Dryers that tumble but do not dry are a waste of energy. If the dryer does not heat up at all, the clothes may eventually dry from being air-tumbled, but only after multiple runs of the dryer. Several issues can cause this problem. Some are easy to fix, while others may require the help of a professional.
Many issues can cause a dryer to dry slowly, such as a build-up of lint in the duct, overloading or high humidity. In such cases, however, there is usually some amount of heat being produced. If your dryer is tumbling but it not producing any heat at all, then there is likely something wrong with the heating element, power supply and fuses, or thermostat.
Gas dryers have a valve that allows the gas to power the heating element. If the valve is closed, the dryer will still run, but it won't heat up. Find and turn the valve so that it is in the open position. If you have an electric dryer, check to make sure the wiring is correct -- you may need an electrician to do this. The circuit may be wired incorrectly, allowing for the dryer to tumble but not heat. Along these same lines, in most cases, one fuse is reserved for the heating element and another for the tumbling. Check to make sure the heating element fuse is not blown or that the circuit breaker has not been thrown.
Dryers have a heating element and thermostat that control the heat levels. Over time, heating elements can partially or completely burn out. How to repair your heating element depends on the brand and model of your dryer. If you are comfortable with home appliance repairs, you can consult your dryer repair manual and replace the heating element yourself, or you can call a professional to do it for you. Faulty thermostats can also cause your dryer to stop producing heat, although it will still tumble. Again, if you are comfortable making the repairs yourself, your dryer manual will tell you how to replace a broken thermostat according to your make and model of dryer.
Sometimes simple human error can cause a dryer to heat but not tumble. Some dryers have an "Air Dry" or "Fluff" selection on their controls. If you inadvertently choose this function, your dryer will tumble without heating. In addition, gas dryers need space around them to properly ventilate the gas burner. Otherwise, it will not function properly and the dryer will not produce heat. Do not put gas dryers flush against a wall or allow rugs or drapes to touch it.