Why do not want to start the heater? What is the temperature in your growing area? Like with calcium and magnesium levels? What nutrients do you add?
Extra cost of electricity is what I'm trying to avoid because they are not in a tent, they are in a decent sized room. Temp in my growing room is currently mid 60's to low 70's F, although there is a heated pocket of air around my plants from the lights and heat mat, so they are in the upper 70's F. I'm not running a fan around the plants for that reason so they can stay in that warm pocket of air under the lights. January/February can get pretty cold down there though and historically it gets into the low to mid 50's F, although with the heat mat I think I can keep the actual plants in the 60's at night.
I use dolomite lime in my soil mix for magnesium and calcium and I add a lot of crushed eggshells. I also top dress with crushed eggshells during the grow. For nutes, it's all organic. Soil is a mix of leaf mold, compost, perlite, worm castings, insect frass, micronized volcanic minerals, pumice granules, biochar, greensand, and graphite powder. I use molasses in aerated water for every single watering. I raise my own worms for castings, breed dubia roaches for the frass and I collect, make, and mill all my minerals locally except for the dolomite lime, greensand, and perlite. The graphite powder is a waste product from my machine shop because I make nozzles for rocket motors. It adds elemental carbon and sulfur as well as beneficial surface area. All that gets vibratory screened/mixed and left to sit for about a month to settle all the nutrients in.
Keep in mind that while this is my first cannabis grow, I've been gardening a long time. I'm basically using the same soil principals that I use on my hot peppers and hop plants.