I picked up two 240v mainframe cooling fans for 50p each at a car boot, with the idea to replace the inadequate ones I have had botched for a few years, to circulate heat from the oil burner in my garaage workshop.
I switched them for three speeds and off, Slow = both in series, medium = 1 on, high = both on. I wasn't sure how they might run in series, but they are fine. Mounted them on a neat frame and at last got around to making a tidier control panel for it, with an LED display for the burner temperature from a t/couple.