Tumble Drier Dryer safety devices TOC and thermostats

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Thermostat heat control

In a typical tumble drier (non electronic) there are at least 2 thermostats which operate within the INLET AIR AND THE EXHAUST AIR zones.

The Inlet thermostat is restricting the ambient air entering the drying process, if its too hot the tumble dryer heater is shut down, as it tries to maintain a controlled drying temperature.


The Exhaust thermostat is monitoring the used air leaving the appliance, most are preset to 60 degrees, at this temperature it switches off the heater.

Once you have loaded the dryer and switched on, your cold wet washing starts to absorb heat from the air flow through the drum. It takes a while for the clothes to start increasing temperature, all the time the exhaust air is taking away moisture, the exhaust temperature will start to rise as the clothes dry (raise their own temperature) and accept less heat. As the exhaust temperature hits approx 60 degrees the heater is switched off, the clothes are still tumbling and the dryer is running to programmed end on its timer. The exhaust thermostat cycles the heater to maintain the temperature within the drum.

Electronic ones essentially do the same but use thermistors as these are much more accurate and quicker responding, also using humidity sensors to sense the wetness dryness of the clothes. Simply passing a very small current through the load, as it dries the conductivity decreases and the electronics controls the heater accordingly.

The heater is approx 2000watts to 2500watts which is a significant power usage, drawing 8 to 10 amps through the plug top. If your plug top is getting hot whilst using the dryer check the socket outlet and the prongs of the plug for heat stress, discolouration and cracking of the plastic, all are sure signs of excessive heat and trouble to come.

Thermal operated cut-out devices

Commonly called a TOC....... THIS IS A SAFETY DEVICE.

The TOC reacts to overheating of the element within the device, why?

1) too high ambient temperature

Check there is sufficient air within the room for the appliance to breathe, if using a vented dryer in a closed room for example. Check the vent hose is OK and not perished and split, not only venting into the room but inside the dryer cabinet. Don't cover inlet vents and exhaust vents on the appliance, don't stand condensor units that vent to the left against a wall.

2) Exhaust airflow restricted

The air needs to get away from the appliance, check the flaps open on the outside vent and that the vent pipe is not choked with lint. A badly run vent hose can also fill with water.

3) No motor action

If the motor does not run the fan blades that push/pull the air flow into and ouf of the unit can't operate. The element still comes on and the TOC will activate.

4) Electrical failure of heater and or wiring

Drawing the current they do its common to find terminals and wiring showing heat damage. Once a joint becomes high resistance to current flow it starts to generate heat within the joint or connection and contributes to its own failure.


Your tumble dryer attracts dust and lint and dog/cat hairs during normal use. Periodically it should be cleaned out, it is combustible material after all.

If your TOC trips and is a resettable device, think before you reset it and walk away. Yes it will trip if someone stops the appliance whilst on full heat and then leaves it so. The last ten minutes of a cycle is air distribution only, to allow

1) the clothes to cool

2) stops permanent creasing of clothes

3) cools the appliance

During a dry cycle the inside temperature of the clothes can be in excess of 100 degrees C, way beyond the flash point of some liquids used in cleaning paint brush and fuels. Always allow the dryer to finish cycle and if you're painting and decorating don't tumble dry your overalls.

Thermal trips are there for a good reason

Please don't just reset the trip and put up with it, there may well be a fault within the appliance and that trip is trying to tell you something!













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