Question: Can one adjust a fan on Shabbos to blow in one direction?
Answer: R’ Moshe Feinstein (Igros Moshe OC 3:49; 4:91:5) writes that electric appliances such as lamps and fans are muktze as they are considered to be kelim shemelachtam leissur, items that serve a forbidden action on Shabbos. The Shulchan Aruch (OC 308:3) writes that such items are allowed to be moved either if one needs the space (letzorech mekomo) or for personal need (letzorech gufo).
Thus, R’ Moshe Feinstein and R’ Ephraim Greenblatt (Rivevos Ephraim 1:235; 5:250; 6:206) allow one to move a fan on Shabbos so that it blows in the right direction. Similarly, one can adjust its mechanical levers to allow it or prevent it from oscillating.
R’ Moshe warns, however, that one must be careful to ensure that the plug doesn’t get pulled out. Likewise, R’ Yisroel Belsky (quoted in The 39 Melochos, p1235:n165) advises that the speed dial is taped down before Shabbos to prevent people from accidentally adjusting it.
R’ Ovadia Yosef (Yabia Omer 7:OC 38:3) writes that while there are poskim who take a stringent view on this, one may follow the lenient authorities.
In conclusion, one may adjust a fan on Shabbos providing that one is careful not to adjust the speed, etc.