If the heat comes on to begin with but then trips out then the thermostat is probably the culprit.
however...if the machine works fine but the heating doesn't come on at all then it is most probably the thermal cut out near the heaters. This usally trips if you do not have the propoer ventage and clear airflow into the machine. Check the vents are clear and search for the cutout. This needs to be reset and away you go.
(it looks like a small inch square unit and usually has a small red button to reset)
Becks, March 2010
