I've installed a 10 year old upflow fuel oil furnace in my garage ( northern minnesota). everything works fine except the fan. it short cycles on and off during and after the time the burner is on. the limit switches for the fan are 2 'snap' switches, one for overheat protection to shut off the burner should the firebox get too hot, and the other one for the fan control itself. I believe it turns the fan on at 130 degrees and off at 110 degrees. I've figured out that the problem occurs due to the cold air (40 degrees or less) being sucked off the floor, up the plenum, hitting the snap switch and cooling off. hence the fan stops. 20-30 seconds later it is heated to the 130 degrees and the fan turns on again. and the cycle repeats until the thermostat is satisfied. the question is... how do I trick the snap switch or replace it with something to solve this. thanks in advance for any help or suggestions you may offer.