After purchasing a 24"x30"x48" tall cold frame for our unheated sun room (Southeastern PA), I knew that I needed both heat and control. This unit controls two 20"x20" seedling mats to perfection. This unit provides continuous readout of the cold frame temperature and indicates when the mats are powered to provide heat. It takes the guesswork out of temperature control and the orchids love it, which makes my wife very happy.