This Chimney-top Power Damper is custom made to the internal dimensions of the chimney flue. A low voltage wire is strung down the outside of the chimney, through the building wall and connected to a 24Vac On-off-On switch and 24Vac indicator light. The On-off-on switch is usually placed near the fireplace hearth and turns on the 24Vac power required for the damper to open.