Nest has power but furnace not kicking on

[Soochick]

Here is our wiring. Thermostat says it has power but although it says heating, the furnace is not coming on. Is it not compatible?

[Antbanks]

I had similar  issue, mine turned out to be the fuses  inside  the furnace...  they kind of look like car fuses 

[CoolingWizard]

Your system does not appear to be a 24VAC based system. That red and white wire and black wires on top of the gas valve is a output of 500 to 700 milliamps that when connected to the input side will turn on the gas valve. Is this a water boiler? I see the thermalcouple. The  thermocouple that the pilot light is heating generates the milliamps needed to operate the gas valve.  This system is not comparable with the nest thermostat.  The nest works with conventional HVAC system that use a 24VAC power source to operate.

Ken, The AC Cooling Wizard

[Houptee]

Where do the 2 thicker black wires go?

Is there a 24v transformer that those black wires originate from?

If there is a transformer then it should work but if this is a millivolt system like Cooling Wizard mentioned then it will not work.

You can confirm if it is a 24v or Millivolt gas valve by reading the data decal on the side of the gas valve it should show the voltage info.

What did the black wire connect to on the old original thermostat?

How did you wire the Nest did you connect that black wire?

If so to what terminal on the Nest?

[Jhonleanmel]

Hey folks, 

We appreciate your help here, @Houptee and @CoolingWizard.
I wanted to follow up and see if you are still in need of any help. Please let me know if you are still having any concerns or questions from here, as I would be happy to take a closer look and assist you further.

Thanks,
Mel

[EdwardT]

Hi folks, 

I wanted to follow up and see if you are still in need of any help. Please let me know if you are still having any trouble from here, as I would be happy to take a closer look and assist you further.

Thanks,

Edward

[Jhonleanmel]

Hi everyone,

I'm just checking in to make sure that you've seen our response. Please let us know if you have any other questions or concerns as I will be locking this in 24 hours.

Thanks,
Mel