Why does the transformer on the furnace keep burning up

hvac

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.

Best Answer

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.