In my experience, the heating mat is only useful for a week or so. In most cases the top of a refrigerator will do just fine. I experimented with leaving triple parallel pairs of 40 watt cool florescent tubes, my usual light source, on for 24 hours as opposed to my usual 16 on and 8 off. The theory that 'maters need a period of darkness was borne out. Timers are reliable and inexpensive. 16 hours might be doable but don't push it farther than that. The important thing is to keep 'em cool (60 Deg or cooler) barely damp, and within 3" of the light source. It seems easier to put the flats on blocks which are removed as the plants grow. Three pairs of florescents can be a hassle to raise up. Chuck Wyatt http://www.heirloomtomatoes,net