Thermostat - using common wire blows the fuse

Replacing my Honeywell thermostat. There’s a blue wire which was not being used and disconnected on the thermostat and HVAC side. When I connect that wire to what I assume to be Common (C on the circuit board) and also goes to my condensor it blows the fuse. Any advice is appreciated on how to work around this.

post pictures of the furnace and thermostat connections.

Thanks, CB is PCBFM1035 (difficult to capture in pic.).

Other Pics:-





Pic labeled ‘Condensor - thermostat closeup’ is how I wired the blue thermostat wire coming out of the thermostat wire (off to the right on the pic). The multi-connector has white going to the condensor from the thermostat cable and connecting to the blue coming from the CB marked ‘C’. The yellow from thermostat goes to the red condensor wire, only 2 wires. The open slot beside the white was where I put the free blue cable from the thermostat and blows the fuse when I restore power.

Thanks again for looking, Ian

Do you have a thermostat wiring picture as well?

Sorry, missed that one:-

Does it blow the fuse when the blue wire is connected at the furnace side and not connected at the thermostat side? If so, your blue wire is likely shorted somewhere to the red wire.

If not, then you’re likely overloading the fuse with the addition of the thermostat to the other circuits powered by it. The transformer in that image shows signs of heating and possible overload, with the discoloration of the bobbin, so it might be undersized for the application, or partially internally shorted.

Correct, when adding the blue wire from the Thermostat to the blue ‘c’ in hvac and not connected to the thermostat it blows the fuse.

Your blue wire is shorted then, you’ll want to use the C adapter.
The app should walk you through how to hook it up, if you tell it your old thermostat has the following wires: Rc, G, Y, W.

If you need any more help, come back here and ask, and i’ll make you a diagram.

Thanks, you’ve been super helpful. Couple of things, is the short somewhere in the thermostat wire ? It’s possible I could run a new wire as this just bugs me - OCD kicking in I guess. Also I will replace the transformer re your comment on the bobbin as a new one is only$20, again just bugs me when something’s not quite right even although that’s not the issue.

Once again, I truly appreciate you taking the time out of your day to help myself and others on this forum - invaluable !

Yes, somewhere the red wire’s insulation is worn through and touching the blue wire.
You could run new wire, but it’s usually quite a hassle, depending on how far the run is and how much you like cutting holes in the walls (and patching the holes up afterwards)

Got it, thanks again for all your help and quick responses. I’ll update when it’s straightened out.