I use a heating pad draped over a wire frame. Works great in my unheated garage (average temps of about 50*). The way it works is that the chicks have to make contact with the heating pad, just like they would if brooded by a hen, to get warmed.
The rest of the brooder is the ambient room temp...
250 watts is too much for inside a house. Chicks need a warm zone, heated to about 95 degrees. The rest of the brooder should be about room temp. The heat plates and heating pad methods are designed to mimic a broody hen, with the chicks going underneath for a quick warm ups throughout the day...