Nest V3 Thermostat Wiring Setup

nokiauk

Active Member
I've had my Nest thermostat installed but am curious about aspects of the wiring if anyone can help.

Coming into the Heatlink are:

2 x Neutral (Grey/Blue)
2 x Live (Brown)
1 x Call for Heat (Black)
1 x Call for Water Heat (Green)
2 x 12v to Thermostat (Red/Blue)

Now this is wired as per pic:

N = 2 x Neutral
L = 1 x Live
2 = Loop wire from L and Loop to 5
3 = Call for Heat
5 = 1 x Live and also Loop from 2
6 = Call for Water Heat
T1/T2 = 12V for Thermostat

My understanding is that the common (2) needs a supply from L and common (5) also needs a Live supply which is normally a loop from (2) which is in place.

Is there any reason why 5 has a direct Live feed? My only reasons for wiring this way would be:

A: The terminals can only support a max of 2 wires, hence 2 x Live + Loop will not fit into L
B: In the case of the above the other Live wire went to 5 and this isn't enough and the Heatlink still needs a bridge from 2 in order to operate?

Anyone got any insights or has it been badly fitted? Needless to say it does work ok.

Thanks
 

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Jamie

Distinguished Member
Where do the 2 live wires go/come from?

I suppose one could be the actual live supply and the other the live feed to the boiler (or maybe another device that requires power) but that does seem a strange way to wire it up.

My Nest Heat link gets its mains supply from the boiler which would be the usual way of powering the heatlink.
 

nokiauk

Active Member
I suspect they come from the wiring centre beside my water tank rather than from the boiler direct. There was a Danfoss TP9000 in place previously.
 

Jamie

Distinguished Member
Your old Danfoss has a very similar wiring schene to the heatlink however the Danfoss is specifically made to use with boilers that use a switched live to call for heat and water. The Danfoss doesn't have the equivalent terminals 2 and 5 as they're already connected to live internally. On the Nest you can use it to control boilers that use a low voltage separate switching circuit as well hence require the link if used for a boiler with switched live controls.

This is the Danfoss wiring diagram. I've added in the location of terminals 2 and 5 on the nest, the red lines are the equivalent of the links on the nest but are internal

Screenshot_20210409_091318_edit_183310382318382.jpg

I'm 99% sure your 2 live cables are a supply and a loop out to somewhere else (to power another part of your system. Originally I'd have thought both browns and both blues would have been together in the L&N terminals on the Danfoss but as you correctly identified you'd struggle to get 3 wires in one terminal on the Nest.
 

Jamie

Distinguished Member
Glad you're sorted. I have actually replaced a TP9000 with a Nest at a mate of mines house, however we took the opportunity to move the heatlink next to the boiler and wiring centre and wired it directly using a multicore cable. I did reuse 2 of the original wires for the Low Voltage power to the Thermostat as we mounted the thermostat where the TP was previously.
 

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