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So a few weeks ago I replaced my thermostat with an ecobee3 smart thermostat. In the process of wiring the thermostat I blew the 3A fuse up in the attic and had to replace the fuse. The thermostat then worked fine for about a week. I do have a C wire.

Then suddenly there was no power to the thermostat. I went back up in the attic and even though the fuse looked ok, I replaced it again anyway and the ecobee lit up right away and worked for another week. At the time I suspected I might not have fully locked the cover panel in place and the kill switch might have just popped open.

Today the thermostat was dead again. I went up in the attic and the fuse was ok but I replaced it anyway. No dice. I tested the fuses from this week and last week with a multimeter and they test just fine. Still, I replaced the fuse yet again but still no power. So I don't think it's a fuse problem.

I verified there is 110v to the power outlet up in the attic where the furnace is plugged in. I used a multimeter to test the thermostat wiring for the ecobee and it appears that no power is making down to the thermostat.

I tried pushing on the kill switch while the unit is plugged in and while doing that no lights go on inside the furnace (I read online a light should go on showing the transformer is working) and the thermostat wiring gets no power on the multimeter. There's only one little red light on the control board but it stays dark no matter what I try.

Does this sound like the transformer would be bad? I can't figure out why the ecobee3 would work for a week and then die, only to work again for another week when all I did was open and close the panels in the furnace (and replace fuses that didn't need replacing).

See pics attached -- is it time to call a professional? I don't feel comfortable replacing a transformer on my own.

fuse inserts into the little blue thing in lower left enter image description here enter image description here kill switch

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    The controller shows 25vac have you measured the voltage to make sure the magic smoke is still in the transformer. – Ed Beal Jul 28 '17 at 18:54
  • Not sure how to do that. Can you give a bit more detail, thanks! – Rocky Jul 28 '17 at 18:59
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    I can't see all the terminals in the photo but there are probably marked AC or ~ , using your meter on AC volts place the leads on those terminals (or at the transformer). You may see a different value 24-27 is what I remember seeing most of the time when working. – Ed Beal Jul 28 '17 at 19:04
  • Should that show voltage without the kill switch being depressed? I might need a second set of hands if I need it pushed in. – Rocky Jul 28 '17 at 19:07
  • Some of the switches can be pulled out, the ones that I have worked on like this are plunger type when the cover is on the switch is made, open the door and the switch opens, pull out on the plunger and it makes the switch for troubleshooting. – Ed Beal Jul 28 '17 at 22:17
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Wanted to follow up in case this helps anyone else. I tested the terminals as Ed Beal mentioned in the comments and was pretty sure the transformer was not working. Since I was uneasy with replacing it myself I decided to call an HVAC professional. Turns out that was a smart idea.

He tested the transformer and confirmed it needed replacing: 120v in, nothing coming out. In the process of replacing it he noticed (and I had totally missed it!) that the power cord from the outlet to the furnace had a significant problem:

this is a problem

So after 20 years with just a bit too much tension on the power cord up in a very warm attic space, it sparked, blew the transformer, and fortunately did not burn the house down.

I have a feeling my installation of the smart thermostat and accidentally blowing the 3A fuse a week earlier was not a pure coincidence and probably made the wiring spark sooner. Me removing the furnace cover and plugging/unplugging a cord that had too much tension on it was probably part of the problem, too. Still, it was an accident waiting to happen.

Anyway, I had the wiring professionally repaired, the 24v transformer has been replaced, and everything is working ok now. Thanks for the pointers, Ed.

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