Replacing Danfoss FP715SI with Nest Third Generation

13 Dec 2023
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United Kingdom
I am wanting to replace my current thermostat/ programmer with the Nest 3rd generation. I have a conventional boiler. Before I go ahead and buy it I want to make sure it will work and what wiring needs to be in place. Any help is appreciated. The large amount of wiring is located beneath the programmer. The wire coming in from the bottom right is for the thermostat downstairs, the black is for the water pump, the one next to the black is for the water tank. The top right wires go into the programmer.

The three coming in on the top left;
Top two go back to the boiler
Bottom wire is the electrical supply.


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It would be a relatively straightforward swap.

The N and L from your current programmer would go to N and L on the Nest Heatlink.

The Green/Yellow wire in terminal 4, would go to terminal 3 - Heating (call for heat) on the HL.

The Brown wire in terminal 1, would go to terminal 4 - Hot water (satisfied) on the HL.

The Blue wire in terminal 3, would go to terminal 6 - Hot water (call for heat) on the HL.

Additionally, short bridging wires would be required between L and terminals 2 and 5 on the HL.

In the wiring centre, move the Green/Yellow wire from the programmer, one space over to the last terminal on the right.
This will bridge out the existing thermostat and allow its cable to be removed completely.

Or, if you want the Nest thermostat to be mounted in the same place as the existing stat - the cable could be reused, with two of the wires connected to T1 and T2.
In this case, the earth terminal on the HL would also have to be connected to an earth.

For reference, your existing wiring doesn't meet current regulations - green/yellow wires can not be used for switched lives any more.
It would also be good practice to oversleeve the switched lives in Brown.
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The heat link would likely be placed next to the wiring centre, and the programmer would be removed, green/yellow is not permitted for anything other than earth bonding, so you have not got enough wires to correct this if the heat link replaced the programmer, so job one is to label all wires at the wiring centre.
Start with cables, orange, grey, white and blue are likely the motorised valve, mid-position-valve.jpg and since N/C terminal used with programmer seems likely Y Plan Y-Plan.jpg there is no standard order of wiring in the wiring centre, so you need to do some detective work working out what wires go where. The Nest book does show Y plan, but you need to work out what wires go to the old thermostat as the new one will be 12 volt, so getting 230 and 12 volt mixed up releases expensive blue smoke.

It would not matter which thermostat you go for, the wiring needs correcting, yes likely Hive or Drayton Wiser will plug straight in without any re-wiring, but wiring is still wrong.

I don't like Nest as it will not connect to any TRV heads, but that's me, I want to control every room independently. But if you don't want that then Nest will work, not happy with the geofencing function, but again if you don't need that function then no problem. But the big question is do you have the skill to fit it?
Thanks so much!

Only thing I’m still confused on is the last point - we do want to keep the thermostat in the same place (downstairs hallway) please could you clarify?

I also think the nest has two ports on the thermostat whereas we currently have three cables connected up to it.

Thanks again
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Only thing I’m still confused on is the last point - we do want to keep the thermostat in the same place (downstairs hallway) please could you clarify?
The cable running to your thermostat appears to have a red, blue, yellow and earth wire.
To reuse this to power the Nest Thermostat, all these wires must be removed from the connections in the wiring centre.
You would then pick two of the wires, i.e. red and blue, place these into T1 and T2 on the HL (and safely isolate the other two wires). This will provide the 12V to power the thermostat.

As said earlier, in this configuration, the HL must be provided with an earth connection from the wiring centre, to its earth terminal.

I have numbered the terminals 1 to 10, and will try and work out what they are:-
1) Line too many brown cables to be likely anything else.
2) Neutral as above.
3) Earth as above.
4) Unused.
5) Unused.
6) Likely live to boiler to make it run.
7) DHW off from programmer
8) DHW on from programmer
9) CH on from programmer, feed to thermostat.
10) From thermostat to three port valve as seems only yellow wire and black/white often call for heat.

To be sure would need to test, and clearly I am not where I can test, because you can set the programming from thermostat or app/program, there is not need for access to the heat link, so no problem being next to wiring centre. This is my best guess from the pictures you have posted.

If my guess is correct then the three and earth is disconnected from wiring centre and goes to T1 and T2 on Nest.
The two cables to the programmer can come out.
6) on wiring centre to 3 on nest.
7) on wiring centre to 4 on nest
8) on wiring centre to 6 on nest
1) on wiring centre to L, 2 and 5 on nest
2) on wiring centre to N on nest
3) on wiring centre to earth on nest

The original thermostat is replaced by nest thermostat, the original programmer is now dead and can be removed.

However as said this is my guess, up to you to verify I have guessed correct.
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