insulation. sticky tape doesn’t provide either.
Also it comes undone leaving a sticky mess after a year or two.

If the cables aren’t quite long enough, moving the Nest a cm or two will probably give you the extra length you need.

OK cleaned it up a bit...

Only wire slightly exposed now is earth, and I connected the other earth to the two others and the heat link earth. What do you think?

I know the problem I also have installed Nest Gen 3, there is not much room to connect the wires, and I do realise the cable to right is only 12 volt, however Nest instructions actually say the earth is only required if the thermostat is hard wired, so yes earth is required, it is called PELV (protective extra-low voltage). An extra-low voltage system which is not electrically separated from Earth, but which otherwise satisfies all the requirements for SELV. Not a clue why, but not ours to reason why.

I had the heat link connected to earth via other cables already. Couldn’t see an earth connection on the thermostat itself though?

 

[Deleted member 267285]

Good evening @shonik,

Well done for sorting this out, however please observe the following:

Although a lot of ignoramus people (not saying you are) think it’s ok just to open up boiler covers, it’s important to know that if you have disturbed a combustion circuit, such as yours, then critical safety checks must be carried out. I’ve attached a screenshot from your boiler manual.

Also twin and earth (grey wire) isn’t usually used inside a boiler as I’m told it’s not heat resistant.

Thanks.