as the library link states, it's important to allow ventilation when it's hot so the buns can cool down. Give them a frozen water bottle and maybe a piece of cloth or towel over it so they can lay against it if they get too hot. Direct fanning can annoy them and be counterproductive if they get stressed from it. If it's a fan that oscillates, set it to do so and angle it so that it circulates the air around the middle and top of the cage. This way the bun can avoid being directly hit with it. If that is not an option, you can direct the fan at a part of the cage and allow for another part of the cage to be free so if the bun wants to get out of the fanned air, he can.