What does a stat or TOC? look like and what do they do?
They both look very similar on most tumble dryers. They tend to be small and round (about the size of a 5p coin) with two wires connected either side. Power flows through them to the heating element (or on the element’s neutral return). When a heat sensitive bi-metal part inside bends under a specific heat temperature it breaks the connection inside. Some TOCs can be totally different though – long and thin.
A stat (or thermostat) is designed to reset when it cools down. They are used to control the temperature of the dryer by regularly switching the heater on and off. A TOC is designed to protect against overheating and normally would not re-set if it triggers. They are designed to only operate under dangerous temperatures.
Generally speaking a device like this is normally closed circuit. If found to be open circuit it has failed. But if it’s a stat and not a TOC a lack of continuity could just be a high resistance through heat sensitive crystals. If you aren’t sure if it is a stat or TOC don’t guess.
A TOC or thermostat should never be by-passed. They are there to stop serious overheating and a dryer could catch fire if run with one by-passed.
Where are the stats and TOCs?
They should be very close to the heating elements. Some dryers have a back panel which can be removed and it’s possible they may be accessible from there, but some will have the heating element and stats at the front of the dryer so it may need stripping down to get to it. Some stats may even be hidden away, for example, monitoring the airflow out (if vented).
Generally speaking if you can’t see one by removing a lid or back panel I’d advise not stripping it down – especially if a condenser dryer – as many dryers can be a right pain to strip and reassemble if you aren’t an engineer.
How to test a stat or TOC
They are tested with a simple continuity test meter. If there is no circuit from one tag to the other it’s generally because it’s failed.
Will replacing a TOC it fix the fault?
Not necessarily. Although these devices can fail for no apparent reason, they usually fail because of overheating due to another fault. In other words, a failed TOC could just be the symptom of an overheating fault in the same way as a normal fuse in a plug is the symptom of a different fault. If a TOC has gone you need to work out why..
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Sometimes they can go for no apparent reason. On vented dryers a blocked or severely kinked vent hose can also cause overheating. Stopping a dryer mid-cycle can cause the overheating stats to fail on some dryers so don’t stop one mid-cycle (see don’t stop a dryer mid-cycle).
Some basic vented White Knight tumble dryers (and possibly others) have a red button on the back which can reset a cut out by pushing it in once it’s cooled down, (it should click when pressed if it has tripped). Again, if this keeps happening there must be something causing it, which needs proper investigation by an engineer.
Faulty Heating element
Another cause for a dryer not heating is of course a faulty heating element. However, replacing an element in a dryer is often much more involved than most people would think. Many tumble dryers would need completely taking to pieces. Some simple vented dryers though have heating elements that are more easily accessible by removing the back panel, and can be tested for continuity or checked for obvious breakage.
Some can just have a small compact heating element inside a metal housing, which slots in and out of the back easily – but most others have large heating elements inside, which aren’t accessible without stripping the dryer down.
A faulty element could be obvious by being physically broken or damaged, but if not, the only way to test one is with a continuity test meter to check that there is a complete circuit all the way through it.
To see what tumble dryer elements look like, and the variety of different types fitted, look at this spare parts catalogue of dryer elements
Micro switches and sensors
Condenser dryer float switches: If you have a condenser dryer it will be designed to cut off the heater via a float switch if the drawer or compartment holding the condensed water gets full. If the dryer stops heating check the compartment has been emptied of water – although most condenser dryers now should have some way of informing the user that the drawer needs emptying such as via a flashing light. It is possible for one of these float switches to go faulty, or stick, therefore triggering the warning when the chamber is empty preventing the heating element from switching on.
Summary of common causes of a tumble dryer not getting hot
- The heating elements can fail (open circuit) or have a faulty connection
- There are cut-outs and stats that can trip out or go open circuit (discussed at length above)
- Faulty connections and burned wires
- Condenser dryers can have float switches or sensors which cut the heater off if the water hasn’t been emptied out of the condenser drawer, check the water is emptied, these sensors and switches can also go faulty