Lindsey, not sure what boiler you have, the only info I got is for a Ferroli boiler. Again, there are numerous of those. If you can a model number would be helpful. On the Ferroli Optimax the F5 error code is:
Fan problem
• Tachometer signal
interrupted, fan connection
• Fan damaged, debris in fan
Possible Solution
• Check the wiring and fan
• Check the fan, clean debris
Hope this helps you to get started. Good Luck.