Yet another Nest thread.

5 Mar 2015
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United Kingdom
Hi all, i've bought another Nest thermostat for our current home, the setup is i believe Y-Plan and was previously fitted with a ST6400C honeywell controller and a generic thermostat.

I've turned the stat to max as i wish to test everything first, when i turn on the heat using the existing ST6400C it fires up the boiler, same for water.

so i thought it was super simple and so i test wired in my heatlink following the instructions, confirmation from a few online posts and the old honeywell label inside the unit.

Please excuse my bad wiring, ill be doing it properly once i have confirmed its working correctly.

So i did the following wires:

Honeywell to Nest:
N -> N
L -> L
4 -> 3
3 -> 6
1 -> 4

I then added a link between 2 & 5 on the heat link.

Now i believe this should be working but when i press the nest button to manually fire it up (Both green lights illuminated on the heatlink) the boiler does nothing.

old stat is at 30 degrees, its no more than 15 in the room so eliminated that possible issue.

Any help much apreciated.


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20 Jul 2005
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United Kingdom
For Y Plan, the two commons 2 and 5 should also be linked to L.

Y Plan.JPG

Without the connection to L, the Heat link switches will be operating, but they won't have any electricity supply to switch.
Last edited:

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