Boy, @Bubbles12345
, as strong an advocate of this method as I am, I don't think even I'd do it in -20 temps, I have to be honest! I learned my lesson about raising chicks - even under a broody hen - in sub zero temps and I'll never, ever risk it again, regardless of the heat source. Scout was my lesson, and I learned it well. (For any who don't know, you can read Scout's story by clicking on the link in my signature. He was my learn on chick for the heating pad method after he got frostbitten feet out with his mom) I did put him back out there when it was below zero with the heating pad, but frankly we weren't sure he was going to survive anyway and we knew if he did he'd need to be a solo bird being integrated with an existing flock if we didn't at least try to put him back out. He was also pretty well feathered by that time.
@ShidelsRestBB
I'm so happy to hear that Mama Heating Pad has worked so well for you. I'm afraid I'm mathematically challenged (have to count the dots on the dice to play Yahtzee) so I don't know how -29 degrees C translates into F, so you'll have to excuse me if I sound dense. But you are correct about no drafts, good ventilation and rapid and easy integration.
Now, @Bubbles12345
Mama Heating Pad can still work very well for you in an indoor brooder setup if you want to go that way. Scout was in his indoors for many days before we moved him out, and it worked perfectly.