New froststat clicking/lighting up but no response from boiler

[Ron S]

I just use mine to check a wire is (ideally not!) live before working on it.

 

[Harry Bloomfield]

The main issue with those type of voltage testers is that you can only test for the presence (or lack) of a live conductor.
For an electric device to work you also need a neutral and those (and the no contact wands) screwdrivers wont tell you that.

Actually, they can, sort of....

If the item in question is switched on, they will show the neutral as being live, if the neutral is not connected to neutral.

They are even useful, to checking without opening up equipment. With a bit of skill, you can run one along the flex, and determine whether 1/3 / 1/2 of the circumference is live, or not. 2/3 live, or entire circumference, suggests no neutral.