Don’t stop a tumble dryer mid-cycle. Especially if it’s been on a while and is very hot. There are two potential consequences. Turning it off mid-cycle could cause a malfunction, or could cause laundry inside to catch fire.
There are overheat protection TOCs inside designed to trip at dangerous temperatures. A higher temperature can be inadvertently reached if you stop a dryer mid-cycle. (Hotpoint and Indesit dryers in particular used to be plagued with this issue). This is because whilst a dryer is running, the fan inside blows air over the element and keeps it at desired temperatures. If you stop a dryer mid way through the drying cycle the fan immediately stops running. The heating element continues to give off latent heat and can suddenly get extra hot inside. This could cause a TOC to trip, or an element could warp.
Heat pump condenser dryers
Some tumble dryers can get extremely hot inside on certain drying cycles. I have a Miele dryer and it warns in the instruction manual against stopping it mid cycle. It specifically says that laundry could catch fire. Laundry could contain grease or other flammable substances due to either being exposed to them and/or not being washed thoroughly enough, or not washed in high enough temperatures to remove it. If so then it is possible for laundry to catch fire (even during normal operation). But if a dryer is stopped manually by a user then a momentary raising of heat caused by the fan suddenly stopping could help laundry to catch fire inside the dryer even when unplugged. Remember we all deposit grease from our skins onto clothes and bedding and if not washed at appropriate temperatures it can build up (presumably not obviously to us).
Use the cancel option
A tumble dryer is one of the appliances you should not leave running if everyone has left the house (or has gone to bed) as it’s a potential fire risk. Therefore it’s sadly ironic that suddenly turning one off mid-cycle could also cause fire risk issues. If you need to stop a dryer mid-flow, turn it to the last 10 minutes if possible so that it enters the controlled cooling down period. This is around 10 minutes where the heater is turned off but the drum and fan remain running to allow a degree of cooling down before stopping. However, many modern dryers are no longer controlled by mechanical timer knobs and are controlled by software. So check the instruction manual to see if there is a way to properly cancel the programme. I’m pretty sure I’ve come across dryers that let you simply cancel without forcing a cool down period first which is arguably a design flaw.
Basically, try not to switch a dryer off mid-cycle for this reason – just in case. Leaving one running when out is also a potential risk so check your instruction manual to see how to cancel a dry cycle. If you do turn one off and there is no cool down cancel mode don’t open the door until it’s cooled down a while in case letting air into the drum facilitates ignition.