Fan systems are necessary for clean, economical performance. The flame produced is concentrated and intense in the small area of the burn pot as a combustion blower introduces air into the bottom of the burn pot, while also forcing exhaust gases into the chimney. While some pellet stoves will be hot to the touch (especially on the viewing window), most manufacturers utilize a series of cast-iron or steel heat exchangers that run along the back and top areas of the visible firebox. With a convection blower, room air is circulated through the heat exchangers and directed into the living space. This method allows for a much higher efficiency than the radiant heat of a hand-fed wood or coal stove, and will in most cases cause the top, sides, and back of the stove to be at most warm to the touch. Along with convection air, an exhaust fan forces air from the firebox through special venting specifically made for pellet fuel. This cycle of circulation is an integral part of the combustion system as well, for the concentrated high temperature flame will quickly overheat the firebox. The possible problems associated with overheating are electrical component failure and flames traveling into the auger tube causing a hopper fire. As safeguards, all pellet stoves are equipped with heat sensors, and sometimes vacuum sensors, enabling the controller to shut down if an unsafe condition is detected. For daily maintenance, an ash vacuum is recommended. These are similar to shop vacs, but are designed for the removal of ash materials. These vacuums are available with a pellet stove kit which enables the cleaning of the interior areas of the stove which improves efficiency.