Nest 3rd Generation Heat Link Wiring Advice Needed


Postby anzuapricot » Tue Nov 21, 2017 4:24 pm

Hi, I'm trying to replace my existing control panel with Nest Heat Link. I've opened the panel and see 2 x yellow/green wires going into earth. What do I do with these wires on Heat Link? Do I need to put them to T1 and T2 on Heat Link or can I just put them to earth again? The photo is my current wiring...
Thank you in advance!
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Postby ericmark » Thu Nov 23, 2017 3:19 pm

Put them into a block connector and do not connect to Nest if Nest is Class II.

Do [u][b]Not[/b][/u] connect to T1 and T2 these connect to the eBUS of boiler when the boiler is OpenTherm enabled.
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Postby anzuapricot » Tue Nov 28, 2017 11:35 am

Thanks, ericmark. When you say put them into a block connector, which one do I put them? Into the earth block?
The nest Heat Link has these connectors.
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Postby Mr White » Tue Nov 28, 2017 6:51 pm

They are earth wires, therefore they should be connected to the earth terminal.

18:51 28/11
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Postby anzuapricot » Wed Nov 29, 2017 2:24 pm

Thanks for the replies.
OK, so I've installed Nest Heat Link. but sadly the boiler doesn't come on when I manually turn on Heat Link!! Does this mean that our boiler is not compatible with Nest? Or is it time to call a professional? Our boiler is a combi boiler.
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Postby Mr White » Wed Nov 29, 2017 7:28 pm

Truth be known most if not all "smart room stats" are just a gimmick. They cost in the region of £200 (Including installation) and you will take years to recoup that. (More than the life of the stat itself)
At the end of the day they just turn on or off (The same as any other thermostat) so if the one you have does not work, it is probably incorrectly connected, in which case you had best engage the services of a person who has installed several before.
Some times it is easier to stand infront of something and physically look at it, this is one of those occasions.

19:27 29/11
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Postby BLAKEY1963 » Wed Nov 29, 2017 10:17 pm

Anzuapricot
Call a person in
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Postby ericmark » Sat Dec 02, 2017 6:11 am

[quote="Mr White"]Truth be known most if not all "smart room stats" are just a gimmick. They cost in the region of £200 (Including installation) and you will take years to recoup that. (More than the life of the stat itself)
At the end of the day they just turn on or off (The same as any other thermostat) so if the one you have does not work, it is probably incorrectly connected, in which case you had best engage the services of a person who has installed several before.
Some times it is easier to stand infront of something and physically look at it, this is one of those occasions.

19:27 29/11[/quote]
Not quite true, modern boilers are modulating and the flame height alters this is what the OpenTherm connections do, when the boiler does not have OpenTherm then it does simply switch on/off but good thermostats use anti-hysteresis software so as it approaches the temperature it uses a mark/space ratio to slowly reduce heat rather than simply switching off. It also has the ability to use IFTTT to connect to wifi TRV heads so either the heads tell the Nest when to turn down or Nest tells the heads according to how set up.

However I have to admit when I fitted the TRV electronic heads I found the house both cooled down and heated up too slow to be able to use the phones location to automaticity turn heating on as I got close to home.

For mothers house I had a problem, the bay window heats the living room up when the sun shines, so the heat requires varies with the cloud cover, using eTRV heads helped cure this problem, next step would be Nest, however since we rarely have the house unoccupied I did not really need Nest.

You show the old programmer base, but you don't say which boiler, how best to connect nest does depend on boiler, some have OpenTherm but most can't use that option.

So likely your best idea it to get some one in, who can work out how you boiler should be wired with Nest.
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Postby ericmark » Sun Dec 03, 2017 8:30 am

I have just read https://www.raypak.com/support/tech-corner/modulation/ and it seems I have made a mistake, it talks about standby loses, so any thermostat which switches the boiler on/off will waste energy, so not sure where the anti-hysteresis comes in as good and where it is bad, as if using a mark/space ratio to reduce the hysteresis then it is losing energy because of standby losses, but gaining because it does not over shoot.

So if Nest is used in a non modulating mode it may not save that much, however fitted in the modulating mode using OpenTherm then it does work better than the old types.

It also seems modulating is not the same across the range, some boilers can reduce output by a 1/3 others can reduce output by a 1/10 of full output, it is likely most houses only use around 5 kW to maintain the heating, yet the boiler is 28 kW to ensure enough domestic hot water, so many boilers are over sized for the house, so use mark/space ratio not modulation to control output.
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