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Nest 3rd generation thermostat heat link no 12 volt

Community Member

Heating System not working. Fault H72. Tested but no 12 volt power supply at heat link output . Mains power to heat link ok. Heat link set to manual green lights on but relays not switching and therefore boiler not starting. Think heat link is defective.I see Google were recalling Heat links and replacing them last year?



Do you have all 3 green lights on when in manual mode?

Have you tested the terminals going out with tester to prove relay not switching?

Only have 2 green lights and one yellow. Can only get manual if I plug a charger cable into heat link. I have tested the outgoing terminals and the unit isn't switching. I've wired in a light switch so I can get some hot water


I assume you mean Yellow light at the top, and 2 green lights below when Heat Link button pressed? This would be manual mode. This is with power going into the Heatlink, ie Live Neutral? You don't mean with the USB cable plugged in you get manual mode do you?

Why would you put a USB cable into the Heat Link? I assume you mean the USB port on the right hand side? That doesn't do anything to solve your issue.

I think your problem is not switching, but faulty Heat Link due to component failure, hence the H72 error. I take it the display screen says H72 error, and you only get to the Tech screen on the display?

Community Specialist
Community Specialist

Hi there @Kenrobson! I'm really sorry to hear that you encountered this issue. Just to confirm, your Heat Link has a yellow light on it, and if you activate manual mode, the other two green lights below turn on? And if so, this is without having a USB plugged in the port on the Heat Link, right?

Community Specialist
Community Specialist

Hi there, just checking in! Did you see the message from above? 

Community Specialist
Community Specialist

Hey there, I haven't heard from you in a while so I'll be locking this thread. If you have any new issues, updates or just a discussion topic, feel free to start a new thread in the community. Thanks, AlexD.