Nest v3 - Help with existing wiring

27 Sep 2012
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United Kingdom
Hi All,

So I recently purchased a Nest v3 and fancied installing it myself. I'm having a bit of trouble identifying the existing wiring and what type of system I have though so was after some help.

Currently, I have a Vailant Ecotec Plus 418 series boiler which is wired up to a cheapo iflo programmer (clone of Drayton LP112). There is what seems to be pump in the airing cupboard upstairs leading up to the 210ltr unvented cylinder in the loft. In the dining room there is an ancient Drayton RTE room thermostat.

From what I'm reading, the Nest heatlink will replace the iflo programmer and then I would need to trace any wires going back to the thermostat from the programmer. From there I would need to connect these to the heatlink to step the voltage down to 12v to allow the nest thermostat to work.

The nest instructions talk about S plan and Y plan systems, and I'm not sure which I have. Also, after taking the front off the programmer and looking at the wiring, im still none the wiser as to which might be going to the thermostat so I'm hoping someone might be able to help figure out what these are and then relate them to where they need to go in the heatlink:

Current programmer wiring:

N (Neutral) 1 Blue (neutral) cable from wall
L (Live) 2 Red (Live) cables from wall
1 (HW off) Empty
2 (CH off) Empty
3 (HW on) 1 Red (Live) cable from wall
4 (CH on) 1 Red/black (Live) cable from wall
The Drayton thermostat internals are as the picture below:
20170724_182744.jpg 20170726_230448.jpg

Airing cupboard pump

Unvented Cylinder

So....Can anyone help me to:
A, Identify the type of system I have (S plan or Y plan)
B, Identify which wires from the programmer go to the nest heatlink
C, Identify which wires are likely to be from the thermostat so they can be reused for the Nest thermostat.

Any help appreciated.




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18 Mar 2010
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United Kingdom
This is not having a dig but if you don't no what your doing give a heating engineer or an electrician a call its not worth the risk of blowing it up or yourself

All the best
27 Jan 2008
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Llanfair Caereinion, Nr Welshpool
United Kingdom
There are C, S, W and Y plans.
C was a modified thermo syphon system with two port motorised valve to turn off domestic hot water when hot enough.
S was a pair of two port motorised valves one for domestic hot water one for central heating could also be used to zone central heating.
W was a three port valve giving domestic hot water priority over central heating.
Y was a three port valve with a mid position giving domestic hot water when at rest, both hot water and central heating in centre position and central heating only when fully powered.
The Honeywell plans also had Honeywell junction boxes with designated wires into the holes, however many builders used socket back boxes with lid and a simple connector block inside, and did not follow standard wiring.
I had one in daughters house and carefully tried to work out a fault, I tried following the wiring diagram.
for the Y system.
However the micro switch shown is actually three micro switches, it was a micro switch that had failed, but the tests I used to try and work out how wired were showing strange results due to extra micro switches not shown, I did sort it, but it was not an easy task.
I am still not sure why people fit Hive and Nest? There are some open plan houses, and some houses using hot air central heating which can be controlled with a single thermostat, but most British houses have internal doors without vents in the doors so each room is controlled by the TRV in that room, the thermostat if fitted is simply to turn off the system in the summer, and to alter the wall thermostat without also altering every TRV is rather pointless, you can with Nest have follow commands where the eTRV follows the Nest or vie versa but since in that case you can control the temperature of each room independently with your phone or PC without using Nest can't really see the point.
There is the EvoHome where each eTRV tells a central hub when to switch on the boiler, but with Nest it is a little hit and miss, I would say first you work out what you want, then you ask how best to do it. I know with eTRV I can set them to auto turn on central heating in the room when my phone is within a set distance from home, however tests found for the room to warm up takes 2 hours, so simple time works better.

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