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Discussion Starter · #1 ·
Trane XL18 suddenly stopped working, the thermostat XL824 is dead. Reset all house circuit breakers, still no power to thermostat. Verified that furnace unit is getting 110 (122.5) from its power source, but got no output on the 24V transformer. Ordered replacement transformer and swapped out. Before connecting the 24v output lines to the motherboard I took a voltmeter and tested the output by touching the 2 output wires/blades at the same time. To my surprise it registered 42v. Took the transformer back to Grainger where they bench tested it and it registered 27v for them on the output.

Reinstalled the transformer (still not connecting output to motherboard) and re-tested. Again I get 42v. Borrowed a neighbors meter and got strange readings of 3, 1, -1, 2, etc. Just jumped around and never showed steady voltage. What could be causing this?


I can provide a link to a quick video showing the voltmeter output while testing.
 

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I can only think that the transformer isn't getting 120 volts to the primary and your voltage reading is not 42 volts but more like 42 millivolts.

Measure the input volts at the transformer primary to verify 120 vac there.
 

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Discussion Starter · #4 ·
Thanks, I disconnected the two 110 input leads to the transformer and they are reading at 122.5 AC steady.

The replacement transformer is the same as what was there before, part TRR01729

I tested both voltmeters on my truck battery and they both read 13.3v

When I can post links, I have a short video showing the transformer test and the meter.
 

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Leave the transformer connected to the board and measure the voltage it is getting under that condition. Do not disconnect it and measure the board lugs. You're blowing the value of the check. Transformers can't just start putting out double their rated voltage unless you double the primary voltage. There is something wrong with your meter or the input voltage to the transformer. I suspect the former.

You also need to ohm out the winding on the transformer that you think failed. If it has an open secondary winding, you may have a short that will blow the new transformer too. It may not be failed and you may have some other problem. Test it to see if it actually is bad. Use a different meter if you can but be sure you have your meter set for AC volts and not DC volts. We need to cover all the bases here since the transformer can't possibly be doing what you say it is.

Some control transformers are internally fused so you can easily ruin one by a momentary short. I always put an in-line fuse in the secondary hookup to temporarily protect the transformer just in case the onboard fuse has a short ahead of its ability to protect.
 

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Hmmm.. I just saw your latest post. Looks like you didn't have a valid voltage test. I suspect more than ever that you never had a transformer problem to begin with.

Tell us what the system was or wasn't doing that made you start working on it.
 

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Discussion Starter · #11 ·
Thanks again. To recap: the system suddenly stopped working a few days ago. No power to the thermostat and the LED on the motherboard that is normally lit up was dead. After checking the input voltage of 110 I checked the output of the transformer and it was zero (although meter was set to DC). Grainger tested the failed transformer and it showed zero for them to. Brought home the new transformer and thats when I started getting the weird 48V reading (because my meter was set to DC).

Setting the meter to AC shows that the transformer blades are outputting 28v. (AC)

Went ahead and connected the 28V outputs to the motherboard and energized. Green LED on the motherboard lit up and started blinking.

But after just a few seconds the LED on the motherboard went dead and then I could smell the 'new' transformer. Powered down and it was hot to the touch. Disconnected the 24V lines from the motherboard and retested and now the 'new' transformer is dead as well.

So I have a short somewhere?
 

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Yes. You should get an HVAC tech to troubleshoot it since you can cause more damage if you are not familiar with going further with this. Smoke testing with a new transformer is expensive and could give you a huge plasma fireball. That's why I do what I described in the last paragraph of my other post.

Stay safe and get some competent help.
 

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In the meantime, can you think of anything that may have damaged the wires going to the thermostat or possibly out to the compressor unit if it's a heat pump.

I'm sorry we can't coach you through this repair but it will require lots of dead circuit tests with an accurate ohmmeter and someone who knows where and what to measure and what readings to expect.

Or he may disconnect certain portions of the system and check with a fused transformer for current draw. Lots of fun that takes time to avoid further damage. Me, I'd put a couple of 12 volt bulbs in series with the secondary and unplug things starting at the far end and working back until the lights went out. To each his own, your tech may have his own plan.

Good luck, SD2
 

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Discussion Starter · #16 ·
Yes, time to call in an expert.

The event logs for the System on the Nexia website show the thermostat "disconnected by System event" 1/6 @ 4:41 then it has some more 'connects' and 'disconnects' until 1/6 @ 5:44 when it finally quit for good. Scrolling back from 1/5 - 1/3 in the logs there are no other connect or disconnect messages so it would appear the problem started 1/6 @ 4:41.
 

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Discussion Starter · #18 ·
Final update:

The tech found the outside contacter was fried, as well as the 110 to 24 transformer. After replacing them, he discovered that the thermostat (XL824) was sending heating and cooling signals at the same time. There were no errors on the thermostat screen. Even with the thermostat set to "off", the outside compressor was turning on.

Replaced the thermostat and now all is good!
 

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That's the trouble with those digital thermostats. The old mechanical ones were more reliable and harder to damage. In your case, the shorted contactor wouldn't normally have damaged the thermostat if you were in heating mode since it isn't R leg connected during that mode. I suspect the tech caused that damage in his testing procedure but so what... smit happens and all is well now.

Thanks for posting back with the update

Regards, SD2
 
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