What are the main culprits of transformer high voltage side burning out?

[BoulderJ]

Greetings,

For the second time in two years I have had the high voltage side of my 120VAC/24VAC 40VA transformer burn out on my furnace (~10 y.o. Amana model). Last time, after much troubleshooting, I isolated this as the bad part, chalked it up as a fluke, and replaced it having no problems with heat or A/C until now. Since it has recurred I am investigating further for the root cause, and there may be a second inter-related electrical issue involved so I'll do my best to describe all the symptoms separately.

The first indication of a problem in both cases has been a bizarre dimming/flickering of the lights in the house on a (presumably) separate circuit (Circuit B), but one that could have some wires in close proximity to the crawlspace circuit for the HVAC (Circuit A). This happens in kind of a 3-step cycle over the course of a few seconds where the house lights progressively dim along with an audible click whenever the furnace is trying to turn on. It's as though the furnace is drawing a progressively bigger load and sort of like if you just kept turning the switch of a 3-way light bulb once a second (but in the reverse direction and ignoring the off position - ...bright, med, dim....bright, med, dim...). In this state, the furnace fails to come on and the cycle continues until the furnace is turned off or somebody goes to switch off the breakers for these circuits. To my knowledge, no breaker has ever tripped on its own during this process, but maybe it would eventually.

I cannot say for sure what is going on between Circuit A and Circuit B. They should be separate circuits and they act like separate circuits at all other times, each controlled by its own breaker with no known shorts or stray voltage between them. Perhaps there could be some kind of induction between them due to the paths of the wires(?), but nothing that should be significant or would plausibly explain why the lights on Circuit B are affected. Because I needed to at least restore heat, I've found on both occasions that I can replace the burned-out transformer and supply 120VAC to the furnace junction box via a temporary extension cord fed from a separate and unaffected circuit (Circuit C). The furnace seems to work just fine like this so it doesn't seem like there's been any damage to the furnace components or control circuitry beyond the bad transformer.

So the first fundamental questions are:
- What kinds of things would cause the transformer to repeatedly burn out on the high voltage side?...a power surge?...a short circuit upstream?
- Aside from some messed-up wiring, is there anything that could explain Circuit B getting affected by Circuit A?
- If the hot wires are crossed from Circuit A to Circuit B, it doesn't seem like that would result in increased or incorrect voltage; it would just potentially mean that switching off the breaker for either circuit might not cut power to it as expected, right?
- If the hot from Circuit A were crossed with the neutral or ground from itself or from Circuit B (or vice versa) wouldn't that result in a short and trip at least one of the breakers?
- Are there any pieces from the puzzle obviously missing here or other things I should check as the first troubleshooting steps?

Just for the record, I have worked for 6 years as an electronics/electrical technician in the marine industry and have done wiring on a few home construction projects. I'm not completely up to speed on shoreside electrical code or HVAC systems, though I have a pretty solid grasp of the fundamentals. Also, the previous owner of my house was a bit of a crackpot at electrical/wiring so I wouldn't rule out the possibility of some shoddy "handiwork" lurking in the walls or attic.

Well, I know this was a long post, but any insight you can give would be great. Thanks in advance!

[Admin]

It sounds like it's an issue with the 120V wiring in the house and not the furnace.  Power surges can cause the primary side of the transformer to fail.  Have you made sure that the furnace is in fact on its own dedicated 15A circuit?  Take a look inside the panel too, make sure nobody has rewired things incorrectly.  You should have an electrician look things over to be safe.  I would probably just run some new wiring to the panel and install a new 15A breaker and see what happens.  Don't forget to wire a switch into the new circuit, and mount it somewhere visible from the furnace.