Nest V3 Thermostat Wiring Setup


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.



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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.


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.


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


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.


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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