Honeywell St9400c to nest 3rd gen

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Hi
have a Y plan heating system with honeywell st9400c controller and a wired honeywell thermostat in hallway .

I have worked out which wire on st9400c goes to which nest heat link terminal.

What I am not sure is how to get 12v power required for nest thermostat . I don't have a direct cable between thermostat and heat link. Also based on st9400c photo attached in don't have any spare cables between controller and main junction box.

Help please..
 

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Do you have a room thermostat anywhere, or just your ST9400C?
 
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Correct, so you need to wire your heat link into the wiring centre and use the existing thermostat wires to connect the Heat Link to the Nest, which will replace your thermostat
 

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