For a forced-air furnace, I put a hinged piece of cardboard over a room vent with a paperclip switch. When the furnace blew, the cardboard would rise and the sw

For a forced-air furnace, I put a hinged piece of cardboard over a room vent with a paperclip switch. When the furnace blew, the cardboard would rise and the switch would open. Simple and effective. It doesn't measure the burn cycle of the furnace exactly, but it was close enough for me.