Generating Heat

The metal heater chimney is located behind and to one side of the drum. The heater element is located inside the chimney. The heater element is composed of coiled resistance wire suspended by ceramic insulators. Air is drawn into the open bottom of the heater chimney, rapidly heated as it passes the element and exits at the top of the chimney into the drum.

Two 120 Volts Alternating Current (VAC) circuits and a common ground come into the dryer. The heater and its controlling thermostats are across both 120 VAC circuits. The motor is across only one 120 VAC circuit. This is important to remember when troubleshooting circuit breakers (or fuses). If the breaker trips (or the fuse blows) only on the 120 VAC circuit that the motor is not on, the motor will run but the heater will not work.

Thermostats are a switchs that open at a specified temperature. Thermostats control the temperature of the air inside the dryer drum. Two strips of metal with different coefficients of expansion versus temperature can be bonded together to form the switch inside a thermostat.

The metal that expands or contracts the fastest with changing temperature is placed on the bottom of the switch leaf. The switch leaf warps up and opens the circuit at the designed temperature. When the air surrounding the thermostat cools down, the switch leaf straightens up and closes the circuit again. Another variation on this design is to use a diaphram made from the bimetalic material that "oilcans" or snaps up and opens the circuit when the operating temperature is reached.

The operating thermostats are located on the exhaust side of the blower. The operating thermostats are in series with the heating element and determine when it is on. There are two ways to configure the operating thermostats to regulate the drum temperature for the "Normal" and the "Permanent Press" cycles. A low temperature thermostat is needed for the Permanent Press cycle and the high temperature thermostat is needed for the Normal drying cycle.

One implementation of the high and low operating thermostats merely has two thermostats; one for the high temperature cycles and one for low temperature cycles. The desired range of drying temperatures are selected by the timer settings. If the "Normal" cycle is selected, the high temperature operating thermostat is switched in series with the heater element,and conversely, the low temperature thermostat is selected for the "Permanent Press" cycle.

Another more clever implementation places one high temperature operating thermostat inside a heater collar. Just the high temperature operating thermostat is switched in series with the heater element for the "Normal" drying cycle. The heater collar encircling the high temperature operating thermostat and the thermostat are both switched in for the "Permanent Press" cycle. The thermostat heater collar warms the thermostat and "biases" it such that it opens at a cooler drum temperature than if the heater collar were not energized. A less expensive (and more reliable) bias heater is substituted for a low temperature operating thermostat.

The safety thermostat or high limit thermostat is mounted near the heating element in the heater chimney. The limit thermostat opens at a higher temperature than the high temperature operating thermostat. Its function is to cycle the heater off if either operating thermostat fails closed. If a heater were allowed to be on continuously, serious damage can be done to the dryer and the clothes; to say nothing about the fire hazard. If the operating thermostat's contacts weld together, the dryer heater is cycling on the limit thermostat. The dryer will smell hot (because it is). Synthetic fibers will be permanently damaged, the paint surrounding the chimney outlet into the drum will become scorched, plastic consoles will melt and the rear access panel paint will discolor.

A thermal cut-off thermostat is embedded in the windings of the drive motor. If the motor winding temperature exceeds the thermostat open circuit temperature, the dryer will stop. The motor's centrifugal switch will open the circuit to the heater element. When the motor temperature drops back down below the open circuit temperature, the thermostat switch will close but the dryer will not restart by itself.