I installed a ceiling fan speed control switch last season on my boiler and the switch failed after a few months. The cheaply made switch couldn't handle the amperage of the motor I was controlling. This season I purchased the following rheostat designed for hand routers. It is working well and is reasonably priced.
http://www.ebay.com/itm/Variable-Speed-Control-Rheostat-potentiometer-motor-AC-/380075545605?ssPageName=ADME:L:OU:US:3160 This link is broken, either the page no longer exists or there is some other issue like a typo.