An electric fan is an inductive load. The relay will be subjected to a high voltage spike when switching off. If the relay isn't designed for handling an inductive load, it will degrade quickly and need replacing. In the summer, the controller will switch the fan on and off repeatedly, exacerbating the issue. There are workarounds, like using a
diode in parallel with the load.
I'm afraid that's about the limit of my understanding in this area.
You should quiz the seller about using it to control a fan, as it might be designed for a refrigeration unit. Aptsys can probably give you more detailed information on whether it will work properly.