A washer dryer not heating on the drying cycle will leave cold wet laundry inside. In this situation you might immediately suspect the heater but (surprisingly) it’s not the main suspect.
Before we start, note that the heater used on the drying cycle is totally separate from the one used on the wash cycle. The element is easily identified from the top of the washing machine and you can clearly see the two connectors for live and neutral. Although not the main suspect you should still check the heater’s continuity first after checking the contacts and connections of course.
Use a continuity tester to check the heating element resistance
Before testing continuity of the element – do not carry out any tests whilst the machine is plugged in! Even if switched off some of the wires may be live. Pull off the wires to at least one of the heating element’s tags to prevent false readings (either live or neutral).
The reading on a continuity meter should have a fair bit of resistance (as with most heating elements). I’d expect a reading of anything between 30 and 50 ohms. It is open circuit, or very high resistance then the element has failed.
If you have a meter capable of doing insulation tests at 500v then check the insulation on each tag to earth. A leakage of up to 2 megs is currently (I believe) the accepted limit. Anything much below that is likely to blow fuses and is not safe. To be honest, any reading to earth is not good
Check the TOCs
If the heater tests out OK, then the TOC’s and thermostats are suspect. They tend to be easily blown on dryers because if they don’t blow in time they can cause a fire. They should be easy to find as all parts for the dryer on a washer-dryer are on the top of the tub. Check for open circuit on the stats/TOC’s as most TOC’s (thermal overload cutouts) are closed circuit and go open circuit when blown.
Occasionally a TOC may be able to be re-set. It’s not common but they can have a small plastic peg sticking out of the centre which when pressed (after cooling down) you can hear a click as it re-sets. If you find one and re-set it but it fails again you need to find out why it is tripping, something is clearly overheating or the device has become faulty.
Don’t mix up a TOC with a thermistor stat
Some devices might look like a TOC but could be a thermistor device (a modern thermostat) so make sure you know it’s a TOC before replacing a device because you tested it and found it open circuit (check my article here for more info and photo of a thermistor). Thermistors are usually slightly larger than a TOC and will usually be open circuit at room temperature, but can give varying resistance readings at increasing temperatures…
If one of the stats (or TOCs) has tripped
If you find one of the safety devices has gone open circuit you should be concerned as to why it has failed. These devices are thermal fuses, which should only fail if the temperature has gone dangerously high. Therefore just replacing one without fixing the reason why it failed may result in the new one failing too. This is also why you should never run a dryer with any of these safety devices bypassed. It could cause a fatal fire!
These devices can and do sometimes fail for no apparent reason, but you must try to see if there is another fault before assuming it is the case. Overheating can be caused by the following –
1: Opening the door or turning the dryer off whilst the dryer is running mid cycle causing the fan to stop running and latent heat from the elements raises the temperature enough to trip the TOC.
2: Overloading. Pay attention to the advised weight of laundry. A washer dryer can always wash more than they can tumble dry. Sometimes quite a lot more. So you probably need to take some laundry out before allowing it onto the drying cycle.
3: No cold water running into condenser chamber during drying cycle (on condenser washer dryers) or a blockage in the chamber stopping the condenser water from draining away.
4: Faulty stat or sensor.
5: Dryer fan faulty or otherwise not running.
6: Unfortunately another common problem is a substantial buildup of fluff and lint inside the heating and fan chamber causing a blockage. This restricts airflow and can cause overheating. It seems to be a design flaw as it affects all different brands of Washer dryer.
Don’t forget when testing a tumble dryer or the dryer section on a washer dryer to see if the heater is working, the first 10 mins on the dial are no-heat, so ensure you turn it fully on to about half an hour or more or you may cure the fault and not realise it because the heaters won’t be energised below 10 minute setting.