Variable speed switch on our fireplace insert fan has stopped working. Can I replace it with a dimmer switch?

the hardware store should have a three position switch(does the motor have set speeds or truly variable?), a dimmer switch wont work because motors are inductive instead of resistive. dimmers work by reducing the voltage to the load, but motors need full voltage all the time or they burn up. a special switch(rheostat) is needed to control motor speed variably. some dimmer switches are rheostats, but not all

Variable Speed Fan Switch

Dimmers switches only work with incandescent bulbs or special fluorescents. The switch will be rated for a range of watts. So the total watts of the bulbs must always be within that range to avoid damage to the switch. If the connections are tight and it still doesn't work properly you need a new dimmer switch.

The manufacture of your insert fan most likely sells replacements. Either visit their website or give them a call and ask; they'll be able to replace the part and give you detailed information on how to replace it. Do not use a dimmer switch; it will damage the motor.

Just replacing the switch with a "Fan speed control" switch may do more harm than good, if the motor isn't "wired" for variable speeds.

DO NOT USE A DIMMER!!!!!!!!