If an electric clothes dryer is not heating up, the cause is often a blown circuit breaker or fuse, or a broken heating element. For a gas dryer, causes for no heat include no gas supply or a broken ignitor.
Other causes for why both electric and gas dryers do not produce any heat include a blown thermal fuse or a faulty power cord. If the clothes dryer produces only small amounts of heat, a buildup of lint can cause internal clogs and prevent the appliance from heating properly. Additionally, if the dryer is positioned too close to a wall, the airflow can get restricted from the appliance by pinching the air duct together, which can cause the dryer to improperly heat up. Also, if the point where the dryer air flows from the house is clogged by lint or debris, or blocked by trees or other large objects, the dryer may not heat up properly.
Some electric dryers have heating elements with two circuits, so if one circuit blows, the dryer can only produce small amounts of heat. Dryers that only produce small amounts of heat may also have a broken cycling thermostat, which can prevent the dryer from heating properly if it fails.