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airheatman, HVAC Technician
Category: HVAC
Satisfied Customers: 9557
Experience:  Furnace, A/C and heat pump specialist.
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Connected new programmable thermostat and now furnace does

Customer Question

Connected new programmable thermostat and now furnace does not turn on. Put back on the old thermostat and furnace still still does not come on
JA: Just to clarify, do you think this is a larger HVAC problem, or something specific to the thermostat?
Customer: We think it is the thermostat. The furnace was working fine until we took out the existing thermostat and connected the new programmable thermostat
JA: Do you plan on doing the work yourself?
Customer: Yes especially since we are in Indianapolis, it is about 17 degrees outside and it is 11 pm
JA: Anything else we should know to help you best?
Customer: Do you need to know the model number of the thermostat
Submitted: 8 months ago.
Category: HVAC
Expert:  airheatman replied 8 months ago.

I will be happy to help. You may have let wires touch or connected on in wrong place and blew a small 3 amp fuse on furnace board. Tell me first about the wiring.What wires you had to what terminals on old and where you put on new.

Expert:  airheatman replied 8 months ago.

The two most common reasons for your current situation are:
1.You did not turn power off at furnace and while replacing wires allowed the R power wire and the C Common wire touch and spark, blowing the fuse on furnace board or
2.If your old thermostat had a B terminal for common and you put that wire on B terminal of new thermostat(New thermostats use C for common and B for a heat pump reversing valve).So if you put the old B to the new B and call for heat , this would again create a dead short and blow the little fuse in furnace.
Below are images of the fuse I am referring to located on control board in blower compartment.
If you simply forgot to turn off power and let the wires touch , blowing the fuse, all you should have to do is replace the fuse and you should be good .If you connected an old B to new B, you will need to mobe that wire to C, then replace fuse and you should be good.

Let me know and we will go from here.

Expert:  airheatman replied 8 months ago.

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