Fuse on furnace keeps tripping

I have the air on and all of a sudden the furnace is not working, called repair man he said it was the fuse. There was an 8 amp in there which the inside of the furnace says not to use more than an 8 amp. So just to keep it working till I could get another fuse he put in a 15 amp with a trigger. That evening it went off again and I pushed the trigger to start it up again. Can't figure out the problem. This fuse box is on the outside of the furnace and only controls the furnace. It is almost impossibe to find an 8 amp fuse in any hardware store. Is it a problem to keep the 15 amp in there? What do you think the problem could be?