I have a 7100FP and replaced the snap disk last season. Even with the new disc, the fan comes on as it should when the firebox is heated. However, if the fire gets real hot the fan shuts down, and the manual override switch will not actifate the fan. I found that if I open the doors and use the screen, the firebox cools a little, and the fan comes back on. Any idea why the override would not work?