Replacing Centaurplus C27 with Nest Gen 3

[jonperks]

Hi
I've purchased a 3rd gen nest to completely replace a horstmann centaur plusc27 programmer. It's controlling a mistral boiler and we currently do not have a thermostat.

The wiring is just to complicated for me to work out. Could anyone help with what wires go where on the heat link?

https://www.tlc-direct.co.uk/Technical/DataSheets/Horstmann/HOC21-C27_installer_Old.pdf

https://dl.dropboxusercontent.com/c...oaEosTtbNYJAQARfNBNTXpR6bXhpuWFpGWm/file?dl=1

I've attached a shot of the current wiring set up.

Any help would be very much appreciated. I may have missed some info!

Thanks

Jon

 

[jonperks]

Ok so I've been reading everything I can and think it goes as follows....

N -> Nest N
L -> Nest L
1 -> Nest 4 (hot water satisfied)
2
3 -> Nest 6 (call for heat HW)
4 -> Nest 3 (call for heat CH)

If that's all correct then what do I need to connect the hot water and central heating commons on the nest heat link. I'm reading about jumpers from the live?

Thanks again

Jon

 

[flameport]

Both common terminals are linked to L.
The rest is as you have already worked out.
The existing programmer has those links internally, the Nest does not.

 

[jonperks]

I thought so but needed the confirmation... Thanks so much!

 

[Guy Lincoln]

I just wanted to thank you guys so much. We wanted a Nest 3rd Gen. We were initially perplexed about the diy bit, and so we asked for 4 quotes to install. One came in at £120, the next at £100 and the the two others did not bother. We followed your wiring instructions on a Y plan and bought a Honeywell junction box from Screwfix at around £7.00 which we positioned in place of the Drayton programmer, and did all the crossover/linkage in that. Job done! One Day NEST or Drayton might come with a built in crossover unit. Crikey, it is only 6 wires!

Guy, Lincoln

 

[dragosheluv]

Hi, I know this is an old post but maybe Someone can help me as well. I have the same centaurus C27 and I'm trying to upgrade to to a nest 3rd gen. My wires are a bit different but not much. I'm new to this type of heating and not very skillful with central heating wiring and electrics but with some guidance I can manage. I also have upstairs a thermostat Drayton connected to 230v compared to nest which must be connected to 12V only.

Any help with the heatlink wiring? Can I put the nest thermostat upstairs and connect to T1 and T2 only or I need to do some modifications? Thanks

 

[stem]

Hi dragosheluv.

Probably no one has answered you because you have added your post to the bottom of someone else's where it is hidden away and not visible as a new post. For future reference, this is also against the forum rules.

However to answer your enquiry the C27 is wired as below:



The diagram above will allow you to identify what the wires in each terminal of the C27 does. Once identified, the wires are moved to the Nest Heat link terminals that have the same function.



So, for example:
N & L are self explanatory.
C27 HW On = Hot Water, Call for Heat at the Heat link
C27 CH On = Heating, Call for Heat at the Heat link

Once you have transferred the wires from the C27 to the Heat link, then the Heat link (L) terminal also needs linking to the two 'common' terminals (2) and (5)

Then, if you wish to use the old thermostat cable to connect the Heat link to the Nest thermostat, you need to disconnect the old thermostat, trace the thermostat cable back to its origin. Once you have positively identified its origin, note where the two thermostat live wires (red & yellow) are connected, then disconnect the live wires and also the neutral wire (blue). Once you have done this, the two terminals where the live wires have just been removed from need connecting together with a wire link.

The thermostat cable should now not have anything connected to it at either end. If so, you can now use two of the wires in the cable to connect T1 at the thermostat to T1 at the Heatlink, and T2 at the thermostat to T2 at the Heatlink.

 

[Fraser2203]

I know this is an old thread but I see lots of help here. I have just moved into a new house which has a combi boiler (previously normal with HW cylinder) however the owner retained the Centaurplus C27 clock and just had hot water set to 24/7 to give a live to that circuit. Coming to replace with a hive single Chanel unit and looking for some help. At present it’s wired up for the centaurplus with a neutral to terminal 3 and live to terminal 4 - nothing in 1&2. Would I be right in saying that 3 is HW on and 4 CH on. If so looking for suggestions, is it just a case of bridging the HW on (neutral) into the perm neutral. Assuming neutral and live given colour of sheathings on the wires. Any help much appreciated.

 

[denso13]

Please don't make two different threads, it can make things very confusing.