Nest thermostat e heat link

[trialnick]

Hi all. My next e heat link is wired with only the c and no, connected to the 24v connector on my vaillant boiler. It works perfectly ok, but the AA batteries for backup only last one day. If I put a meter earthed to the boiler chassis I get 26 volts on either c or no when the boiler is running. I have moved the heat link next to the boiler and wired directly. Should the heat link be earthed ? Or what’s wrong?
Help required please. 

[CoolingWizard]

@trialnick ,

The Nest HeatLink needs 240VAC 50HZ power on the L and N terminals. When the Nest Thermostat display needs power it can be hardwired to the HeatLink or powered by USB port when attached to the Nest Stand. 
The thermostat and heatlink are bluetooth paired to each other from the factory.  It is possible to replace either and then bluetooth paired them again. When the Nest Thermostat display is hardwired to the HeatLink T1 and T2 it receives 12VDC power from the HeatLink. The A0001 thermostat does not have replaceable batteries.

The Nest HeatLink is essentially a relay pack. It allows for the connection and control of heating boilers. Each connection has Normally Closed(N.C.) Common (C), Normally Open (N.O.) terminal connections.  And low voltage connections for Open Therm (OT) boiler control and Nest Thermostat (T1/T2).  T1 and T2 are 12V DC power non-polarized. 

The Cooling Wizard

[trialnick]

Thanks for the response. There are no L and N connections on the heat link only the open therm.

As the AA batteries are running down rapidly it must be operating from those and not the internal battery. Sorry if I’m being dumb??

[CoolingWizard]

Here is a picture from the official Google Nest E and Heat Link installation manual. Note on the left side are the Line and Neutral terminals. 

 The Cooling Wizard

[trialnick]

Hi my heat link e has the following connections

c                       Qt1

no                     Qt2

fp