Good afternoon, I have a weathermaker 8000 carrier and recentlly had to change out 24 volt transformer by the circuit board, it runs for 20 minutes or so then pops the 3amp fuse on circuit board, I also changed to 5amp fuse still the same could it be thermostat problem?

Could be but probably not, you have a short in the thermostat wiring or possibly burnt coil on the contactor in the condensing unit, disconnect the two wires (thermostat wires) to the out door unit and see if it stills blows the fuse , Mike

acetc is correct. Also check the low voltage wiring (be 2 wires in brown insulated covering) from air handler to the outdoor unit. They could be shorted and rubbing together causing this problem. Also the stat wiring from the thermostat to air handler.

Could be there is just very small break in insulation and a vibration or small amount of movement is causing is causing this. It will not be difficult to check these things out, just a little time. Anytime the fuse blows it is from the low circuit side. This fuse protects the transformer from blowing. Check in all out. Good Luck. Post back with results

Thanks so much doug, I will try tonight and post back.

To ace and doug the problem is fixed thank to you guys also found were mice have gotten in.