Thanks for the suggestions Boon, but I'm looking to heat the soil not the air, or possibly makes a heat platform to set the trays on. I ran across several designs that use incandescent rope light as a heat source. I can buy 24' for around $17 with my discount at Lowes. These were all used in a hot bed where the light string was mounted to foam board in channels and the trays set on top. An adjustable thermostat with a remote sensor runs about &12. A 24' soil heating cable runs $50 or so with a thermostat that's built in line and not adjustable. I'm still comparing specs, wattage and $/ ft. You know me....I like to do my research.Really I'd like to heat the soil and once sprouted in the plugs, transfer them to the soil to really get established prior to transplanting them.Last night the temp in the bed went right down to ambient....23*. That ain't gonna work. Tonight I'm going to try covering the house with a tarp or something and see what happens. Rg