If a clothes dryer is not heating properly, the most likely source of the problem is a defective heating element. Often a break in the coil is visible, but if it isn't, use of a multi-meter reveals problems that are invisible to the naked eye. Other potential causes of the problem include the upper limit switch or the timer. With gas dryers, the igniter or heat sensor may be defective.
The upper limit switch is a safety device that prevents overheating. If the temperature of the dryer rises above the set point of the fuse, the switch breaks the circuit leading to the heating element to prevent a fire. Manufacturers design the fuse for replacement each time it stops the device from overheating.
The dryer timer starts the heat and turns it off to allow clothing to cool down before the end of the drying cycle. A defective timer sometimes initiates drum rotation but the heat element does not turn on. In this case, clothing tumbles in a cold machine without drying.
Gas dryers include safety devices to prevent a gas leak if the burner does not ignite properly. Both an electronic igniter and a heat sensor are present, according to The Family Handyman. If the igniter burns out, the heat sensor stops the gas flow. Owners need to have both devices checked if a gas dryer does not heat.