Slow a fan's speed

I've got a 230v extractor fan, pretty standard and cheap, that's quite noisy and too powerful for the room under the stairs that houses a fridgefreezer
and 4 computers that are on permanantly. Without the fan the computers get too hot in summer.
I'd like to slow the fan down to a slower but constant extraction so the room never builds up heat and have seen special fan controlers to do this but they seem expensive. Is a light dimmer switch practically the same thing? Could I use one without any undesirable complications?
The light in the small room comes on via a mains PIR when someone opens the door. With the fan running slower and quieter, I'd like it to go to full speed when someone enters. Could I wire the mains line from the light to the fan directly, so when door closed it receives a small voltage from the dimmer, and when door open it gets full mains from the light? Would the two power sources overload the fan? Would the power coming from the light affect the dimmer? Would it need a diode or am I just getting too complicated?
I'm not an electrician (as you've probably gathered) but I'm a good DIYer and am seeking informed advice from people who know more about electrics than me. TIA, James
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A light dimmer would not be recommended. There are similar devices that are rated for motors-they do cost more because motor loads are inductive-presenting more of a problem for the controller. You don't indicate the power/ speed rating for this motor. That is important. A device that simply changes the voltage may not actually be satisfactory. Why not replace the fan with a smaller unit and have a second unit come on when the light comes on? It would be more efficient and possibly cheaper in the end.
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Don Kelly snipped-for-privacy@shawcross.ca
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