At a little over 7 weeks I took away the heat lamp for my chicks in the coop. It did get down to 29 one night, and there were no problems whatsoever. My grandmother is the one who assured me they would be fine, and that the heat was not necessary. Now at nearly 9 weeks, they leave the coop at sunup, while it's probably 40 degrees and are as happy as they can be.
Right now I have 3.5 weekers in my sunroom and I'm pretty sure it's about 75-80 in there during the day, and probably 65-70 at night (I leave the door to the kitchen/dining open at night so our central heat can circulate in there). They have no heat lamp and I cover their brooder with a blanket at night......all are as feisty and as healthy as they can be. They are also feathering beautifully.
I'm thinking these little birds are a lot tougher than we give them credit for.