How to slow down 240V AC bathroom fan
Simply use capacitor in series. You are talking about bathroom fan, so I think this power is about 20...30W, it means current is about 0.1A, so to loose 100V you need reactance about 1kohm - so something between 1.8...3.3uF should be enough. Of course, DO NOT use electrolytic capacitors - only HV MKSE, MKS or polypropylene capacitors with breakdown voltage > 600V.