Your chickens aren't dying from the cold. If you have standard, cold hardy breeds, there is something else going on in your flock. How old are your chickens? How many do you have? What is your setup like? How big is your coop in feet by feet? Same with your run? Is your coop well ventilated? Did the ones that died show any symptoms before they died? If so, what were they? The more you can tell us, the more we can try to help.
I live in MN. I don't use heat. It gets to the 20's - 30's below (sometimes for days on end) here in the winter. I have yet to lose a bird to the cold. When I first started raising chickens, I would shut the coop up tight and have a heat lamp in there to keep the water thawed and the chickens warm. They suffered from frostbite and respiratory problems every year. When I finally learned about no heat and good ventilation, my chickens have been healthy throughout the winter. Now, I don't even put the windows in until it's well below freezing. Even then I keep them open a couple of inches at the top. Pop door stays open until it's in the teens below zero. I do keep smaller-combed birds because of the potential for frostbite. (When it gets that cold, frostbite can happen even in the best of circumstances.)
One of the problems with artificial heat is, your birds get dependent on it. They don't grow the necessary downy layers to keep themselves warm. If you lose that heat, it's harder on your birds than being without.
