NEST Thermostat (3rd Gen) - Advice Please

10 Dec 2017
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United Kingdom
Hi everyone,

I’m asking for advice on how to install the NEST Thermostat (3rd Gen).

I currently have:
  • Potterton Suprima boiler
  • Potterton EP2002 Programmer
  • Potterton PRT100.ST2 Room Thermostat
It’s all wired up onto an S-plan system controlling, both, the CH and HW.

Please find attached pictures of the current wiring on the EP programmer, the thermostat and also the junction box (upstairs inside the airing cupboard).


Programmer Diagram:


Junction Box:

My plan would be to replace the programmer with the HeatLink, straight forward hopefully, specially after coming across this post.

Pretty much the same setup and very helpful, however, I would like to use the existing thermostat wires to power the NEST thermostat and place it in the same location.

I believe these wires are going to the junction box and then being sent back down to the the programmer, so how do I go about getting that wired?

Please let me know if any further clarification of the wires is needed.

Much appreciated!
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Stage 1 Install the Heatlink in place of the EP2002

N and L are straightforward they are the same for the Potterton and the Heatlink, so move them across accordingly.

Move the wire in the EP2002 terminal 3 'hot water on' to the Heatlink terminal that has the same function, which is terminal (6) 'Hot water call for heat'

Move the wire in the EP2002 terminal 4 'heating on' to the Heatlink terminal that has the same function, which is terminal (3) 'Heating call for heat'

The wires presently in the EP2002 terminal 5 will be removed, but should remain connected together. This should be done in a new separate terminal. Do not connect them to the Heatlink.

Link together the Heatlink (L) terminal and the two 'common' terminals (2) & (5)

Connect the Heatlink earth terminal to earth.

Stage 2 Remove the Existing Thermostat

Because the Nest also carries out the functions of the room thermostat, the old wired thermostat needs to be decommissioned 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, they are.

1. Trace the thermostat cable back to its origin notice where the wires are connected and then disconnect it, then link the terminals where the red and yellow wires originally came from together. The blue neutral can be simply disconnected.

2. Remove the existing thermostat and replace it with a junction box, connect the red and yellow switching wires together, and isolate the neutral.

3. Leave the existing room thermostat in place, and remove the yellow and red wires and connect them together in a separate connector located inside it.

Stage 3 Get Power to the Nest Thermostat

This can be done by using a separate plug in power supply, or by connecting the Heatlink terminal T1 to the Thermostat T1 and connecting the Heatlink terminal T2 to the Thermostat T2.

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