I seem to be having this problem every season. Two years ago the 24V transformer in my gas furnace failed. Under an urgent repair time line, I replaced it with a 24V door bell transformer which worked fine until about a year ago. I realized that this wasn't the proper part and replaced it with a heavy duty transformer more appropriate for the job. At the time I added a 3A fuse in-line with the transformer and plugged the furnace itself into a heavy duty surge suppressor. Everything worked fine until this weekend again when the heat wouldn't come on.

This time I found that the 3A fuse I added last year had blown. :mad: At least the transformer was protected this time. Replacing the fuse has restored the heat for now. I feel like this is a bit of bandaid however and I am wondering what could be causing these apparent spikes that are blowing the transformers and now the fuses?:confused:

Make sure you have the proper Va. rating on the transformer. :)