It is time to make some changes to your furnace or you may have a leaking roof vent. It is the first thing that will catch the fire if the furnace is accidentally turned on. If the furnace runs for quite a while, there is no reason for the fan blades to be stationary. The blades come in contact with the hot air passing through them on a regular basis and it is possible that they are not moving. You should check these to make sure that they are not moving and there is no build up of dust in the area.
It is also a good idea to ensure that there is no debris blocking the vents. This is especially important if you have a chimney in the roof. The chimney can block the vents for a number of reasons. For instance, if it is made from clay, the debris can cause the chimney to collapse. This could even lead to serious structural damage. If your roof is made of clay, you may want to consider changing it to a more fire resistant material such as tile. If you are not sure which type of material will suit you best, you should consult with your roofer or roofing contractor.
You should make sure that the roof vent is clear of debris. Even if you think that it is clean, it is still something you need to check out. There are many times when the chimney can leak and the debris will start to collect in the area where the ventilation is. When this is the case, the debris will stop the chimney from working effectively. You should clean out this area and you will find that your furnace will work much better as well as you.