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Heat link gen 2 switches heating in reverse (on when should be off and vice versa)

Community Member


I have used my thermostat for many years with much success, but now my central (city) heating valve short circuited and had to be replaced. (New type: danfoss)

Following that, my thermostat turns on the heating when it should be off, and vice versa. I suspect this is just a wiring issue, but we didn't change the wires from the previous valve, so I guess this one works differently.

Can anyone help? Should I change the number 1 and 3 wires?

I am using a second gen thermostat with gen 2 heat link. It basically just controls the valve that lets hot water flow to my radiators.

Many thanks for your help!




Community Specialist
Community Specialist

Thank you for the information @Sartaxi and sorry for the late reply. The Heat Link has the 1,2 and 3 terminals for the heating. You will usually see a wire from live to terminal 2, or a separate live wire, to provide power to the relay. From there you will generally see a wire from terminal 3 going to the valve or boiler at the location.


When the heating is off the relay is closed between terminals 1 and 2, so no voltage to 3. When heating is demanded from the thermostat the relay will close between 2 and 3 providing power and leaving 1 empty. 


You mentioned changing the number 1 and 3 wires. Currently which terminal has a wire in it and which is empty? If you are comfortable in checking.

Community Member

Thanks @AlexD,

So I managed to get some more info. The Heating was connected to terminals 1 & 2 and working in reverse, as would be expected based on your explanation above.

When I change the wire from terminal 1 to 3, the system should work correctly, however, it doesn't do anything. (Note, the heating system is low voltage and doesn't need additional power, so no need/no go to bridge L & 2)

Using a multimeter, we measured the circuit between terminals 2 & 3, and there is no voltage running through, (we manually switched the heat link with the button to make sure it wasn't due to the circuit being intentionally open).

That leads me to believe that there is a failure in the heat link (and Nest phone support confirmed they think so too).

Unfortunately, it is a 2nd gen heat link which has been out of production for a while, and Nest/Google don't have spares. They suggested buying a new one.

If you have any other suggestions of what we can try, we'd appreciate it, as it seems to be just the number 3 terminal that has been fried (the circuit between terminals 1 & 2 works as intended, just not what we need 😉 ).



Community Specialist
Community Specialist

Hey @Sartaxi. I'm sorry to hear about what happened. If Google Nest support said that the Heat Link is faulty, I'd suggest to buy a new thermostat too. The 3rd generation thermostat will also control your hot water if you have a system boiler. 

Community Specialist
Community Specialist

Hi there. I'm going to lock this thread. Should anything come up, feel free to open a new one.