Pitbull Puppies love heater

My puppies love the heater.  I made sure I got a heater without the burning rods.  Instead I got a heater that blows heated air.  They love to gather in front of it.  And sometimes they compete to be in front and often topples the heater down.  And in these cold days, they seems to love it even more.  I'm not exactly sure what type of temperature they can withstand, but I don't let them sleep outside.  I bring them in the house to sleep.  They stay in the kitchen and I block all entrance for the rest of the house.  If not, we will wake up with a lot of dog shit in the living and else where.