Blower in but no heat (no flame)

[Southbasin]

My home thermostat wiring only has 3 wires. W1, Rh and Y.  Nest thermostat was sorta working. It was also turning on my air conditioner…..it’s -10C here. 
But the real problem is that it calls for heat and immediately turns on the blower fan but the furnace doesn’t fire up (no flames). Switched back to old thermostat and everything works fine.  Do I need the Nest power connector?

Any ideas?

Thanks in advance. 

[Houptee]

Did you take a pic before you removed the old tstat? Can you post it if you took one.

3 wires is very strange for a Heat and AC system.

Are you sure there was no G wire?

Do you have a C common wire connected?

Usually you put the red wire to Rc and then connect a C wire for the constant power but it has to be connected to the C screw on the furnace also.

[Southbasin]

Thanks for your response. Yes - 3 wires only - no C wire - the current thermostat runs the forced air furnace and the A/C unit. 
Wondering if I should buy the Nest power connector. 

[Houptee]

If you have a spare wire in the wall that was not connected to anything then you can use that as a C wire but you have to also connect it to the furnace C terminal screw. Can you take cover off the furnace and take a pic of the thermostat wires connected there so we can see if you have a C terminal.

[Jhonleanmel]

Hey folks, 

We appreciate your help here, Houptee.
Southbasin, 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

[Jhonleanmel]

Hey there,

Checking back in should you still have some questions here. Let us know by replying to this thread. 

Best, 
Mel

[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 thread after 24 hours.

Thanks,
Mel