Most radiant floor heat systems take about a day to come up to full temperature. The reason for this is due to how the radiant heating system stores energy. Before a radiant floor can emit energy (heat) into a space, it first has to raise the floor temperature. Depending on the floor construction and the initial floor temperature, this start up time may be anywhere from a few hours to a few days. Slab on grade floors will see the largest start up time, mainly because they will have the highest mass value.