The answer is: thermal fuses / thermostats / switches. I recently had to repair (big word) my Yaesu FP-800 power supply. The fan didn’t switch on anymore, which is not a good thing when you’re busy DX’ing. I didn’t have to search long to find the defect: the thermostat (a.k.a. thermal switch), which is bolted onto the heat sink. When the temperature of the heat sink exceeds 50 degrees Celsius, the switch closes and the fan starts spinning.
At first I tried to get the original part, but I never got an answer from my Yaesu dealer. Instead I used a KSD 301, rated 50 degrees Celsius and N.O. (Normally Open), bought on eBay for less than $2. For a few bucks more I ordered one of those modern silent fans too and threw away the (very noisy) old one. Installation was a piece of cake.
When I discovered that my condenser dryer didn’t warm up, I remembered my Yaesu repair. I knew that dryers use these things too. The only difference is that a dryer uses a N.C. (Normally Closed) type and that these switches, for safety reasons, don’t return to their initial state after they cool down. You can reset them though, and that’s just what I did. It probably saved me a lot of money, as my dryer has been out of warranty for years.