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Using 24VAC thermostat with UK 240v boiler

feridoun

Aug 4, 2013
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Dear forum

Apologies in advance - this may be well out of my depth. I only have a GCSE in electronics from many years ago.

Basically I have a UK boiler (Vaillaint Ecotec 837) which can operate a 240v thermostat as is the norm here.

Unwisely, I've bought a US Nest Thermostat which operates on 24V.

However, there is a device (Honeywell RC840T-240/U) which is a transformer and relay and potentially could solve this issue - but it costs $90 incl. shipping from the US.

Is there a way I could replicate the function of this device?

I've tried to copy the schematic in the Honeywell device as below - it may well be complete rubbish.

If it is correct, 1, which tranformer could I use (this one?) . 2) Which relay could I use?

Thanks in advance!

jnNxcL4
 

Harald Kapp

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Sorry, the diagram doesn't make sense to me at all. I'm not an electrician versed in setting up boilers an thermostats, but obviously the 240V mains connection is missing from the diagram. Also I see no reason why the 240V subcircuit and the 24V subcircuit should be coupled using a common return (the signal with the small triangle on it).

SInce, as you state, you are not experienced in this matter, I suggest - for reasons of your own safety - that you either get the expensive controller or get help from an experienced friend or technician near you.
 

CDRIVE

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I don't disagree with what Harald told you but I'd like to add that the link you provided states that the transformer is designed for 60Hz. I believe you're mains are 50Hz.

Chris
 

feridoun

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Hi everyone, just wanted to share that I got my Nest thermostat working with a 24v transformer and 24vac relay using the following wiring. My boiler is a Vaillant Ecotect plus 837. http://imgur.com/JjHJQq2 .
 

CDRIVE

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Glad to hear it. Apparently the 60Hz xfmr is working fine on your 50Hz mains. Any signs of it getting hot?

Chris
 

philgritsr2

Oct 21, 2013
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in my experience most transformers can work with 50/60 hertz as a general use and i've never had issues with over heating
 

CDRIVE

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No argument there. I would have liked the manufacturer to spec it though. ;)

chris
 

feridoun

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I'd say it's warm to touch, probably 30-35 degrees. It says rated for 50/60hz
 

CDRIVE

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Sorry, I must have missed that when you posted the links.

Chris
 

krowlys

Jan 3, 2014
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Hi everyone, just wanted to share that I got my Nest thermostat working with a 24v transformer and 24vac relay using the following wiring. My boiler is a Vaillant Ecotect plus 837. http://imgur.com/JjHJQq2 .
Hi,

I'm trying to accomplish the same thing and wondered if you could give me some advice.

I have ordered a Nest from the US and will get it in about a week hopefully. I sort of get the diagram but am not an electrician. I think I'd best have one take a look at your diagram :)

Do you have any advice on the transformer \ relay to purchase. I'll probably have to hide this in the wall somehow or find a new place for the thermostat. Any help you can proved would be appreciated.

Thanks
Jamie
 

feridoun

Aug 4, 2013
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Hi,

I'm trying to accomplish the same thing and wondered if you could give me some advice.

I have ordered a Nest from the US and will get it in about a week hopefully. I sort of get the diagram but am not an electrician. I think I'd best have one take a look at your diagram :)

Do you have any advice on the transformer \ relay to purchase. I'll probably have to hide this in the wall somehow or find a new place for the thermostat. Any help you can proved would be appreciated.

Thanks
Jamie

Hi

I did a little write up for novices like myself :

https://sites.google.com/site/feridounspage/home/nestlearningthermostatintheukeurope240v
 
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