Wiring help. Not sure what to do.

[JR1313]

When I removed the old thermostat it had two sets of wires hooked up to it. Each set had one red, one black, and one green. Both wires of each color were in the corresponding slot on old thermostat. I only have a furnace, no a/c. When I looked at the circuit board in furnace, there is only one set of wires coming off of it. I'm not sure where the other set is coming from. Anyway, I noticed only one wire can fit in slots on Nest Learning Thermostat 3rd Generation. Am I able to connect both wire of each color together then use pigtail to connect to Nest? Thanks.

[CoolingWizard]

@JR1313 yes, indeed you can wire three wires together giving yourself a little pigtail to give you three wires for your thermostat. The real question is what is your thermostat controlling because normally it would only be controlling one device but yours looks like it controls two separate devices. Can you upload a picture of your old thermostat that you took before you disconnected the wires? 

AC Cooling Wizard

[JR1313]

 this was the old one

[CoolingWizard · Accepted solution]

@JR1313 Go ahead and use a wire nut and add a third solid copper conductor for Rh, G, and W. Since this is an heating only system, attach the red wire to Rh of the Nest Learning Thermostat 3rd Generation. Connect the Green wire to G.  The last wire is for heating so attach it to W.  

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AC Cooling Wizard