Connect US Nest to Vaillant Ecotec Plus 618

5 Oct 2020
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United Kingdom
I have a US Nest Learning thermostat that I want to connect to my Vaillant gas boiler (Ecotec Plus 618 - Vu186/3-5 R1).

The boiler accepts both 24V and 230V thermostats as well as eBus. Below I am attaching a diagram with the connections as well a photo from my Nest Thermostat Installation Manual.

My understanding is that the US version of the Nest thermostat operates as a normal 24V thermostat so in theory I should be able to wire it directly to the boiler 7-8-9 inputs on the PCB (unlike the UK version of the thermostat that I need to have the Heatlink module as well). Am I correct here or am I missing something?

If I am correct can someone help me with the wiring? In my understanding I will have to bridge the high voltage thermostat in order to use the 24V one (or the eBus) but I am a little bit confused on how the rest of the wiring would work.

It is worth mentioning that I am just using the boiler for heating (i.e. turning the boiler ON and OFF) and I do not care about hot water.


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You can't for a few reasons. For starters, you need to be Gas Safe Registered to remove the front cover from your boiler. As you can't access the electronics without removing said cover, this is not a DIY job.

The American Nest requires 5 wires, I believe, but there are only three 24v connections in your boiler and there's nothing to say which goes where in relation to a control which was never designed to be connected to it.

A standard heating system has zone valves which activate the boiler, and these need 230v power to open. You clearly can't power one open with your 24v Nest, so you'd need to find an alternative for that too.
Thank you very much for the response. I have contacted Nest who have told me that the Nest thermostat can be used like any other 24V thermostat. In regards to the 5 wires I can only use 3 (Line, Neutral, and Ground) to the respective C, W1 and * ports of the Nest. So assuming this would work like any other 24V thermostat, wouldn't it be sufficient to use it with the 24V ports?
No, because you still haven't solved the other two points I made
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In addition, the US Nest is designed for US furnaces that have 24 V AC control circuits.

The 24 V thermostat shown connected to the ecoTECH plus in the installation manual is 24 V DC, and the boiler terminals only give you access to half of the supply for switching purposes. (I imagine it will be the +, but it doesn't say)

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Thank you all for your responses. It is clear now.

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