I'm going to jump in on this thread as well if you don't mind. I just today changed out the fan on my heater, and would like to be able to lower the speed if it gets to warm. In your responce Eric, you said you can use this PWM speed controller, but if you put up a picture or something I didn't see it. Here is the fan I just put in, they don't show the amps, 140x38mm 2 ball bearing High Speed 5200RPM at 308CFM, 12Volts

This is the controller I found, and wonder if it is likely to do the job for me if you don't mind giving your opinion. Please bear with me, I'm electroniclly challanged. (some would say in other ways as well) :-) JImChuk
Input supply voltage: 1.8V-15VDCThe maximum output power: 30W
The maximum continuous output
current :2A