Was having transformer problems on residential unit as previously mentioned. Put 3 amp inline fuse on 24v circuit as suggested. Indoor fan comes on now with manual ON switch, but switching T-stat to cool blows the fuse. Getting 27 volts TO T-stat on all wires. Getting 239V on outside line side of contacter. Push in on contacter and comes on. Getting continuity on contacter coil. T-stat will not send low volts to outside unit, just blows fuse. Also replaced wire to outside contacter. At a loss... help!