Furnace comes from a Greek word for "oven". Today's furnaces are metal boxes that create and contain heat that can melt stones (yes, they're that hot). Many modern furnaces run on natural gas, others on electricity. The heat generated creates high amounts of heat which is then fed through a ventilation system throughout a small house or the largest building. Small furnaces are attached to boilers to provide hot water. These units are more efficient than traditional water heaters.
A number of these or similar problems can occur. The best thing to do is contact Five Star Comfort and we'll be happy to investigate and restore your equipment to full functionality.