Why does the 115V->24V transformer keep blowing on my Trane XB80?

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It would be pretty unusual for lightning to keep destroying the same single transformer in an HVAC unit without also detroying other things like the electronics board in the same furnace, or other appliances, eqpt, etc in the house. I've seen heard of lots of stuff inside a house getting hit by a surge, but never just a transformer, which should be among the most surge immune devices, for a variety of physics based reasons.
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On 4/2/2011 2:10 PM, Steve Turner wrote:

A few years ago mine went and I happened to have a little radio shack transformer hanging around. I wanted heat right away so I installed it (not using the center tap). Still works. Not rated for watt I'm using it for but I've never seen a fused or non-fused transformer start a fire so I sleep well. I did fuse it though I forget the value.
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Steve Turner wrote:

TWO is (probably) a coincidence; THREE would be a trend.
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On 4/4/2011 7:33 PM, HeyBub wrote:

mistake. Make the same mistake twice, and it's jazz.
Jeff
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If the transformer has any bare metal, keep in mind that IR thermometers usually do not get good readings on bare metal. Stick on a 3/4 inch square of masking tape.
--
- Don Klipstein ( snipped-for-privacy@misty.com)

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Frequently as accessories are added the VA value of the transformer can be exceeded, or run at the top end for too long. Next time you replace the tra nsformer increase the VA rating by 25VA. If it is a 50 VA go to 75VA.
When you turn the unit on, measure the voltage across the transformer. it s hould be above 20VAC when the unit is on and fully loaded. If it is not is in an indicator that there is a partial short in the 24VAC system. This can be traced down by eliminating some of the 24Volt accessories until the vol tage returns above 20VAC.
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Next tme you reply to a post, check the date.
Mark
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replying to Steve Turner, YosemiteDave wrote: Check the contactor in the outside unit. These short out on the 24 volt side and taking out the transformer. But if the fan board on a XB80, the fuse should have blown if that was it. But older ones did not have a fuse, I think. If the fuse didn't blow, I'd say ya just got another bad transformer.
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