"fan" on the computer start regularly
Open Activity Monitor and check the "Energy" tab. Click on "Energy Impact" to see what is using the most energy. This is likely what is causing the heat. Close this program when not in use. You can monitor the temp with Temperature Monitor (built into OS X). Spotlight > Temperature Monitor  

Also, this 3rd party program can help, SMCfanControl: https://www.eidac.de