I was changing the pipe insulation on my exterior units( now I hear this is not necessary) when the power went off on one unit. I checked the breakers, turned off the power and checked the small wires outside. The insulation on the wires had deteriorated so they may have touched each other causing a fuse to burn out on an interior board. This I was told over the phone by a electrician. My problem is which fuse, what does it look like? The system is about a 12 years old Tempstar. I'd be grateful for any feedback. Pat

Sometimes there are no fuses and the 24 VAC transformer burns out. Look for automotive style fuses on the circuit board. Around 3 Amps and possibly blue.

There were no fuses. It did turn out to be the transformer. Thank you for your expertise.

I would also replace or repair the uninsulated wires or it will happen again

Add a 3A automotive blade type fuse on the secondary (24 V side).