I got a little PTC heater the other day. Got a 120V model.

I wired it up and plugged it into the outlet strip in my office. I waited for about half an hour and it barely got warm.

Hmmmmm..., so I called them up. Hey, this thing ain't working. Guy asks what the ambient temp is. About 70F. He said it's self regulating and won't kick out at that temp.

So, the next morning when the building was pretty cold I plugged it in. Low and behold, it was kicking out some heat. As the building warmed up it regulated itself down to about nothing. I'll try it in the freezer and see how it works.