Nest Thermostat - VoltFree Switching

aMusetti · 9 Apr 2021

Hi All,

we are looking to install a Nest thermostat in our home but having looked at the installation instructions, we became confused as we only have two wires connecting the current thermostat to the boiler. The instructions show 4 wires to install.

The boiler is Atag E22S boiler and current thermostat is Atag BrainQ RSC/2.

I have emailed Atag and they have said that NEST can be used IF I can confirm that nest control does VOLT FREE switching. We would need a qualified engineer to install as well as they would need access inside the boiler to wire in an external control that isn’t Atag branded.

can anyone confirm if this is possible before we start to look for a qualified installer?

Thanks
Andy

ianmcd · 9 Apr 2021

Nest does support volt free switching, but your boiler if really an E22s is a conventional system boiler , not a Combi, so will depend on how your HW requirements are set up

aMusetti · 9 Apr 2021

Hi Ian, thanks for the reply. We have a hot water tank upstairs - I'm not very technical with this so if i can provide any more info for you, or someone else, to advise if Nest can be fully utilised, please let me know what i can provide?

ianmcd · 9 Apr 2021

post a pic of the HW cyl and any motorised valves you valves you have, do you use the BrainQ to control the HW settings or do you use another control somewhere

aMusetti · 9 Apr 2021

Hi Ian,

see attached. Not sure about motorised valves. We do use the BrainQ to control HW settings.

Andy

muggles · 9 Apr 2021

It can be done but it won't be simple. The VC2011 is 24 volt and will be connected back to the boiler. That needs removing and replacing with a pair of two-port valves, which can provide volt-free switching provided the person installing them knows what they're doing. A full system drain down will be required. It'll be a good half a day's work plus materials. Budget about £500

CBW · 9 Apr 2021

@muggles I’m Not fully up on electrics, but can the 24v side of Honeywell be used with the Nest?

muggles · 9 Apr 2021

@Chris_W in some setups it can, yes, but not here. The issue is that the original BrainQ used its 2 wires in the same way that Nest uses T1 & T2, as both power and communication, but using a different control language. The BrainQ would tell the boiler when to fire and also which position to put the 24v 3-port valve in according to whether heating, hot water, or both are demanded. The boiler knows the temperature of the cylinder as a cylinder sensor is wired back to it.

In this instance I think the OP needs to strip out the 24v Honeywell kit, install an S-plan, and wire the grey & orange across the volt-free connections, with no permanent Live to grey. A standard cylinder thermostat would be needed between Nest terminal 6 and the brown of the hot water zone valve.

CBW · 9 Apr 2021

Appreciate that thanks @muggles

ianmcd · 9 Apr 2021

Typical of @muggles, gets me to gather all the evidence then jumps in for the glory ,

, Atag controls can be a law unto themselves but great boilers in my opinion,

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Nest Thermostat - VoltFree Switching

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