Using existing thermostat wiring to link to nest heat link

30 Nov 2018
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United Kingdom
Hi hope someone can help me. I’m planning to install a nest 3rd generation thermostat to control my Vailliant ecotec combi boiler. I will be locating the nest thermostat in an alternative location to the existing thermostat so no wiring for thermostat other than plugging it it is required. My current thermostat is a simple Danfoss wired dial thermostat.

Whilst I’d be happy to remove the boiler casing to access the internal connections, if it’s not necessary to connect the heat link I’d rather not. My question is:

I can power the heat link by wiring it to the fused spur that provides power to the boiler which is straightforward enough but is it possible to use the existing wiring of the Danfoss thermostat to make my control connections to the heat link? I would just need to identify the existing thermostat cable leaving the boiler and cut it to facilitate connection to the heat link. Will this work?
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If you have to open up a room sealed compartment to access the boiler terminals (on some boilers you do, on some you don't) then it should be done by a Gas Safe engineer anyway so they can make sure it is properly sealed up again afterwards.

However, you may be able to simply reroute the existing thermostat cable to the Heat link. It depends on the model of the existing thermostat and how it is wired. If there are three wires present at the existing thermostat (excluding any earth) it's should will be OK to use, because two of the wires will probably provide a 230V supply, and the third will be the switched live to control the boiler.

If, on the other hand, there are only 2 wires at the thermostat (excluding any earth), then you can wire them to the Heat link terminals (2) and (3) and then get the 230V from the 3A fused spur for the (N) and (L)
Thanks for your response. I want to make this as simple as possible and avoiding removing the boiler cover is my objective.
If you need anymore help, post back details of the model of the existing Danfoss thermostat and the wires that are connected to it.
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I’m not at the property until next week but will take you up on your offer then. Thanks
Im at the property now, it’s not Danfoss it’s Drayton RTS1 which is 240v. The connections (connector blocks) for the thermostat cable are in the fused spur that powers the boiler.
At the thermostat the connections are Red-Live, Black-Neutral, Black-Call.
I can identify that the black call wire from the thermostat is connected to the gray wire from the boiler.
The RTS1 is wired as below:


Unfortunately with the wiring colours, there is no fixed colour scheme it depends on the personal preferences of the installer and the cables they had to hand. One may use a black cable for a certain connection another may use grey. However, it can be worked out from where the wires are connected. Your Heat link won't need any extra wires than those already there.

The wire presently in L is the live and should go to the Nest Heat link (L) and also be linked to terminal (2) 'Heating Common'
The wire presently in N is the neutral and should go to the Nest Heat link (N)
wire presently in 3 Call for heat is the switched live, and should go to the Nest Heat link (3) 'Heating Call for Heat'

As the RTS 1 is just a basic thermostat without any time control built in. I assume there is some time control elsewhere. If so, this will need to be set to be permanently 'on' 24/7, otherwise it will interfere with the operation of the Nest.
Thanks for your swift response. Makes perfect sense. Expecting nest to be delivered today then it’s a visit to Wickes to buy a couple of bits and get the job done.
Thanks for your help.
A final question if you would be so kind. Reading the nest literature it says the heat link should be placed near the boiler (>30cm). Is there any reason for this? It’s just that rather than change the wiring in the fused spur switch (contains thermostat wiring too) and fiddle around with connector blocks in what is a small space I think the simplest solution would be remove the existing thermostat and put the heat link in its place using existing wires. It wouldn’t be close to the boiler but I can’t see why it should be.
The distance between the Heat link and the Thermostat is the critical distance as they need to communicate wirelessly with each other.

For the same reason the Heat link should not be too close to the boiler. This is because the boiler being a large metal object would screen the Heat link and may prevent good communication with the thermostat. For this reason, it should be more than 30cm from the boiler.

30cm is a minimum, distance not a maximum.


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