I have described a solution an overheating problem with my IC2200H at Cooling an IC2200H.
Another solution for control of the fan to minimise nuisance draft and noise is one of the inexpensive digital thermostats on eBay.
I reviewed one of these things at Review of inexpensive Chinese thermostat – MH-1210. It had its issues, but as modified for 12VDC operation, I tested it for control of the fan in the above pic.
Above is the thermostat, though modified, purchased for about A$12 including post and the sensor thermistor. Don't buy the MH-1210 as some at least are shipped with incorrect calibration.
Nevertheless, the test demonstrated that the approach performs quite well. The thermostat was configured for cooling, setpoint=50° and differential=5° (to minimise the nuisance of short run cycles).
The above NTC 10k thermistor was wedged in between heat sink fins at the hottest part of the heatsink using a piece of closed cell foam which kept the thermistor in contact with the heatsink and protected it from direct cooling by the fan. For more permanent installations, I use thermal adhesive (sold on eBay as “heat sink plaster”.
The solution works a treat, the thermostats are inexpensive and give a direct reading of heat sink temperature as a bonus. The difficult part is finding a 12VDC one on eBay as most sellers do not describe them accurately (I have a replacement being shipped as I write, a result of the seller shipping a 230VAC model).