This seems to be the other end where the programmer is (the white cable with the blue wire with yellow tape around it) lt;li>Operating periods per day: 3 per day for Central Heating and 3 per day for Hot Water</li> This is the Horstmann CentaurPlus C27, a two channel programmer which gives independent control over hot water and heating with up to 3 ON/OFF settings for each day of the week. It also has a 1 hour manual boost button for the hot water and an advance button for the central heating. As you have figured, the old room thermostat can't simply be disconnected as this would leave the heating wiring 'open circuit' and it would not operate, the integrity of the circuit needs to be maintained. As not all thermostats are the same, some working out will be required.

CentaurPlus C27 Series 2 User Operating Instructions CentaurPlus C27 Series 2 User Operating Instructions

So I have finally got round to doing this today and now having finally removed the C27, I only have wires on 3 & 4, Looks like I just connect these to B? But I believe I need a loop from the Second L to AThe Nest Heatlink will need the usual Live and Neutral, from the same supply as the programmer etc, for it's power. There is a separate immersion switch I can use to heat the water, but this can become expensive I I forget to turn it off. We’ve got a very old centaur TC7 that is no longer readable, can I do a straight swap to this product? You can now disconnect all of the wires going to the old thermostat and remove the cable and thermostat.

Trianco Aztec/ Centaurplus C27 problems? | DIYnot Forums Trianco Aztec/ Centaurplus C27 problems? | DIYnot Forums

The honeywell system should then take care of the thermostat, however, there is a room thermostat that needs to be removed, and that part Im not sure of. I suppose I could set it on full and let the evohome do the job, but Id rather remove. Any tips? or anything I havent got right? 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.

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Having a few issues wireing in the Nest Heatlink - mainly trying to work otu what the current Common and Call-For-Heat connections are - have been looking at http://www.horstmann.co.uk/files/86..._guide_CentaurPlus_C21-C27_installer_web1.pdf but havent managed to work it out my self , we also have one thermostat on the system which will be replaced if that helps. note it says Centaur Plus is volt free, so it seems the centaur plus is same as Hive single channel and standard one is same as Hive duel channel, so I would be hunting on the actual device to see if there is a diagram. Your picture clearly shows a brown sleeve on the blue wire going to 3 so clearly a line not neutral. Just done that and its all up and running - Thank you so much - Saved me alot of money there getting it installed.