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Why does the transformer burn up, about 2 minutes after i turn on the power to the furnace? Nothing gets turned on at the thermostat. I just plug it in and push in the blower door switch and it burns up. I know after a lightning strike I heard a "slap" sound and then smelled burning. I know it was the transformer because there was no power light on on the control board. When I replaced the transformer the lights came on, then the transformer burned up. So I replaced the contactor and the transformer, the lights came on again, then the transformer burned up.

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    It sounds like your control board is having a bad day. What make and model is this furnace, and do you have a meter handy? – ThreePhaseEel May 16 '17 at 3:29
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    Are you using a large enough transformer? That is to say did you accidentally downsize when you replaced it? Second consider adding a fast acting fuse on the low voltage side that is smaller than the transformer current rating, that way you eat fuses instead of transformers while you troubleshoot. – Harper May 16 '17 at 3:46
  • I have just a cheap voltage meter. It is a Rheem criterion gas furnace. Model RGDG-07EAUER. And I replaced the transformer with the exact specs. – Bobbie May 16 '17 at 3:59
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    There is a shorted circuit somewhere in they wiring, (a burned out part, a loose or pinched wire, bad circuit board, or something. If you are good at trouble shooting and have knowledge about your furnace, GREAT. If you don't call a service tech before you make a big and costly mistake. – d.george May 16 '17 at 9:43
  • Maybe the lightning strike got the control board? Would the power light still come on if it was bad? – Bobbie May 16 '17 at 13:55
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Your transformer is undoubtedly burning up because something in the rest of the transformer load circuit also got fried in the lightening strike and is placing a load on the transformer that greatly exceeds its normal operating specifications. Could be your control board, could be the thermostat, anything that is a load on the transformer output.

The idea posted in comments to fuse one leg of the transformer output is a good trouble shooting scheme - fuses are going to be cheaper than transformers. So it seems like you will be replacing a few fuses whilst you try to isolate what component is severely loading the transformer. I would place my bet on a relay coil that has burned the insulation on the coil to a near short or a semiconductor that has suffered on chip meltdown to a short.

  • You can also upgrade your transformer with a small circuit breaker on it. Just make sure you have the right voltages and the right KVA rating. You definitely have something else burnt up in you controls. – Retired Master Electrician May 16 '17 at 14:48
  • I took the jumper out on my thermostat, then I put a fuse on my transformer just in case that wasn't the problem. I turned the power on to the furnace and the transformer didn't blow the fuse. I pushed the reset button on my thermostat and switched it on cool, and it came on and is working great!! Why would that have been the problem? – Bobbie May 22 '17 at 22:04
  • @Bobbie - We have absolutely no idea of what you mean by "took the jumper out on my thermostat". That could be many different things. It could even possibly have been a short across your transformer output if someone had totally forked up the thermostat wiring. – Michael Karas May 23 '17 at 0:05
  • @Tester101 and Michael- The jumper that is on both RC and RH on the thermostat. I just took that out and only have the wire on the RC terminal. – Bobbie May 25 '17 at 11:06
  • It was working fine until the lightning struck that morning. I just heard a loud slap sound and then smelled something burning coming from my furnace. – Bobbie May 25 '17 at 11:10
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As others have mentioned, the transformer is burning up because the load on it is too great. This is likely caused by a damaged component within the control circuit, which is drawing far too much current for the transformer. Rather than burning up more transformers, and/or blowing through a crate of fuses, why not try troubleshooting the circuit with the power off.

Turn the power off to the equipment at the breaker or serviceman switch, and insure it's off before poking around in the equipment.

Start by visually inspecting all the low voltage components (relays, circuit boards, limit switches, etc.), looking for obvious signs of damage (broken, burnt, discolored, etc.). Look for things that are out of place, especially things that might be shorted to the chassis or other grounded items.

If you have a thermal imaging camera (not likely), you could look for hot spots when the equipment is powered up. Using an inline fuse, this method might only cost one or two fuses.

If nothing looks bad, it's time to start poking around with a meter. Grab the schematic, and start isolating and testing each component individually.

  • When I test the voltage outside on the contactor, the voltage coming in to the contactor reads 277v. Shouldn't it read 240v? – Bobbie May 17 '17 at 1:16
  • @Bobbie if you're supplied by a single split phase 120/240V system, then yes you measure ~240 volts line to line. – Tester101 May 17 '17 at 1:23
  • It is 120/240. So why would it read ~277v instead of ~240v? There is no power coming from the thermostat or furnace. – Bobbie May 17 '17 at 1:50
  • Are you getting 277 volts at the service panel as well? Are the batteries in your meter fresh? Is it a quality meter, or a cheapie? – Tester101 May 17 '17 at 10:29
  • I got a meter with a continuity tester so i checked my wires. I only get a beep when i touch the red wire on my thermostat and the red wire on the control board for my compressor. when i went out to the compressor, it goes off when i touch the 2 sides of my contactor, which the inside wires hook to. what does this mean? – Bobbie May 18 '17 at 21:15
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There is a short somewhere. Typically it is the thermostat wire grounding on the cabinet but after a lightning strike anything could be causing it. I would ohm out all the components to ground. Or you could install fuses on both sides of the transformer and take one thing out of the circuit at a time, when the fuse doesn’t blow that is your problem. I would start by disconnecting all the thermostat wires from the furnace and see if that is the problem.

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