Abstract: An adaptor is designed to mount a duty cycling control switch for a furnace or air conditioning system which has flexible duct work. The adaptor includes two opposite cylindrical sections with a central section which includes an arcuate portion and a planar rectangular portion. The planar rectangular portion is tangential to the surfaces of the first and second cylindrical sections. The duty cycling control switch is mounted to this planar portion. Thus, air passing through the duct work will pass through this adaptor altering the temperature of the rectangular plate, thus, altering the temperature of the duty cycling switch causing it to activate.