that's why i use heat emitters. basically it's a ceramic 'bulb' with a heating element in it. it puts out NO light and as much heat as you desire (by the wattage of the emitter bulb)... in the house, i use a 75 watt emitter in a 2x4 brooder, about 10" above the bedding for new chicks. when they start spreading out because they're too warm i swap it out for a 40 watt. after about 4 weeks i turn it off all together. if they're outside, if it's likely to be below 40 degrees, i use a 150 watt emitter and set it about 12-14" from where they sleep. if it's going to be warmer than 40 over night, i'll use a 90 watt. i don't change the outside ones much, if at all, but go by what the weekly average is likely to be, and will opt for the hotter one than worry about cold chicks. in the summer, outdoor babies get a 40 watt bulb and it's usually turned off by the time they're 4-5 weeks old. and they last practically forever, short of breaking/dropping one. i've only had 1 in the last 20 years just stop working, and it may have been dropped or damaged when i moved.