I just replaced my old thermostat and timer for a Google Nest, but every time I turn the electricy back on, it goes off shortly after. I have an oil boiler for heating.

Before I call an electrician, what could be the problem? How can I troubleshoot it?

I'm looking for guidance more than for a straight answer if that's possible.

Thanks!enter image description hereenter image description hereenter image description here

  • 2
    Are you referring to a breaker at the panel or a breaker in the heating system? Either way, what was the thermostat you replaced? How many wire system do you have? Did you run a common wire? The NESTs instructions come with a series of alternative-situational wiring schemes you follow. Also, wire color is NOT always correct, so it is best to verify the colors and the connections at the control board side **the furnace side. What could be wrong? You wired a current carrying wire to a neutral/common.
    – noybman
    Aug 20, 2017 at 14:25
  • 2
    Post pictures of what you changed, maybe someone can suggest specifically what to test. It appears you've created a dead short.
    – Tyson
    Aug 20, 2017 at 14:26
  • 2
    Oh - you also mention "thermostat AND timer" did you replace two things? Please tell us what they were, and ideally, their model numbers
    – noybman
    Aug 20, 2017 at 14:27
  • 2
    Can you post photos of the thermostat wiring at both ends? (t-stat end and furnace end) Aug 20, 2017 at 14:35
  • 1
    Please add photos as @ThreePhaseEel notes. Definitely want to see what you are working with behind those components. What is under that rectangular panel? Also, while it appears you have it hooked up "the way it was", does it work? Would be good to ensure there isnt something ELSE tripping the breaker
    – noybman
    Aug 20, 2017 at 14:43

2 Answers 2


The guidance we are/have been providing in the comments.

The direct answer, is that you have a current carrying wire directly tied the return. (E.g., Neutral & Live In wired together). While this may not be exactly what happened, its an example.

Unfortunately, the diagram the book shows you is possibly misleading and therefore it is likely you were able to either misinterpret the wiring, or just juxtaposed a wire. It i also even possible that the Nest could be damaged or faulty, but vetting the wiring used will help ensure that is not the case. If it is, return it for a new one.

The pictures help to ensure any guidance given is sound, since there are many possible things that could actually come into play to cause an issue like this.

If you have a multi-meter it is invaluable. It is best when doing these types of connections to always ensure without any doubt each wire is what you think it is. So performing OHM tests between L & N, or End to End on a wire is a great way to have this surety.

Only then, would you turn the power on. Nearly eliminating any chance of damage to the equipment, or you.


The object in the photos is a 230V "immersion-heater timer". Most traditional UK heating systems have a "programmer" instead but I guess you could have an installation that just uses the timer. I'm assuming IE practices are similar to the UK.

In that case, wire 1 on the old timer would have provided switched-live to a 230V thermostat and the 230V thermostat would switch that live power to the boiler when needed.

If you just took the wires off terminals 2&3 on the timer and attached them to 2&3 on the heat-link, the heat-link would, when calling for heat, short neutral to live and trip the breaker. But I doubt you've done that because you wouldn't then have power to the heat-link itself.

I assume that you repurposed the wires that previously carried 240V live out to the thermostat and switched 240V live back so that these now carry 12V power to the Nest (as shown by T1, T2 in their diagram)? Presumably you disconnected any redundant wire(s) at both ends and made it safe (e.g. connect to ground at both ends).

I would turn off the breaker, disconnect the wire from terminal 3 of the heat-link, isolate that wire in a connector block by itself, turn the breaker back on and then carefully use a voltage tester to check that all the wires were as expected and that terminal 3 is initially dead (near 0v) but goes live when the heat-link/Nest realise that heat is needed.

Not the answer you're looking for? Browse other questions tagged or ask your own question.